Introduction to Constitution & Preamble | 1 GEOGRAPHY MASTER SERIES UNIT 1

Climatology

The ‘Atmosphere’ Around Us

What is Climatology? ●● It is due to the atmosphere that living beings can perform photosynthesis and respiration Climatology is the study of the atmospheric conditions and related climate & weather phenomena. which is essential part of survival of all life on the Earth. The Earth has a radius of 6400 km, and possesses a ●● As part of the hydrologic cycle, water spends narrow skin called atmosphere which is the air that a lot of time in the atmosphere, mostly as envelopes the earth which stretches upwards upto a water vapour. The atmosphere is an important maximum thickness of about 500 km. Ninety-nine per reservoir for water. cent of the gases that constitute the atmosphere, are ●● Ozone in the upper atmosphere absorbs high- located below a height of 32 km. energy ultraviolet (UV) radiations coming Earth’s atmosphere protects us from incoming space from the Sun. This protects living beings debris like comets, asteroids that burn up before on the Earth’s surface from the Sun’s most reaching the planet’s surface, and blocks harmful harmful rays. short wavelength radiations from the Sun. The lower ●● Along with the oceans, the atmosphere keeps boundary of the atmosphere is considered to lie on the Earth’s temperatures within an acceptable Earth’s surface, the upper boundary is the gradational range. Without an atmosphere, Earth’s transition into space. temperatures would be frigid at night and It blocks the outgoing long wave radiations to keep the scorching hot during the day. Earth from cooling to below freezing temperatures, ●● The atmosphere also takes care of extra- thus creating greenhouse effect, which is essential for terrestrial objects like meteors, which get burnt the life over the Earth surface. up while passing through the atmosphere due The weather we experience at the land surface is to the friction. largely determined by the interaction of different ●● Weather is another important phenomenon wavelengths of incoming solar radiations with the which dictates the direction of a number of atmosphere. The solar radiation supplies the energy natural and man-made processes, like plant necessary for cloud formation, precipitation, and local growth, agriculture, soil-formation, human weather conditions. settlements, etc. Various climatic factors join together to create weather patterns. ‘Utility’ of The Earth Composition of Our ●● All living beings need some of the gases in air for their life support. Without an atmosphere, Atmosphere the Earth would be just some another lifeless The atmosphere is composed of a mix of several rock. different gases in differing amounts. 2 Master Series : Geography The permanent gases whose percentages do not ●● As the atmosphere is shallow, its motions over change from day to day are nitrogen, oxygen and large areas are primarily horizontal. argon. ●● The horizontal wind speeds are many times greater than the vertical wind speeds, but the small vertical Composition of Gases in the Atmosphere displacements of air have significant impact upon Gases Amount in Percentage the state of our atmosphere. Nitrogen 78.1 Oxygen 20.9 Aerosols: The Suspended Dust Particles Carbon Dioxide 0.9 in Atmosphere Hydrogen 0.03 ●● With all the gases, the atmosphere contains a wide Neon 0.01 variety of suspended articles, known collectively as Helium 0.0018 aerosols. Ozone 0.0005 ●● Aerosols may be liquid or solid and are small enough that they may require very long time to Others 0.00006 settle out of the atmosphere by gravity. ●● It occurs up to 120 km but up to Oxygen: ●● Common aerosols include suspended soil or desert 6 km as O2 forms or as unstable ozone while sand particles, smoke particles from wildfires, above it occurs in dissociated. It amount to salt particles from evapourated ocean water, plant 20.95%. pollen, volcanic dust, and particles formed from the ●● Nitrogen: Accounts for 78% of the pollution created by coal burning power plants. atmosphere. ●● Aerosols significantly affect atmospheric heat ●● Gases like carbon dioxide, nitrous oxides, balance, cloud growth, and optical properties. methane, and ozone are trace gases that ●● The particles in aerosols cover a wide range of sizes. account for about a tenth of one per cent of Raindrops in clouds are 1-6 mm in diameter. Fine the atmosphere. desert sand and cloud droplets range in diameter down to about 0.01 mm. Sea salt particles and ●● It is unique as its Water Vapour: smoke particles are 1/100th of this, about 0.0001 concentration varies from 0-4% of the mm, or 0.1 micrometre, in diameter (1 micrometre atmosphere depending on location and time of = one thousandth of a millimetre). the day. In the cold, dry arctic regions, water ●● Smallest of all aerosols are the particles that form vapour usually accounts for less than 1% of the when certain gases condense—that is, when several atmosphere, while in humid, tropical regions gas molecules come together to form a stable cluster. water vapour can account for almost 4% of These are the Aitkin nuclei, whose diameters can be the atmosphere. Water vapour content is very measured down to a few nanometers (1 nanometre important in predicting the weather. It contains = one millionth of a millimetre). huge numbers of solid and liquid particles, ●● The size of some aerosol particles allows them to collectively called ‘aerosols’ or the particulate efficiently scatter sunlight and create atmospheric matter. Most of the solid particles are kept in haze. suspension in the atmosphere. These particulate ●● Under some conditions, aerosols act as collecting materials help in scattering the solar incoming points for water vapour molecules, encouraging the radiation which lead to the sunset and sunrise growth of cloud droplets and speeding the formation colour appearance over the horizon. of clouds. ●● Aerosols may also play a role in Earth’s climate. Facts Related to Atmosphere ●● Aerosols are known to reflect a portion of incoming ●● Most of the atmospheric mass is confined in the solar radiation back to space, which lowers the lowest 100 km above the sea-level. temperature of Earth’s surface. Cont... Climatology 3

The Past, Present and Future ●● Now, over billions of years, a considerable excess “Atmosphere” built up this way, so that oxygen now makes up over ●● At the time of formation of the Earth, 4.5 billion 20% of the atmosphere (and carbon dioxide makes years ago, the present atmosphere was not there, it up less than 0.033%). was just like those of other planets. ●● It is difficult to predict the future changes to the ●● The atmosphere of the Earth that was present after atmosphere. Earth was formed about 4.5 billion years ago was ●● It is now argued that the human activity may be probably much different than that of today, with altering the atmosphere to the point that it may abundance of gases like hydrogen, ammonia and affect Earth’s climate. methane. Layers of Atmosphere ●● The Earth at the beginning would had just resembled those of the outer planets—Jupiter, Saturn, Uranus, Earth’s atmosphere is divided into five main layers. and Neptune—with an abundance of hydrogen, The exosphere, the thermosphere, the mesosphere, methane, and ammonia gases. the stratosphere and the troposphere. There is ●● The present atmosphere did not form until after this no distinct boundary between the atmosphere and primary atmosphere was lost. space, but an imaginary line about 62 miles (100 ●● Probably, the primary atmosphere was blasted from kilometres) from the surface, called the Karman line, Earth by the Sun. If the Sun is like other stars of is usually where scientists say atmosphere meets the its type, it may have gone through a phase where outer space. (Ref. Fig 1. 1) it violently ejected material outward toward the planets. All of the inner planets, including Earth, (a) The Troposphere: It is the layer closest would have lost their gaseous envelopes. to Earth’s surface. It is 8 to 18 km thick near ●● A secondary atmosphere began to form when gases poles and equator, and contains half of Earth’s were released from the crust of the early Earth atmosphere. Air is warmer near the ground by volcanic activity. These gases included water and gets colder higher up. Nearly all of the vapour, carbon dioxide, nitrogen, and sulfur or water vapour and dust in the atmosphere are sulfur compounds. in this layer that is why clouds are found here. ●● Oxygen was absent from this early secondary atmosphere. Within the troposphere, temperature decreases ●● The large amount of water vapor released by the with altitude at a rate of about 6.5° C per volcanoes formed clouds that continually rained on kilometre. The boundary between troposphere the early Earth, forming the oceans. and the overlying stratosphere is known as the ●● As carbon dioxide dissolves easily in water, the new tropopause and temperatures at this altitude oceans gradually absorbed most of it. typically approach –50°C. ●● Nitrogen, being unreactive, was left behind to (b) The Stratosphere: It is the second layer of become the most common gas in the atmosphere. atmosphere. It starts above the troposphere and ●● The carbon dioxide that remained began to be used by early plant life in the process of photosynthesis ends about (12-50 km) above ground. Ozone is two to three billion years ago, probably in an ocean abundant here and it heats up the atmosphere or aquatic environment. while also absorbing harmful ultraviolet (UV) ●● At that time, there appeared aerobic (oxygen using) radiation from the Sun creating very stable bacteria and other early animal life, which consumed atmospheric conditions. The air here is very dry, the products of photosynthesis and emitted CO2. and it is about a thousand times thinner here than ●● The cycles for CO2 and O2 were completed; the two gases stayed in balance as all plant material at sea level. This is where jet aircraft and weather was consumed by an oxygen-breathing organism. balloons do fly. There is a presence of high ●● But, some plant material was inevitably lost or altitude cirrus clouds. Maximum temperatures buried before it could be decomposed. This in total approach 0°C at the stratopause that separates removed carbon dioxide from the atmosphere and the stratosphere and the overlying mesosphere. left a net increase in oxygen. Cont... 4 Master Series : Geography (d) The Thermosphere: It literally means “Heat Sphere” extends from about 90 km to between 500 and 1,000 km. Temperatures can get up to 1,500° C at this altitude. The thermosphere is considered part of Earth’s atmosphere, but air density is so low that most of this layer is what is normally thought of as outer space. In fact, this is where the space shuttles fly and where the International Space Station orbits the Earth. (e) Ionosphere: Isolated gas molecules in the thermosphere are broken into ions as solar radiation strips electrons from oxygen and nitrogen molecules. These ionized gases make up the ionosphere, from 80 to 600 km. This is also the layer where the auroras occur. Charged particles from space collide with atoms and (a) molecules in the thermosphere, exciting them into higher states of energy. The atoms shed this excess energy by emitting photons of light, which we see as the colorful Aurora Borealis North Pole and Aurora Australis at the South Pole. (f) The Exosphere: It is the highest layer, is extremely thin and is where the atmosphere merges into outer space. The exosphere extends upto 10,000 km above the Earth’s surface. It is composed of very widely dispersed particles of hydrogen and helium. These light gases escape into the space from here. This is the area where many satellites orbit the Earth. Solar Radiation and the Heat Budget ●● The earth receives heat energy from different (b) sources viz. solar radiation, gravity, and Fig. 1.1 (a) & (b). Layers of the Atmosphere endogenetic activities coming from within (c) The Mesosphere: It literally means ‘Middle the Earth. However, solar radiation is the Sphere’ starts at 50 km and extends to 85 most prominent form of energy that the Earth km high. The top of the mesosphere, called receives. the mesopause, is the coldest part of Earth’s ●● All objects emit heat, and the hotter an object, atmosphere, with temperatures averaging about the shorter the wavelength of radiation it minus (minus 90°C). This layer is hard to study. emits. Jets and balloons don’t go high enough, and ●● Solar radiation occurs in a range of wavelengths represented by the electromagnetic spectrum. satellites and space shuttles orbit too high. Incoming short and intermediate wavelength Climatology 5 radiation may be absorbed by gases in the There is a temporal variance in the amount atmosphere, reflected back into space from of insolation received at different latitudes at the atmosphere or the Earth’s surface, or get different periods of the year. absorbed by the Earth’s surface. ●● Regions within the equator and 40° N and S ●● Incoming and outgoing long-wavelength latitudes receive abundant sunlight and hence radiation is absorbed by water vapour, carbon more heat will be gained than lost. Hence, dioxide, and other gases in the atmosphere. they are energy surplus regions. Regions The greenhouse effect occurs when long beyond 40° N and S latitudes lose more heat wavelength radiation is absorbed in the than that gained from sunlight. Hence they troposphere. are called energy – deficit regions (This is ●● Visible light and near-infrared radiation because of slant sunlight and high albedo of (wavelength 0.7-1.5 micrometres) make up Polar Regions). over 90% of all solar radiation reaching the Earth’s atmosphere. Less than 10% of solar Factors Affecting Distribution radiation is short-wavelength ultraviolet (UV) of Insolation radiation (0.01-0.4 micrometer). There are various factors affecting the distribution of ●● The relative proportion of wavelengths is important as atmospheric gases absorb solar insolation. (Ref. Fig 1. 2) These are: specific wavelengths of radiation. Ozone in the stratosphere absorbs ultraviolet radiation between 0.2 to 0.35 micrometre, effectively blocking the majority of incoming UV rays. In contrast, water vapour and carbon dioxide in the troposphere absorb longer wavelength infrared radiation in the range of approximately 1 to 2 micrometers.

Key Terms in Atmospheric Study Infrared radiation: Similar to visible light but of slightly longer wavelength, sensed as heat. Ionosphere: The region of the atmosphere above 80 Fig 1.2. Insolation Pattern on the Earth km, with raised levels of charged atoms and molecules called ions. 1. Angle of the Sun’s Rays: The angle between the rays of the sun and the tangent Lapse rate: The rate at which the atmosphere cools to the surface of the earth at a given place with increase in altitude. largely determines the amount of insolation Ultraviolet radiation: Radiation just as visible to be received by that place. The rays of sun light but of shorter wavelength, and thus possesses a are perpendicular at one place, while they are higher energy. slanting at the other. The greater the distance X-ray radiation: Light radiation with wavelengths they travel, more slanting they become. The shorter than the shortest ultraviolet, these are very more slanting are the rays, the greater distance energetic and harmful to living organisms. they will spread into, leading to lesser heat. On the other hand, the perpendicular rays have to Distribution of Insolation Over pass through a smaller portion of the atmosphere, therefore they spread into smaller extent. Hence, the Earth the regions receiving direct and perpendicular ●● The amount of insolation received by the rays of the sun are warmer than the others. Earth varies from equator towards the pole. 6 Master Series : Geography 2. Length of the Day: Insolation has a direct relationship with the duration of the sunlight or the hours when the sun is shining. Because the earth is tilted at an angle of 66.5 ° and the orbit of the earth is an ellipse, the circle of illumination is ever shifting in its position due to rotation and revolution. The result is that duration of sunlight varies with latitudes and seasons. The distribution of insolation also differs accordingly, from latitude Fig 1.4. The Sunspots to latitude, and from season to season. The seasonal change is, however, the least at the equator and the 5. Effect of the Atmosphere: The incoming insolation is the maximum at the equinoxes. electromagnetic solar radiation when passes through different layers of the atmosphere, is 3. Distance Between the Earth and the partially reflected, partially absorbed, partially Sun: The distance between sun and the earth scattered and partially transmitted to the earth changes during the course of the year because surface. Reflection from dust, salt, and smoke the earth revolves in an elliptical orbit around the particles in the air is an important mechanism sun. (Ref. Fig 1. 3) Earth is about 147.1 million for returning shortwave solar radiation to space. kilometres from the Sun at perihelion around

January 3, in contrast to about 152.1 million 6. Land and Sea Surface Differences: The specific heat of water is 2.5 times higher kilometres at aphelion around July 4 resulting than landmass, therefore water takes longer to into a difference of about 5.0 million kilometres. get heated up and to cool down, also the Albedo Due to this, winters in northern hemisphere is of land is much greater than albedo of oceans 7% less intense and similarly summers are 7% and water bodies. Especially snow covered areas more intense in southern hemisphere. reflect up to 70%-90% of the insolation. This creates difference in heating of land and water bodies. The temperature over the earth surface depends much on the construct of the surface. Sea water is almost homogeneous in character. Terrestrial Radiation ●● The insolation received by the earth is in the form of short waves and heats up its surface. Fig 1.3. The Changing Position of the Earth and the The earth after being heated itself becomes a Insolation radiating body and it radiates energy to the 4. Sunspots’ Activity: Sunspots are darker, atmosphere in long wave form. This energy cooler areas on the surface of the sun in a heats up the atmosphere from below. This region called the photosphere. (Ref. Fig 1. 4) process is known as the terrestrial radiation. The photosphere has a temperature of 5,800 (Ref. Fig 1. 5). degrees Kelvin. Sunspots have temperatures of about 3,800 degrees K. They look dark only in comparison with the brighter and hotter regions of the photosphere around them. Sunspots can be very large, up to 50,000 kilometres in diameter. The energy radiated from the sun increases when number of sunspots increases and therefore the amount of solar insolation increases and vice

versa. Fig 1.5. The Temperature Zones Climatology 7 ●● The long wave radiation is absorbed by the the atmosphere (6 units absorbed directly by atmospheric gases, particularly by carbon the atmosphere, 9 units through convection dioxide and the other greenhouse gases. and turbulence and 19 units through latent Thus, the atmosphere is indirectly heated heat of condensation). 48 units absorbed by by the earth’s radiation. The atmosphere in the atmosphere (14 units from insolation turn radiates and transmits heat to the space. +34 units from terrestrial radiation) are also Finally the amount of heat received from the radiated back into space. Thus, the total sun is returned to space, thereby maintaining radiation returning from the earth and the constant temperature at the earth’s surface atmosphere respectively is 17+48=65 units and in the atmosphere. which balances the total of 65 units received from the sun. This is termed the heat budget Heat Budget or heat balance of the earth. ●● The earth as a whole does not accumulate or loose the heat. It just uses heat to maintain Distribution of Temperature its temperature. This can happen only if Over the Earth’s Surface the amount of heat received in the form of Temperature indicates the relative degree of heat of insolation equals to the amount lost by the earth a substance. Heat is the energy which make things through terrestrial radiation. (Ref. Fig 1. 6) or objects hot, while temperature measures the ●● If we assume the insolation received at intensity of heat. Although quite distinct from each the top of the atmosphere as 100 per cent, other, yet heat and temperature are closely related while passing through the atmosphere some because gain or loss of heat is necessary to raise amount of energy is reflected, scattered and or lower the temperature. Let us study it under: (a) absorbed. Only the remaining part reaches The horizontal distribution of temperature (b) The the earth surface. Roughly 35 units are vertical distribution of temperature. reflected back to space even before reaching the earth’s surface. Of these, 27 units are (a) Horizontal Distribution of reflected back from the top of the clouds Temperature and 2 units from the snow and ice-covered Distribution of temperature across the latitudes areas of the earth. over the surface of the earth is called horizontal distribution. The horizontal distribution of Insolation and temperature is commonly shown by “Isotherms”, lines connecting points that have equal temperatures. The factors responsible for the uneven distribution of temperature are as: (i) Latitude: The temperature of a region depends upon the amount of sunshine it gets. Since the angle of incidence goes on decreasing from equator towards poles, higher the angle of incidence, higher is the temperature lower Fig 1.6. The Heat Budget of the Earth angle of incidence leads to the lowering of temperature. It is because of this that higher ●● The remaining 65 units are absorbed, 14 units temperatures are found in tropical regions within the atmosphere and 51 units by the and they generally decrease at a considerable earth’s surface. The earth radiates back 51 rate towards the poles. Temperature is below units in the form of terrestrial radiation. Of freezing point near the poles almost throughout these, 17 units are radiated to space directly the year. and the remaining 34 units are absorbed by 8 Master Series : Geography (ii) Land and Sea Contrasts: Land and higher latitude to a lower latitude. The winds sea contrasts affect temperature to a greater blowing from ocean towards the land bring extent. Land gets heated more rapidly and to marine effect to the region. Sometimes local a greater degree than water as in daylight. It winds can change the temperature dramatically also cools down more rapidly than water during for example Warm Chinook winds can raise night. Hence, temperature is relatively higher the temperature of the region by 20-30 °F on land during day-time and it is higher in within few minutes along the eastern slopes water during night. In the same way there are of the Rockies mountain seasonal contrasts in temperature. (vi) Vegetation Cover: The soil devoid of (iii) Relief and Altitude: Relief features such vegetation cover receives heat more rapidly as mountains, plateaus and plains control than the soil under the vegetation cover. the temperature by way of modifying its Because vegetation cover absorbs much of distribution. Mountains act as barriers against sun’s heat and prevents quick radiation from the movement of winds. The Himalayan ranges the earth whereas the former radiates it more prevent cold winds of Central Asia from rapidly. Due to this the temperature variations entering India, during winter. Because of this, in dense forested areas are lower than those Kolkata is not as cold as Guangzhou (Canton) in desert areas. For example, annual range in winter though both are situated almost on of temperature in equatorial regions is about the same latitude. 5°C while in hot deserts, it is as high as As we move upwards from sea level, we 38°C. experience gradual decrease in temperature. (vii) Nature of the Soil: Colour, texture and Temperature decreases at an average rate of structure of the soils modify temperature 6.5 °C per 1000 m altitude. It is known as the to a great extent. Black, yellow and clayey normal lapse rate. The air at lower elevations soils absorb more heat than the sandy soils. is warmer than that of higher elevations Likewise heat radiates more rapidly from because it is closest to the heated surface of sandy soils than from black, yellow and the earth and it is also the densest. As a result clayey soils. Hence temperature contrasts are mountains are cooler than the plains even relatively less in black soil areas than those during summers. of sandy soils. (iv) Ocean Currents: Ocean currents are of (viii) Slope and Aspect: Angle of the slope two types – warm and cold. Warm currents and its direction, both control the receipt of make the coasts along which they flow warmer, insolation. The angle of incidence of sun’s while cold currents reduce the temperature rays is greater along a gentler slope and of the coasts along which they flow. The are smaller along a steeper slope. The ray North-Western European Coasts do not freeze in both the cases carry an equal amount of in winter due to the effect of North Atlantic solar energy. Greater concentration of solar Drift (a warm current), while the Quebec on energy per unit area along gentler slope raises the coast of Canada is frozen due to the Cold the temperature while its lesser concentration Labrador Current flowing along it, though the along steeper slopes lowers the temperature. Quebec is situated in lower latitudes than the For such reasons, the southern slopes of the North-West European Coast. Himalayas are warmer than the northern (v) Winds: Winds also affect temperature ones. At the same time the slopes, in terms because they transport heat from one region of aspect, exposed to the sun as receive more to the other. The winds blowing from low insolation and are warmer than those which latitudes to high latitudes raise the temperature are away from the direct rays of the sun. over the region however they can bring down The northern slopes of the Himalayas for the temperature if they are blowing from a example, not facing the sun as are exposed Climatology 9 to cold northerly winds, and are obviously colder. On the other hand the southern slopes of the Himalayas are sun-facing and are also sheltered from the northerly cold winds, are warmer. Hence we observe settlements and cultivation largely on the southern slopes of the Himalayas while the northern slopes are more under forest area. (b) Vertical Distribution of Temperature The permanent snow on high mountains, even in the tropics, indicates the decrease of temperature with altitude. Observations reveal that there is a fairly regular decrease in temperature with an increase in altitude. The average rate of temperature decrease upward in the troposphere is about 6.5°C per km, extending to the tropopause. This vertical gradient of temperature is commonly referred to as the standard atmosphere or normal lapse rate, Fig 1.7. The Phenomenon of Temperature Inversion but it varies with height, season, latitude and other ●● An inversion acts as a cap on the upward factors. Indeed, the actual lapse rate of temperature movement of air from the layers below. As a does not always show a decrease with altitude. result, convection produced by the heating of Following are the reasons that could be associated air from below is limited to levels below the with this phenomenon. inversion. Diffusion of dust, smoke, and other (i) Heat is transferred through the process of air pollutants is likewise limited. In regions conduction, convection and radiation. Thus, where a pronounced low-level inversion is with an increase in altitude, the amount of heat present, convective clouds cannot grow high transported becomes lesser. enough to produce showers and, at the same (ii) The air pressure is higher at the ground time, visibility may be greatly reduced below level, thus the air density is maximum, the inversion, even in the absence of clouds, but it decreases as we go higher, leading to by the accumulation of dust and smoke thin air. particles. Because air near the base of an inversion tends to be cool, fog is frequently (iii) The quantity of heat absorbing materials like present there. water vapour, dust particles, etc are constantly decreasing. As a result, less heat is captured, Ideal Conditions for Formation of leading to low temperature. Temperature Inversion A. Long nights, so that the outgoing radiation is Temperature Inversion greater than the incoming radiation. ●● As we go up in the atmosphere, the B. Cloudless and Clear skies, which allow temperature decreases, but under specific unobstructed escape of radiation. conditions, a condition opposite to this C. Calm and stable air, so that there is no vertical occurs. Sometimes, the temperature in the mixing at lower levels. lower layers of air increases instead of decreasing with elevation. (Ref. Fig 1. 7) D. Presences of dry air near the surface, so that there is no mixing of heat at the lower surface. 10 Master Series : Geography E. Snow covered ground surface, so that there is maximum reflection of solar radiation from the ground surface. Types of Temperature Inversion 1. Ground Inversion: Develops when air is cooled by contact with a colder surface until it becomes cooler than the overlying atmosphere. This occurs most often on clear nights, when the ground cools off rapidly by radiation. If the temperature of surface air drops below its dew point, fog may result. Topography greatly affects the magnitude of ground inversions. If the land is rolling or hilly, the cold air formed on the higher land surfaces tends to drain into the hollows, producing a larger and thicker Fig 1.8. Illustration of a Frontal Inversion inversion above low ground and little or none 5. Valley Inversion It generally occurs above high. in the mountainous valley due to radiation 2. Turbulent Inversion: Often forms when and vertical movement of air. This is also quiescent air overlies the turbulent air. Within called as vertical advectional inversion of the turbulent layer, vertical mixing carries temperature. (Ref. Fig 1. 9) During winter heat downward and cools the upper part of the nights, the temperature at the upper part of layer. The unmixed air above is not cooled and the mountain becomes very low due to rapid eventually is warmer than the air below; an loss of terrestrial radiation. Consequently, the inversion, then exists. air which is in contact with this cool surface 3. Subsidence Inversion: Develops when a becomes very cold and heavy on the other widespread layer of air descends. The layer is hand the air at the valley floor is respectively compressed and heated by the resulting increase warmer than the upper layer of the air. During in atmospheric pressure, and as a result, the day – time the peaks receive the first ray and it lapse rate of temperature is reduced. If the air starts warming while the air at the valley floor mass sinks low enough, the air at higher altitudes is cooler in comparison to the surrounding becomes warmer than at lower altitudes, air. Then cold dense air descends, pushing the producing a temperature inversion. Subsidence warm air out of valley. As a result, valley floors inversions are common over the northern are not an ideal place for human habitation or continents in winter and over the subtropical cultivation, while the upper parts of the vallies oceans; these regions generally have subsiding are ideal places for humans. air because they are located under large high- pressure centers. 4. Frontal Inversion: Occurs when a cold air mass undercuts a warm air mass and lifts it aloft; the front between the two air masses then has warm air above and cold air below. This kind of inversion has considerable slope, whereas other inversions are nearly horizontal. In addition, humidity may be high, and clouds warming of mountainsides may be present immediately above it. (Ref. Fig 1. 8). Climatology 11 2. Isotherms take sudden bends where land- water contrasts are maximum. 3. The spacing of isotherms indicates the latitudinal thermal gradient i.e., steepness or slow gradual nature of temperature change. Thus, close spacing indicates a rapid change in the temperature and wide spacing means a slow change. The isotherms are irregular over the northern cooling of mountainsides hemisphere due to an enhanced land-sea contrast. Because of predominance of land over water in the Fig 1.9. Inversion of Temperature in a Valley north, the northern hemisphere is warmer. The thermal equator lies generally to the Regional Distribution of north of geographical equator. While passing (Worldwide) through an area with warm ocean currents, the Temperature isotherms show a pole-ward shift. This shift is Isotherms are lines connecting points that have equal very much marked over the eastern side of the temperatures. North Atlantic due to the combined effect of Isotherms have three general characteristics. North Atlantic Drift and Gulf Stream, coupled with the Westerlies. It is marked over the North 1. Isotherms tend east-west, generally following Pacific due to the combined effect of Kuroshio the parallels. Current and North Pacific Current and the Westerlies. Air Pressure and Atmospheric Circulation Atmospheric Pressure density increases. The higher the density of air, the greater is the air pressure and vice ●● The weight of a column of air contained versa. The mass of air above in the column in a unit area from the mean sea level to of air compresses the air under it, hence its the top of the atmosphere is called as the lower layers are more denser than the upper atmospheric pressure. The atmospheric layers. As a result, the lower layers of the pressure is expressed in units of milibar. At atmosphere have higher density, thus, exert sea-level the average atmospheric pressure is more pressure. 1,013.2 milibar. ●● Conversely, the higher layers are less ●● Due to the gravity, the air at the surface compressed and, hence, they have low density is denser and hence has a higher pressure. and low pressure. The columnar distribution Air pressure is measured with the help of a of atmospheric pressure is known as vertical mercury barometer or the aneroid barometer. distribution of pressure. The pressure decreases with height. At any elevation it varies from place to place and its Horizontal Distribution of variation is the primary cause of air motion, i.e., wind which moves from high pressure Pressure areas to low pressure areas. ●● The distribution of atmospheric pressure over the globe is known as horizontal Vertical Distribution of distribution of pressure. It is shown on Pressure maps with the help of isobars. An isobar is a line connecting points that have equal ●● Air is a mixture of various gases. It is values of pressure. Isobars are analogous highly compressible. As it compresses, its 12 Master Series : Geography to the contour lines on a relief map. The convergence experience low pressure spacing of isobars expresses the rate and while those of divergence have high direction of change in air pressure. pressure. ●● This change in air pressure is referred to (iii) Pressure of Water Vapour: Air as the pressure gradient. Pressure gradient with higher quantity of water vapour is the ratio between pressure difference and the actual horizontal distance between two has lower pressure and that with lower points. Close spacing of isobars expresses quantity of water vapour has higher steep pressure gradient while wide spacing pressure. In winter the continents are indicates gentle pressure gradient. relatively cool and tend to develop high pressure centres; in summer they stay ●● The horizontal distribution of atmospheric pressure is not uniform in the world. It varies warmer than the oceans and tend to be from time to time at a given place; it varies dominated by low pressure, conversely, from place to place over short distances. the oceans are associated with low The factors responsible for variation in the pressure in winter and high pressure in horizontal distribution of pressure are as: summer.

(i) Air Temperature: As we know that the earth is not heated uniformly because Pressure Belts of unequal distribution of insolation, The horizontal distribution of air pressure across the differential heating and cooling of land latitudes is characterized by high or low pressure and water surfaces. Generally there belts. This is however, a theoretical model because is an inverse relationship between air pressure belts are not always found as such on temperature and air pressure. The higher the earth. They migrate with the march of the sun the air temperature, the lower is the air northwards and southwards. The pressure belts are pressure. Along the equator lies a belt of divided as: low pressure known as the “equatorial low or doldrums”. Low air pressure (a) The Equatorial Low Pressure in equatorial regions is due to the fact Belt that hot air ascends there with gradual decrease in temperature causing thinness ●● The sun shines almost vertically on the equator of air on the surface. In polar region, cold throughout the year. As a result, the air gets air is very dense hence it descends and warm and rises over the equatorial region and pressure increases. produces the equatorial low pressure. This belt extends from equator to 10°N and 10°S (ii) The Earth’s Rotation: The earth’s latitudes. (Ref. Fig 1. 10). rotation generates a centrifugal force. This results in the deflection of air from ●● Due to excessive heating horizontal movement its original place, causing decrease of of air is absent here and only conventional pressure. It is believed that the low currents are there. Therefore, this belt is pressure belts of the sub-polar Regions called doldrums (the zone of calm) due to and the high pressure belts of the sub- virtual absence of surface winds. These are tropical regions are created as a result the regions of convergence because the winds of the earth’s rotation. The earth’s flowing from sub-tropical high pressure belts rotation also causes convergence and converge here. This belt is also known as-Inter divergence of moving air. Areas of Tropical Convergence Zone (ITCZ). Climatology 13 with cargo of horses passing through these belts found difficulty in sailing under these calm conditions. They used to throw the horses in the sea in order to make the vessels lighter. Henceforth, these belts or latitudes are also called ‘horse latitudes’. These are the regions of divergence because winds from these areas blow towards equatorial and subpolar low pressure belts. ●● The subsiding air is warm and dry; therefore, most of the deserts are present along this belt, in both the hemispheres. A calm condition (anticyclonic), with feeble winds, is created in this high pressure belts. Fig 1.10. Major Pressure Belts and Wind System Hence most of the deserts are present in ●● This belt is characterized by extremely this belt. low pressure with calm conditions. This is (c) The Sub-Polar Low Pressure because of the absence of Surface winds since winds approaching this belt begin Belts to rise near its margin. Thus, only vertical ●● The sub-polar low pressure belts extend currents are found. As the larger part of between 45°N and the Arctic Circle in the the low pressure belt passes along the northern hemisphere and between 45°S and the oceans, the winds obtain huge amount of Antarctic Circle in the southern hemisphere. moisture. Vertical winds (convection) carry They are known as the North sub-polar low the moisture form cumulonimbus clouds and and the South sub-polar low pressure belts, lead to thunderstorms (convectional rainfall). respectively. In spite of high temperatures, cyclones are ●● Winds coming from the sub-tropical and not formed at the equator because of ‘zero’ the polar high belts converge here to Coriolis force. produce cyclonic storms or low pressure (b) The Sub-tropical High Pressure conditions. Belts ●● This zone of convergence is also known as ●● The sub-tropical high pressure belts extend polar front. This pressure belt is dynamically from the tropics to about 35° latitudes in both induced. The surface air spreads outwards the Hemispheres. In the northern hemisphere from this zone due to the rotation of the earth it is called as the North sub-tropical high thus, produce low pressure. pressure belt and in the southern hemisphere ●● During winter, because of a high contrast it is known as the South sub-tropical high between land and sea, this belt is broken pressure belt. The existence of these pressure into two distinct low centres – one in the belts is due to the fact that the uprising air vicinity of the Aleutian Islands (equatorial of the equatorial region is deflected towards low pressure belt), and the other between poles due to the earth’s rotation. After Iceland and Greenland (the Circum – Polar becoming cold and heavy, it descends in low pressure belt). This zone is marked by these regions and gets piled up. This results ascent of warm Sub–tropical air over cold in a high pressure. polar air blowing from poles. During summer, ●● Calm conditions with feeble and variable a lesser contrast results in a more developed winds are found here. In olden days vessels and regular belt. 14 Master Series : Geography ●● The area of contrast between cold and warm (d) The Polar High Pressure Belts air masses produces polar jet streams which In Polar Regions, the Sun never shines vertically. encircles the earth at 60 degrees latitudes and Sun rays are always slanting here, resulting in is focused in these low pressure areas. Due to low temperatures. Because of low temperature, air a great contrast between the temperatures of compresses and its density increases. Hence, high pressure is found here. In northern hemisphere the the winds from sub-tropical and polar source belt is called the North polar high pressure belt while regions, extra tropical cyclonic storms or it is known as the South polar high pressure belt lows’ (temperate cyclones or frontal cyclones) in the southern hemisphere. Winds from these belts are produced in this region. blow towards sub-polar low pressure belts. Winds, Monsoon and Jet-Streams

Wind is defined as air moving horizontally over the left in the Southern Hemisphere, derived from Earth’s surface. Air motions can also be vertical, but the application of the Coriolis effect to air these are known by other terms, such as updrafts Masses). The deflection is more when the wind or downdrafts. Wind direction is identified by the velocity is high. This deflection force does not direction from which the wind comes. seem to exist until the air is set in motion and increases with wind velocity, air mass and an Factors Affecting Wind increase in latitude. The Coriolis force acts perpendicular to the pressure gradient force Strength and Direction (pressure gradient force is perpendicular to ●● Pressure Gradients: Which are usually an isobar). (Ref. Fig 1. 11) As a result of these two forces operating perpendicular to associated with air temperature differences, each other, in the low-pressure areas the wind act to push air from high- to low-pressure areas. The pressure gradient is strong where blows around it. the isobars are close to each other and is weak where the isobars are apart. It affects the speed of the wind. It is greatest at the surface and its influence generally extends up to an elevation of 1-3 km. The vertical pressure gradient force is much larger than that of the horizontal pressure gradient. But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience strong upward winds. The wind direction follows the direction of change of pressure, i.e. perpendicular to the isobars. ●● The Coriolis Effect: Generated by the Fig 1.11. The Effect of Earth’s Rotation on Wind Direction Earth’s rotation, it turns the path of moving Due to the absence of Coriolis force there is absence air sideways, changing the direction of flow. of cyclones around the equator, since at the equator This deviation is the result of the earth’s (Coriolis force is zero) wind blows perpendicular to rotation and is called the Coriolis Effect or the isobars. The low pressure gets filled instead of Coriolis force. Due to this effect, winds in getting intensified i.e., there is no spiraling of air the northern hemisphere get deflected to the due to zero Coriolis Effect. The wind directly gets right of their path and those in the southern uplifted vertically to form thunderstorms. hemisphere to their left, following Ferrell’s ●● Friction: Surface slows the wind in the lower Law (the law that wind is deflected to the atmosphere and acts in a direction opposite to right in the Northern Hemisphere and to the air motion. The irregularities of the earth’s Climatology 15 surface offer resistance to the wind movement towards the centres of rotation (e.g., low and in the form of friction. (Ref. Fig 1. 12) It high pressure centres). This force produces a affects the speed of the wind. It is greatest circular pattern of flow around centres of high at the surface and its influence generally and low pressure. Centripetal acceleration is extends up to an elevation of 1 - 3 km. Over more important for circulations smaller than the sea surface the friction is minimal. Over the mid-latitude cyclone. uneven terrain, however, due to high friction, the wind direction makes high angles with Classification of Winds isobars and the speed gets retarded. ●● The winds blowing almost in the same Maximum Deflection at Pole direction throughout the year are called prevailing or permanent winds. These are also called as invariable or planetary winds, because they involve large areas of the globe. ●● On the other hand winds with seasonal changes in their directions are called seasonal winds e.g. monsoonal winds. Winds blowing in a particular locality are called local winds example Chinook, sirocco, Harmattan, bora-bora, blizzard, etc. ●● Winds blowing from hilltops to the valley and from Valley floor to the hill tops are called Mountain and Valley breeze. Winds blowing from Land to Sea and from sea to land are called land and sea breezes. ●● Thus the winds are classified into two broad Maximum Deflection at Pole categories first permanent winds or invariable Fig 1.12. Deflection of Winds with Latitude winds or prevailing winds, and second ●● Centripetal Acceleration: Centripetal variable winds, which are further classified acceleration creates a force directed at right into Mountain and Valley breezes, Land and angles to the wind movement and inwards Sea breezes, etc. Types of Winds

Permanent or Planetary movement, the pressure belt systems provide pressure gradient for movement of the winds. Winds Their direction is affected by the Coriolis ●● Considering the fact that the location of high force. and low pressure belts remains stationary on Geostrophic Wind the globe, consequently, the winds blow from Air under the influence of both the pressure gradient high pressure to low pressure belts. Winds force and Coriolis force tends to move parallel to blow at the same direction throughout the isobars in conditions where friction is low (1000 year, remain little affected by the seasonal meters above the surface of the Earth) and isobars cycle. are straight. Winds of this type are usually called ●● These cover large areas and distance over Geostrophic winds. (Ref. Fig 1. 13). the earth. They are guided by the Horizontal 16 Master Series : Geography pressure belt), and as they reach the equator, they become humid and warmer after picking up moisture on their way. The trade winds from two hemispheres meet at the equator, and due to convergence they rise and cause heavy rainfall. The eastern parts of the trade winds associated with the cool ocean currents are drier and more stable than the western parts of the ocean. Fig 1.13. Effect of the Pressure Gradient Force and ●● In the Southern hemisphere winds originating Coriolis Force on Wind Direction from the Sub-tropical high pressure and Geostrophic winds come about because pressure blowing towards the Equatorial low pressure gradient force and Coriolis force come into balance are similarly deflected westward to become after the air begins to move. A Geostrophic wind the prevailing South-east trades. flows parallel to the isobars. ●● Trade winds are noted for their consistency, both in force and direction in many areas Winds in the Tropics especially over open seas and derive their ●● The trade winds are those blowing from the name from the nautical expression ‘to blow sub-tropical high pressure areas towards the trade’ meaning ‘to blow along a regular track’. equatorial low pressure belt. (Ref. Fig 1. 14) ●● Zones of sub-tropical highs in latitudes Therefore, these are confined to a region between about 30°-35°N and 30°-35°S are areas of 5°N-30°N and 5°S-30°S throughout the earth’s descending air and are characterize by calms surface. They flow as the north-eastern trades in light variable winds, comparatively dry air the northern hemisphere and the south-eastern and quiet, stable weather conditions. trades in the southern hemisphere. The Westerlies ●● The Westerly Winds blow across latitudes 35°-60° of both the hemispheres. The air streams flowing polewards from the Sub- tropical high pressure areas deflects eastward in the Northern hemisphere to form South- Westerlies. ●● Similar winds in the Southern hemisphere are known as North-Westerlies. Unlike the trade winds, the Westerlies are very variable in force and direction, especially in the Northern hemisphere. In the Southern hemisphere, on the other hand, the Westerlies blow with great strength and regularity throughout the year over the almost uninterrupted expanse of ocean and have given the name Roaring forties, Furious fifties and Shrieking Sixties Fig 1.14. The Tropical Wind System to the region specially between latitudes ●● This deflection in their ideally expected 40°S and 60°S. Sometimes the name is north-south direction is explained on the basis applied to the winds themselves as they give of Coriolis force and Farrell’s law. a roaring sound on account of high speed. ●● Trade winds are descending and stable The poleward boundary of the Westerlies is in areas of their origin (sub-tropical high highly fluctuating. There are many seasonal Climatology 17 and short-term fluctuations. These winds surface is also chilled with the result that produce wet spells and variability in weather. there is a marked high pressure over land. Thus the cooler, heavier, denser air over the The Polar Easterlies land flows towards the sea and land breeze ●● The Polar easterlies blow from the Polar high occurs. pressure area to the Temperate low pressure ●● See Breeze: Unlike land breeze, it blows area. On their equatorward journey they the opposite way from the sea to the land. are deflected westwards to become North During the day the land becomes quickly easterlies in the Northern hemisphere and heated compared to sea with the result there is South easterlies in the Southern hemisphere. a marked low pressure over the land. Thus air ●● The Polar easterlies are dry, cold prevailing is drawn into the land from the comparatively winds blowing from north-east to south-west high pressure area of the adjacent seas or direction in Northern Hemisphere and south- oceans. The southerly sea breeze in summer east to north-west in Southern Hemisphere. is highly welcome in Kolkata. They blow from the polar high-pressure areas ●● Fohn: Fohn is a warm dry wind that blows of the sub-polar lows. down the valleys of the north facing slopes of the Alps and is most common in spring and Local Winds autumn. It occurs when a depression to the Winds, caused by local factors and confined to a north of the Alps draws air from the south limited area compared to planetary winds, are called over the mountains. as local winds. (Ref. Fig 1. 15) Some well-known examples of local winds are: Famous ‘Winds’ of the World & their Regions Names Nature Region Fohn Warm Alps Chinook (Snow eater) Warm Rockies Kalbaisakhi Warm North India Berg Warm S Africa Zonda Warm Andes Loo Warm Indian subcontinent (a) Santa Ana Warm Coastal Southern California Southerly Cold New South Wales Burster Khamsin Warm Egypt Harmattan (Doctor) Warm Guinea Coast Mistral Cold S E France Samun Warm Iran (b) ●● Chinook: Chinook is a warm dry Fig 1.15 (a) & (b). Land & Sea Breeze southwesterly wind, similar to the Fohn in ●● Land Breeze: It is a common local origin and character. Chinook or the Fohn wind that affects only the Coastal areas. is relatively much warmer than the air into During the night the land becomes very which it is invading, but in the absolute sense, much cooler than the sea as land is quickly it may be even colder at times below freezing chilled than the sea. The air adjacent to the point of water. 18 Master Series : Geography ●● Mistral: It is one of the local names given to such winds that blow from the Alps over France towards the Mediterranean Sea. It is channeled through the Rhine valley. It is very cold and dry with a high speed. It brings blizzards into southern France. ●● Sirocco: It is a Mediterranean wind that comes from the Sahara and reaches hurricane speeds in North Africa and Southern Europe. It arises from a warm, dry, tropical air mass that is pulled northwards by low-pressure cells moving eastward across the Mediterranean Fig 1.16. The “Location of Region’ of Famous Sea, with the wind originating in the Arabian Wind Systems on the Globe or Sahara deserts. The hotter, drier continental air mixes with the cooler, wetter air of the 5. The Loos are hot and dry summer westerly maritime cyclone, and the counter-clockwise winds that sweep the Upper and Middle Ganga circulation of the low propels the mixed air Plains. across the southern coasts of Europe. The Sirocco causes dusty dry conditions along Tricellular Meridional the northern coast of Africa, storms in the Mediterranean Sea, and cool wet weather in Circulation of Atmosphere Europe. ●● The Tricellular model is made up of three ●● Loo: In the plains of northern India and Pakistan, sometimes a very hot and dry different air masses, these control atmospheric wind blows from the west in the months of movements and the redistribution of heat May and June, usually in the afternoons. It energy. The three air masses, starting from is known as loo. Its temperature invariably the equator, are called the Hadley cell, Ferrell ranges between 45°C and 50°C. It may cause cell and the Polar cell. sunstroke to people. ●● The Tricellular model also contains the Local Winds : Fact Sheet (Ref. Fig 1. 16) ITCZ (Inter-Tropical Convergence Zone), this is the meeting place of the trade winds 1. Harmattan is a very dry and dust-laden West from both the northern hemisphere and the African wind that blows south-east from the Sahara to the coast especially between Octobers southern hemisphere. The ITCZ is a low to February. Its incursion into the coast of Gulf pressure area where the trade winds, which of Guinea gives relief to stifling humidity. have picked up latent heat as they crossed 2. The Sirocco, a hot dry south wind blowing oceans, are now forced to rise by convection from the Sahara to the Mediterranean, is most currents. These rising convection currents are unpleasant. In Egypt this wind is called Khamsin. then cooled adiabatically to form massive 3. The Mistral and Bora are cold north winter cumulonimbus clouds. It is believed that there winds. Mistral is experienced in southern is cellular circulation of air at each meridian France, especially in the Rhone delta and Bora (longitude). Surface winds blow from high blows down the mountains to the east coast of pressure areas to low pressure areas but in Adriatic Sea and North Italy. the upper atmosphere, the general direction 4. Norwester is a squall occurring during hot season of air circulation is opposite to the direction (April-June) in North India and may bring rain of surface winds. or hail. Climatology 19 Cells of Tri-Cellular Meridional ●● These winds (which diverge equator-ward) again descend near horse latitudes (30°- Circulation (Ref. Fig 1. 17) 35° latitudes) to reinforce subtropical high 1. Tropical Cell or Hadley Cell pressure belt. After descending, these winds ●● Tropical cell is also called as Hadley cell again blow polewards as surface Westerlies because G Hadley first identified this thermally and thus a complete cell is formed. in­duced cell in both the hemispheres in the ●● Winds blow from subtropical high pressure year 1735. The winds after being heated due belt to subpolar low pressure belt but the to very high tempera­ture at the equator ascend winds become almost westerly due to Coriolis upwards. These ascending warm and moist winds release latent heat after condensation­ force. It may be mentioned that the regularity which causes further ascent of the winds and continuity of Westerlies are frequently which after reaching the height of 8 to 12 disturbed by temperate cyclones,­ migratory kilometres in the troposphere over the equator extra-tropical cyclones and anticyclones.­ diverge northwards and southwards or sky ●● Contrary to the existing view of upper air polewards. tropospheric easterly winds in the zones ●● The surface winds in the name of trade extending between 30°-60° latitudes Rossby winds blow from subtropical high pressure­ observed the exist­ence of upper air Westerlies belts to equatorial low pressure belts in order in the middle latitudes due to poleward to replace the ascending air at the equator. decrease of air temperature.According to G.T. The upper air moving in opposite direction Trewartha the middle and upper tropospheric to surface winds (trade winds) is called the Westerlies are associated with long waves and antitrade. These upper air antitrades descend jet streams. Warm air ascends along the polar near 30°-35° latitudes to cause a subtropical front which is more regular and continuous high pressure belt. in the middle tropo­sphere. It may be pointed ●● These antitrades after descending near a 30°- out that this new concept does not explain the 35° latitudes again blow towards the equator cellular meridional circulation in the middle where they are again heated and ascend. Thus, one complete meridional cell of air circulation latitudes. is formed. This is called tropical meridional 3. Polar or Sub Polar Cell cell which is located between the equator and ●● Polar cell involves the atmospheric circulation 30° latitudes. It may be pointed out that the prevailing between 60° and poles. Cold winds, regularity and continuity of the antitrade wind known as polar easterlies, blow from polar systems in the upper air has been refuted by high pressure­ areas to sub-polar or mid-latitude a host of meteorologists on the basis of more low pressure belts. The general direction of upper air data being available during and after surface polar winds becomes easterly (east to Second World War. west) due to Coriolis force.These polar cold 2. Polar Front Cell or Mid-Latitude Cell winds converge with warm Westerlies near 60°-65° latitudes and form polar front or mid- or Ferrel Cell latitude front which becomes the centres for ●● Polar front cell or mid-latitude cell, according to old concept surface winds, known as the origin of temperate cyclones. The winds Westerlies, blow from the subtropical high ascend upward due to the rotation of the pressure belt to sub polar low pressure belt earth at the sub-polar low pressure belt and (60°-65°). The winds ascend near 60°-65° after reaching middle troposphere they turn latitudes because of the rotation of the earth poleward and equator-ward. The poleward and after reaching the upper troposphere upper air descends at the poles and reinforces diverge in opposite directions (poleward and the polar high pressure. Thus, a complete equator-ward). polar cell is formed. 20 Master Series : Geography western Pacific. Higher up in the atmosphere, west-to-east winds complete the circulation.

Fig 1.17. Tri-cellular Meridional Circulation ●● The subtropical high pressure and sub-polar low pressure belts are dynamically induced due to subsidence and spreading of air caused by the rotation of the earth respectively. Upper air anti-trades are not uniformly found over all the meridians. If the trade winds are exclusively of thermal origin, then the thermal­ gradient must be present boldly throughout the tropics but this is not true. At the height of 500 to 1000 m, in the atmosphere, the winds become almost parallel to the isobars which are generally parallel to the latitude. If this is so, the meridional cell of air circulation may not be possible. Walker Circulations Fig 1.18. The Ocean-based System of Air Circulation ●● The Walker circulation is an ocean-based ●● The warm waters of the western Pacific system of air circulation that influences Ocean in East Asia heat the air above it and weather on the Earth. (Ref. Fig 1. 18) supply it with moisture. On average, the air ●● The Walker circulation is the result of a rises, forms clouds, and then flows to the east difference in surface pressure and temperature across the Pacific, losing moisture to rainfall. over the western and eastern tropical Pacific The air then sinks off the west coast of South Ocean. Normally, the tropical western Pacific America and returns to the west along the is warm and wet with a low pressure system, surface of the ocean, back to the western and the cool and dry eastern Pacific lie under Pacific Ocean. The Walker circulation a high pressure system. This creates a pressure contributes to normal weather conditions in gradient from east to west and causes surface the tropical Pacific Ocean: warm, wet weather air to move east to west, from high pressure in the western Pacific and cool, dry weather in the eastern Pacific to low pressure in the in the eastern Pacific. Climatology 21 3. The mean resultant wind velocity in at least one of the months should exceed 3 m/s. 4. There should be less than one cyclone anticyclone alternation every two years, in either month, over a five degree latitude/longitude grid. On the basis of above four criteria, the area of the monsoon region is as a rectangle roughly extending from 35° N to 25° S latitudes and 30° W to 173° E longitudes. The Thermal Concept of Fig 1.19. The El Nino and La Nino Impacts ●● The Walker circulation reverses every few Monsoon years, as part of a phenomenon called the ●● According to this view monsoon is a result El Nino – Southern Oscillation (ENSO). of differential rates of heating and cooling When the Walker circulation weakens, the of land and sea. (Ref. Fig 1. 20) The sun is winds also weaken and the warm water of vertical over the Tropic of Cancer in summer the western Pacific spreads to the east. These season of northern hemisphere and the Indian conditions are called El Nino. During times landmass at this time gets heated to a greater when the Walker circulation is particularly extent than the neighboring sea. This leads to strong, called La Niña, the winds are stronger formation of low pressure conditions over the across the Pacific. (Ref. Fig 1. 9) These Indian subcontinent in comparison to that over strong winds cause cooler ocean temperatures Indian Ocean. Therefore, thermally induced because of upwelling in the eastern Pacific. El pressure gradient is produced from ocean Nino and La Nina impact the weather in North towards Indian sub-continent leading to the and South America, Australia, and Southeast onset of southwesterly winds blowing from Africa, and can cause flooding, droughts, and Indian Ocean towards India. These winds, increases or decreases in hurricane activity. called southwest monsoon, blowing from sea towards land, carry a large amount of Monsoon in India moisture and cause copious rainfall over the landmass. Origin and Mechanism of Monsoon The term monsoon has been derived from the Arabic word mousim or the Malayan word monsin, both of which mean season. Monsoon is characterized by a seasonal reversal of wind direction. They flow from sea to land during the summer and from land to sea during the winter. The Asiatic seasonal wind reversal is notable for its immense extent and the Fig 1.20. The Summer and Winter Monsoon in India penetration of its influence. C S Ramage (1971) has ●● The land not only gets heated faster, it also identified the four main features of monsoon winds. cools faster to a greater extent in summer 1. The prevailing wind direction should shift by at than the ocean. Hence, the Indian Ocean least 120 ° between January and July. is warmer than the Indian subcontinent in 2. The average frequency of prevailing wind winter. This causes the pressure gradient to directions in January and July should exceed 40 be reversed towards sea. This altered pressure per cent. gradient leads to the onset of winds blowing 22 Master Series : Geography from northeast to southwest, i.e., winds and the southern hemispheres converge here. blowing from Indian subcontinent towards When ITCZ is situated close to the equator, Indian Ocean. This wind system is called the Trade Winds converge near the equator. It the northeast monsoon. Since the winds are at is also known as the ‘doldrums’ or calm area. this time blowing from land towards sea, they In this equatorial zone, the planetary winds carry little moisture. The winter season over are equatorial Westerlies. the Indian landmass thus remains largely dry. Criticism of the Thermal Concept This concept visualizes monsoon winds as regional surface winds only. It fails to explain the inherent uncertain and irregular character of dynamic monsoon. Modern climatologists express doubt about the thermal origin of low (summer) and high (winter) pressure areas over the land (the Indian Fig 1.21. Dynamic Concept of Shifting of Inter Tropical sub-continent). According to them, the position of Convergence Zone low and high pressure areas change suddenly. These ●● During summer solstice sun’s rays are vertical sudden changes are not exclusively related to thermal over the Tropic of Cancer. Therefore, all conditions rather than to dynamic factors. Another wind and pressure belts of the globe shift criticism is about the low pressure areas which towards the north. At this point of time, are not stationary as stated in the thermal concept. ITCZ shifts northwards and becomes NITCZ The rainfall is not only convectional, but a mix of (Northern Inter Tropical Convergence Zone). orographic, cyclonic and convectional rainfall. It extends up to 30º N Latitude in South and South-East Asia. The excessive heating Dynamic Concept: Shifting of of Indian sub-continent further intensifies this process. According to Flohn, at this Inter-Tropical Convergence point of time the equatorial Westerlies of Zone (ITCZ) doldrums shift northwards and get extended ●● This concept was propounded by H. Flohn as southwest monsoon winds. Some other of German Weather Bureau in 1951. He has scholars consider south-west monsoon winds suggested that monsoon system experienced as an extension of south-east trade winds of in tropical Asia is a result of the seasonal southern hemisphere towards NITCZ. They changes in the planetary wind system resulting become south-westerly under the influence from the seasonal swing of temperature and of Coriolis force as they cross the equator. NITCZ also result into tropical disturbances pressure belts in this region in association which play significant role in surface weather with the changes in overhead position of sun. conditions. Heavy rainfall is received during ●● The planetary winds of tropics are trade winds. summer season because south-west monsoon In the months of March and September, when winds are on-shore. sun is overhead in equatorial area, low pressure belt is created near equator and north-east trade ●● During winter season due to southward winds of northern hemisphere and south-east shifting of ITCZ the pressure and wind belts, trade winds of southern hemisphere converge the planetary system of north-east trade winds in this belt of low pressure. This zone is gets re-established over this region. These known as Inter-tropical Convergence Zone or are called north-east winter monsoons. They ITCZ. (Ref. Fig 1. 21) The ITCZ is associated prevail over majority area as off-shore winds. with the zone of highest temperature and the Therefore, are generally dry and devoid of lowest pressure. It is due to the low pressure rains. But on Tamil Nadu coast they are here that the Trade Winds of the northern on-shore and bring precipitation in winter Climatology 23 months. The SITCZ (Southern Inter Tropical ●● Koteswaram, supported by Flohn, feels that Convergence Zone) position is associated because the Tibet Plateau is a source of heat for with north-west monsoon rainy season over the atmosphere, it generates an area of rising northern part of Australia. When the ITCZ air. During its ascent, the air spreads outwards shifts towards Tropic of Capricorn in winter, and gradually sinks over the equatorial part of the Trade Winds of northern hemisphere will the Indian Ocean. At this stage, the ascending cross the equator, will be deflected to left hand air is deflected to the right by the earth’s side and the southern hemisphere tropical rotation and moves in an anti-clockwise zone will experience northwesterly winds. The direction leading to anticyclonic conditions in reversal of wind direction thus occurs in both the upper troposphere over Tibet around 300- hemispheres in the tropical zone. 200 mb (9 to 12 km). It finally approaches the west coast of India as a return current Jet Stream Concept of from a south-westerly direction and is termed as equatorial Westerlies. It picks up moisture Monsoon from the Indian Ocean and causes heavy ●● Jet stream is a band of fast moving air from rainfall in India and adjoining countries. west to east usually found in the middle ●● The south-west monsoon in southern Asia latitudes in the upper troposphere at a height is overlain by strong upper easterlies with of about 12 km. (Ref. Fig 1. 22) The wind a pronounced jet at 100 to 200 mb. These speeds in a westerly jet stream are commonly easterly winds, which often record speeds 150 to 300 kmph. with extremes reaching 400 exceeding 100 knots are known as the kmph. Jet stream is the latest theory regarding Easterly Jet Stream of the tropics. the origin of the monsoons and has earned ●● The periodic movements of the sub-tropical worldwide acclaim from the meteorologists. jet stream provide a useful indication of ●● In winter the western jet stream flows along the the onset and subsequent withdrawal of the southern slopes of the Himalayas but in summer monsoon. In fact, northward movement of the it shifts northwards, rather dramatically, and subtropical jet is the first indication of the flows along the northern edge of the Tibet onset of the monsoon over India. Plateau. The periodic movements of the Jet ●● The easterly jet does not come into existence stream are often indicators of the onset and if the snow over the Tibet Plateau does subsequent withdrawal of the monsoon. not melt. This hampers the occurrence of rainfall in India. Therefore, any year of thick and widespread snow over Tibet will be followed by a year of weak monsoon and less rainfall. Southern Oscillation and the El Nino Recent studies have revealed that there seems to Fig 1.22. Jet Stream Concept of Monsoon be a link between meteorological events which are ●● Tibet is an ellipsoidal plateau at an altitude separated by long distances and large intervals of of about 4,000 m above sea level with an time. They are called meteorological teleconnections. area of about 4.5 million sq km. This plateau The one which has aroused considerable interest is surrounded by mountain ranges which rise among the meteorologists is the difference between 6,000 – 8,000 m above sea level. It gets heated an El Nino and the Southern Oscillation. El Nino (EN) is a narrow warm current which appears in summer and is 2°C to 3°C warmer than the off the coast of Peru in December. In Spanish, it air over the adjoining regions. 24 Master Series : Geography means The Child Christ because it appears around Christmas. In some years this warm current is more intense than usual. The El Nino phenomenon, which influences the Indian monsoon, reveals that when the surface temperature goes up in the southern Pacific Ocean, India receives a deficient rainfall. However, there have been some years during which the El Nino phenomena did not occur, but India still got deficient rainfall, and conversely, India received sufficient rainfall during an El Nino year. Southern Oscillation (SO) is the name ascribed Fig 1.23. El. Nino, Walker Circulation and to the curious phenomenon of sea-saw pattern of Southern Oscillation meteorological changes observed between the Pacific This is the difference in pressure between Tahiti in and Indian oceans. This great discovery was made French Polynesia, representing the Pacific Ocean by Sir Gilbert Walker in 1920. and Port Darwin, in northern Australia representing While working as the head of the Indian the Indian Ocean. The positive and negative Meteorological service, he noticed that when the values of the SOI i.e. Tahiti minus the Port Darwin pressure was high over equatorial south Pacific, it pressure are pointers towards good or bad rainfall was low over the equatorial south Indian Ocean and in India. (Ref. Fig 1. 24) the vice versa. The pattern of low and high pressures Scientists of India Meteorological Department over the Indian and Pacific Oceans (SO) gives rise to (IMD) joined an international study programme vertical circulation along the equator with its rising called the Tropical Oceans and Global Atmosphere limb over low pressure area and descending limb (TOGA) in 1985. This is an interesting and ambitious over high pressure area.This is known as Walker programme, which investigates both teleconnections Circulation. The location of low pressure and hence effects and the internal variability. As a follow up the rising limb over Indian Ocean is considered to to TOGA, the climate variability (CLIVAR) was set be conductive to good monsoon rainfall in India. In up in January 1995, to develop an internationally other words, when there is low pressure over the operational climate forecasting system. Indian Ocean in winter months, the chances are that the coming monsoon will be good and will bring sufficient rainfall. (Ref. Fig 1. 23) Its shifting eastward from its normal position, such as in El Nino years, reduces monsoon rainfall in India. Due to the close association between an El Nino (EN) and the Southern Oscillation (SO), the two are jointly referred to as an ENSO event. Some of the predictors used by Sir Gilbert Walker are still used in long-range forecasting of the monsoon rainfall. The main difficulty with the Southern Oscillation is that its periodicity is not fixed and its period varies from two to five years. Different indices have been used to measure the intensity of the Southern Oscillation, but the most frequently used is the Fig 1.24. The Soutwest Monsoon Southern Oscillation Index (SOI). Climatology 25

ENSO distribution of solar heating. The duration and amount of rainfall depends upon the moisture The Indian monsoon is also influenced by El-Nino, southern oscillation and Somalian current. We know content of the air, and on the configuration that El Nino is the reversal of normal condition in the and strength of the atmospheric circulation. Pacific Ocean’s sea surface temperature. Though there is no direct correlation between bad monsoon and El Nino, but both are generally associated. There are years when India faced severe drought and those are not El Nino years and vice-versa. Southern Oscillation is the see-saw pattern of atmospheric pressure between the eastern and western Pacific Ocean. The oscillation has a period varying from 2-7 years. It is measured with Southern Oscillation Index (SOI) by measuring pressure difference between two points in Pacific Ocean (Tahiti and Darwin). A negative value of SOI implies high pressure over north Indian Ocean during the winter Fig 1.25. Monsoon Climate Change Impacts season and a poor monsoon. ●● The regional distribution of land and ocean The Somalian current changes its direction of flow after every six months. During the North- East Monsoon also plays a role, as does the topography. the Somali Current flows to the south-west, while For example, the Tibetan Plateau — through during the South-West Monsoon it is a major western variations in its snow cover and surface boundary current, comparable with the Gulf Stream. heating — modulates the strength of the Normally, there remains a low pressure area along the complex Asian monsoon systems. Where eastern coast of Somalia. In exceptional years, after moist on-shore winds rise over mountains, every six or seven years, the low pressure area in as they do in southwest India, monsoon western Arabian Sea becomes a high pressure area. rainfall is intensified. On the lee side of such Such a pressure reversal results into a weaker monsoon mountains, it lessens. in India. ●● Since the late 1970s, the East Asian summer monsoon has been weakening and Monsoon and Climate Change not extending as far north as it used to in ●● Monsoons are the most important mode of earlier times, as a result of changes in the seasonal climate variation in the tropics, and atmospheric circulation. That in turn has led are responsible for a large fraction of the annual to increasing drought in northern China, but rainfall in many regions. Their strength and floods in the Yangtze River Valley farther timing is related to atmospheric moisture south. In contrast, the Indo-Australian and content, land–sea temperature contrast, land Western Pacific monsoon systems show no cover and use, atmospheric aerosol loadings coherent trends since the mid-20th century, and other factors. (Ref. Fig 1. 25) but are strongly modulated by the El Nino- ●● Overall, monsoonal rainfall is projected to Southern Oscillation (ENSO). become more intense in future, and to affect larger areas, because atmospheric moisture content increases with temperature. However, the localized effects of climate change on regional monsoon strength and variability are complex and more uncertain. ●● Monsoon rains fall over all tropical continents Asia, Australia, the Americas and Africa. The monsoon circulation is driven by Fig 1.26. The Monsoon Impact in India the difference in temperature between land ●● The land surface warms more rapidly than and sea, which varies seasonally with the the ocean surface, so that surface temperature 26 Master Series : Geography contrast is increasing in most of regions. The in future — and how its effects on monsoon tropical atmospheric overturning circulation, will change — also, both remain uncertain. however, slows down on average as the climate However, the projected overall increase in warms due to energy balance constraints in monsoon rainfall indicates a corresponding the tropical atmosphere. These changes in increase in the risk of extreme rain events in the atmospheric circulation lead to regional most of the regions. changes in monsoon intensity, area and timing. ●● Surface heating varies with the intensity of Jet Streams solar radiation absorption, which is itself ●● The jet stream is a river of wind that blows affected by any land use changes that alter horizontally through the upper layers of the the reflectivity (albedo) of the land surface. troposphere, generally from west to east, at an Also, changing atmospheric aerosol loadings, altitude of 20,000 - 50,000 feet (6,100 - 9,144 such as air pollution, affect how much solar meters), or about 7 miles (11 kilometres) up. radiation reaches the ground, which can change (Ref. Fig 1. 27) the monsoon circulation by altering summer solar heating of the land surface. Absorption of solar radiation by aerosols, on the other hand, warms up the atmosphere, changing the atmospheric heating distribution. ●● The strongest effect of climate change on the monsoons is the increase in atmospheric moisture associated with warming of the atmosphere, resulting in an increase in total monsoon rainfall even if the strength of the monsoon circulation weakens or does not change. Climate model projections through Fig 1.27. Jet Stream the 21st century show an increase in total ●● A jet stream develops where air masses of monsoon rainfall, largely due to increasing differing temperatures meet. For this reason, atmospheric moisture content. (Ref. Fig 1. 26) surface temperatures determine where the jet ●● The total surface area affected by the stream will form. The greater the difference monsoons is projected to increase, along in temperature, the faster the wind velocity with the general poleward expansion of the inside the jet stream. tropical regions. Climate models project ●● They flow at very high speeds since air at from 5% to an approximately 15% increase the equator rotates around the earth’s axis of global monsoon rainfall depending on much faster than they do at more northerly scenarios. Though total tropical monsoon or southerly latitudes. Thus, as the warmer rainfall increases, some areas will receive less air is drawn toward the poles, it moves faster, monsoon rainfall, due to weakening tropical relative to the earth’s surface. The rising warm wind circulations. air feeding the jet stream happens all along ●● Monsoon onset dates are likely to be early the equator, the effects accumulate, giving or not to change much and the monsoon rise to high-speed winds. retreat dates are likely to delay, resulting in ●● A jet stream develops where air masses lengthening of the monsoon season. Future of differing temperatures meet, so surface regional trends in monsoon intensity and temperatures help determine where they will timing remain uncertain in many parts of form. The jet stream is snakelike, undulating the world. Year-to-year variations in the like a river, because of the pressures on either monsoons in many tropical regions are side from the warm and cold air masses. (Ref. affected by ENSO. How ENSO will change Fig 1. 28) Climatology 27 heat as it moves toward the Equator. The existence of these waves explains the low- pressure cells (cyclones) and high-pressure cells (anticyclones) that are important in producing the weather of the middle and higher latitudes.

Fig 1.28. The Flow of Jet Stream ●● When the jet stream is pushed south by a cold air mass, it allows high pressure to sink and create colder-than-normal weather in the South. In the opposite situation, when northern regions get warmer than normal, Fig 1.29. The Formation of Rossby Waves the jet stream has been pushed north by the ●● In temperate region, Westerlies moves from tropical air. west to east. When these winds move towards ●● Meandering of jet streams are due to the fact pole where landmass is less to balance the that when the temperature gradient is more, angular momentum, then they form meanders jet stream flows in near straight path, but or wavy structure called Rossby waves & when temperature gradient reduces, the jet follow the cycle. stream starts to follow a meandering path. So, the meandering depends upon the temperature ●● At higher latitudes, the warm air cools and contrast (temperature gradient). A meander is sinks, drawing more warm air in behind called peak or ridge if it is towards poles and it. The cooled air flows back towards the trough if it is towards equator. equator, creating a loop or convection cell. ●● The reason for very high speeds of the winds Rossby Waves of jet streams is that the Pressure gradient ●● Rossby waves are formed when polar air increases with altitude and creates high moves towards the Equator while tropical velocity winds at higher altitudes. The friction air is moving polewards. Because of the in the upper troposphere is quite low due to temperature difference between the Equator less denser air. Hence the Jet streams flow at and the poles due to differences in the great velocities. Temperature also influences amounts of solar radiation received, heat the velocity of the jet stream. The greater the tends to flow from low to high latitudes; difference in air temperature, the faster the jet this is accomplished, in parts, by these air movements. (Ref. Fig 1. 29) stream, which can reach speeds of up to 250 ●● Rossby waves are a dominant component of mph or greater, but average about 110 mph. the Ferrel circulation. The tropical air carries The jet streams have an average velocity of heat polewards, and the polar air absorbs 120 kilometers per hour in winter and 50 km 28 Master Series : Geography per hour in summer. The speed of Jet streams The STJ exists all round the year in the southern in winter is faster than in summer hemisphere. However, it is intermittent in the northern hemisphere during summer when it migrates north.

Fig 1.30. The Polar and Subtropical Jets Types of Jet Streams There are five types of jet stream. (i) Polar Front Jet Stream: They are formed above the convergence zone of the surface Fig 1.31. Various Types of Jet Stream polar cold air mass and tropical warm air mass. (iii) Tropical Easterly Jet Streams: This The thermal gradient is steepened because of jet occurs near the tropopause over Southeast convergence of two contrasting air masses. It has Asia, India, and Africa during the summer. The a more variable position than the sub-tropical jet. strongest winds are over southern India, but they In summer, its position shifts towards the poles are not as intense as the winds encountered in and in winter towards the equator. (Ref. Fig 1. 30) polar-front or subtropical jet streams. This jet (ii) Subtropical JET Streams (SJT): These is closely connected to the Indian and African jets, like the polar-front jets, are best developed summer monsoons. The existence of this jet in winter and early spring. During summer, in implies that there is a deep layer of warm air to the Northern Hemisphere, the subtropical jet the north of the jet and colder air to the south weakens considerably, and it is only identifiable over the Indian Ocean. This warm air is of course in sporadic velocity streaks around the globe. associated with the maximum heating taking During winter, subtropical jets intensify and can place over India in summer, while the colder be found between 20° and 50° latitude. Their air is over the ocean. The difference in heating maximum speed approaches 300 knots, although and cooling and the ensuing pressure gradient these higher wind speeds are associated with is what drives this jet. Tropical easterly jet their merger with polar-front jets. The core is streams are responsible for bursting of monsoon most frequently found between 35,000 and in India. (Ref. Fig 1. 31) 40,000 feet. A subsidence motion accompanies subtropical jets and gives rise to predominantly fair weather in areas they pass over. These jets are also remarkably persistent from time to time, but they do fluctuate daily. Sometimes they drift northwards and merge with a polar-front jets. Over Asia in summer, the subtropical jet is replaced by the tropical easterly jet stream. During winter, the Sub-Tropical Jet stream (STJ) is nearly continuous in both hemispheres. Fig 1.32. The Jet Stream and Monsoon in India Climatology 29 (iv) Polar Night Jet Stream: This jet meanders remainder of the planet. When the jets streams through the upper stratosphere over the poles. are warmer, their ups and downs become more It occurs only during the long winter night, extreme, bringing different types of weather since the night is 6 months long over the to areas that are not accustomed to climate pole in which winter is occurring. The polar variations. If the jet stream dips south, for stratosphere undergoes appreciable cooling due example, it takes the colder air masses with it. to the lack of solar radiation. The horizontal ●● Jet streams also have an impact on air travel temperature gradient is strongly established and are used to determine flight patterns. An between the equator and the pole, and the airplane can travel much faster, and save fuel, pressure gradient creates this westerly jet. The by getting “sucked up” in the jet stream. That temperature gradient breaks down intermittently can also cause a bumpy flight, because the during middle and late winter in the Northern jet stream is sometimes unpredictable and Hemisphere; therefore, the jet is intermittent can cause sudden movement, even when the at these times. In the Southern Hemisphere the weather looks calm and clear. temperature gradient and jet disappear rather ●● The world’s jet streams are also impacted by abruptly near the time of the spring equinox. El Nino and La Nina. During El Nino for example, precipitation usually increases in They are formed due to (v) Local Jet Streams California because the polar jet stream moves local thermal and dynamic conditions and have farther south and brings more storms with it. limited local importance. Conversely, during La Nina events, California dries out and precipitation moves into the Jet Streams and the Weather Pacific Northwest because the polar jet stream ●● Jets streams play a key role in determining the moves more north. In addition, precipitation weather because they usually separate colder often increases in Europe because the jet and warmer air. Jet streams generally push stream is stronger in the Northern Atlantic, air masses around, moving weather systems and is capable of pushing them farther east. to new areas and even causing them to stall if ●● In winter the sub-tropical westerly jet streams they have moved too far away. (Ref. Fig 1. 32) bring rain to the western part of India, especially ●● While they are typically used as one of the Himachal Pradesh, Haryana and Punjab. factors in predicting weather, jet streams ●● The end of the monsoon season is brought don’t generally follow a straight path — the about when the atmosphere over the Tibetan patterns are called peaks and troughs — so Plateau begins to cool; this enables the Sub- they can shift, causing some to point at the Tropical Jet-Streams to transition back across poor forecasting skills of meteorologists. the Himalayas. This leads to the formation of ●● Climatologists say that changes in the jet a cyclonic winter monsoon cell typified by streams are closely tied to global warming, sinking air masses over India and relatively especially the polar jet streams, because there moisture-free winds that blow seaward. This is a great deal of evidence that the North gives rise to relatively settled and dry weather and South poles are warming faster than the over India during the winter months. Humidity, Condensation, Precipitation and the Air Masses

Humidity In our atmosphere, water vapour is converted Humidity defines the amount of water vapour in back into liquid form when air masses lose heat the air. The amount of humidity found in air varies energy and cool. This process is responsible for the because of a number of factors. Two important development of most of clouds, and also produces factors are the evaporation and condensation. the rains that fall to the Earth’s surface. 30 Master Series : Geography The atmospheric humidity can be expressed in a temperature. Specific humidity at 20°C is 15g number of ways. per kg. At 30°C, it is 26 g per kg and at -10°C, it is 2 g per kg. (i) Relative Humidity: It can be simply Specific humidity is a constant property of air, defined as the amount of water in the air therefore, it is frequently used in meteorology. relative to the saturation amount the air can The value of the specific humidity changes only hold at a given temperature, multiplied by 100. if the amount of water vapours undergoes any Temperature and evaporation are positively change. But it is not affected by the changes related and hence humidity and temperature in pressure or temperature of air. It is directly are also directly positively related. Air with proportional to the vapour pressure of air and a relative humidity of 50%, contains a half of inversely proportional to the atmospheric the water vapour it could hold at a particular pressure. temperature. (Ref. Fig 1. 33) It decreases from equator to polewards. In arctic, The above figure illustrates the concept of it is 0.2 gm per kg while in equatorial region it is Relative Humidity. Relative Humidity (RH) is 18 gm per kg. always expressed as percentage. Suppose an air mass of 1 kg contains 9 gm of water vapours at (iii) Absolute Humidity: It is defined as the a given temperature and constant pressure. But weight of water vapours in a given volume of 1 kg of an air mass has the capacity to contain air. It is expressed as grams of water vapours per 12 gm of water vapours at the same temperature cubic meter of air (g m–3). Absolute humidity and pressure. is rarely used because it varies with the RH = 9/12 × 100 = 75% expansion and contraction of air. It varies with temperature, even though the amount of water vapours remains constant. Temperature and evapouration are positively related and hence humidity and temperature are also directly Water Water Water positively related. Vapour Vapour Vapour Condensation ●● Condensation is the process by which water vapour in the air is changed into liquid water. Condensation is crucial to the water cycle Fig 1.33. Relative Humidity and Temperature because it is responsible for the formation There is inverse relationship between air of clouds. These clouds may produce temperature and relative humidity i.e., precipitation, which is the primary route for relative humidity decreases with increasing water to return to the Earth’s surface within temperature while it increases with decreasing the water cycle. Condensation is the opposite temperature. of evaporation. (Ref. Fig 1. 34) Absolute Humidty Relative Humidity = ●● In free air, condensation results from Humidity Capacity cooling around very small particles termed as hygroscopic condensation nuclei. Particles (ii) Specific Humidity: It is the ratio of mass of of dust, smoke, pollen and salt from the water vapours actually present in the air to a unit ocean are particularly good nuclei because mass of air including the water vapour (dry air they absorb water. Condensation also takes + moisture). It is expressed as grams of water place when the moist air comes in contact vapour per kg of moist air mass. The amount with some colder object and it may also take of water vapour that air can hold depends upon Climatology 31 place when the temperature is close to the cause droplets to grow through the processes dew point. Condensation, therefore, depends of collision and coalescence. One initial upon the amount of cooling and the relative condition, however, must be met for this humidity of the air. process to begin-droplet size in the cloud must be variable. This initial condition allows larger and heavier droplets to collide and coalesce with lighter smaller droplets during downdraft periods. If enough atmospheric mixing occurs, the larger droplets can expand by up to 250 times and can become heavy enough to fall to the Earth’s surface. The other mechanism of precipitation development involves clouds whose temperature is below freezing. In these clouds, large ice crystals grow due to the differences in vapour pressure between ice crystals and supercooled water droplets. Vapour pressure Fig 1.34. Sublimation and Condensation differences between ice and super cooled water cause a net migration of water vapour from water droplets Dew Point and Frost Point to ice crystals. The ice crystal then absorbs the water Associated with relative humidity is dew point (if the vapour, depositing it on their surface. At the same dew point is below freezing, it is referred to as the frost time, the loss of vapour from the water droplets point). Dew point is the temperature at which water vapour causes them to shrink in size. A necessary initial saturates from an air mass into liquid or solid usually requirement for this process is the presence of both forming rain, snow, frost, or dew. Dew point normally condensation nuclei and deposition nuclei. While occurs when a mass of air has a relative humidity of 100%. deposition nuclei form ice crystals at temperatures This happens in the atmosphere as a result of cooling through a number of different processes. just below zero degrees Celsius, condensation nuclei can remain liquid (super cooled) to temperatures as low as –40° Celsius depending upon size. Because Precipitation of this phenomenon, cold clouds can contain both ●● Precipitation refers to any liquid or solid ice crystals and super cooled water droplets. The aqueous deposit that forms in a saturated relative proportion of these two types of particles atmosphere (relative humidity equals 100%), determines whether snow crystals grow to a size to and falls from clouds to the ground surface. overcome atmospheric updrafts. It is important to recognize that most clouds do not produce precipitation. In many clouds, Types of Precipitation water droplets and ice crystals are just too (A) Orographic Precipitation small to overcome the natural updrafts found ●● This type of precipitation occurs when the in the lower atmosphere. As a result, the moist air mass rises on the windward side of tiny water droplets and ice crystals remain the mountains. (Ref. Fig 1. 35) The moist air suspended in the atmosphere until they are mass is lighter than the dry air mass, therefore, converted back into vapour. buoyancy forces push the air mass along the ●● Water droplets and ice crystals can only fall slope of the mountain and cools at the dry adiabatic rate. When cooling is sufficient, air to the Earth’s surface if they grow to a size mass becomes saturated and the condensation that can overcome updrafts. Conditions for starts. As a result, lifting condensation level growth can develop in clouds via two different is reached and clouds begin to form. When processes: In clouds with temperatures above the mountains act as barrier to the flow of freezing, turbulent atmospheric mixing can air mass, the air cools adiabatically, as a 32 Master Series : Geography result, clouds and precipitation occur. This is unequally. During the day, the air above the called orographic precipitation. This type of bare soil will grow warmer than the air over precipitation occurs on the windward side of the adjacent forest. Warm air is less dense the mountains. as compared to cold air. Convection currents are set up forcing air to rise. The air is cooled adiabatically and its temperature will decrease as it rises. The air mass will continue to rise as long as it remains warmer than its surrounding air. ●● Rising air mass becomes saturated as it gets cooled adiabatically. Condensation starts and the rising air column becomes a puffy cumulus cloud. If the convection continues strongly, the cloud develops into a dense cumulonimbus cloud. Heavy rainfall is always associated with this type of cloud. Fig 1.35. The Orographic Precipitation Convective type precipitation is a warm ●● But on the leeward side, there is abrupt weather phenomena. It is generally associated decrease in precipitation due to the descending with thunder, lightning and strong surface air mass which gets heated at dry adiabatic winds. Sometimes hails are also associated lapse rate. The descending air mass becomes with it. dry and hot. As a result, the clouds on the leeward side disappear. Therefore, dry areas always exist on the leeward side of the mountains. These are known as rain shadow areas. This is due to the reason that moist air prevails on the windward side and warm dry air prevails on the leeward side. In India, south-west monsoon causes heavy rains on the windward slope of Western Ghats, whereas on the leeward side there are extensive rain shadow areas. There is a continuous increase in precipitation on the windward side up to

a certain height beyond which the rainfall Fig 1.36. Convectional Precipitation starts decreasing. This is called the inversion of rainfall. (C) Cyclonic or Frontal Precipitation It occurs when deep and extensive air masses are (B) Convectional Precipitation Two made to converge and move upwards so that conditions are required to cause this type of their adiabatic cooling takes place. For this type precipitation: (Ref. Fig 1. 36) of precipitation lifting of air mass is required. (i) Intense heating of the ground surface. (Ref. Fig 1. 37) Cyclonic precipitation can be achieved in two (ii) Abundant supply of moisture. ways ●● Solar radiation is the main source of heat to produce convection currents in the air. (i) When two air masses with different temperature This process starts, when surface is heated and moisture content meet at a certain angle, Climatology 33 the warm and moist air is forced to rise over has a diameter greater than 0.5 millimetres. the heavier cold air mass. The maximum size, of a rain drop is about 5 (ii) When air masses from different directions millimetres. Beyond this size, inter-molecular converge to the centre, some of the air is forced cohesive forces become too weak to hold the mass of water together as a single drop. up. ●● In the tropical region, there is little difference ●● Freezing Rain: This takes place when falling liquid water droplets encounter a in the temperature and humidity of the surface with a temperature below 0° Celsius. converging air masses. The lifting is almost Upon contact with this surface, the rain vertical and is accompanied by convection. In quickly turns into ice. Another important such a condition the convergence provides the condition required for freezing rain is that initial upward movement of unstable air mass the atmosphere where rain develops must and causes large clouds and heavy showers. be above freezing. A situation where warm air is found on top of cold air is called as the temperature inversion. Temperature inversions are not the common state of the lower atmosphere. Usually, air temperature decreases with an increase in altitude in the troposphere. In the mid-latitudes, we often find temperature inversions developing along the moving front edge of a cold air mass that is overtaking the warmer air. This condition causes the less dense warm air to be pushed up and over the more dense cold air. ●● Ice Pellets (or Sleet): These are transparent or translucent spheres of frozen water. (Ref. Fig 1.37. Frontal Precipitation Fig 1. 38) They have a diameter smaller than 5 millimetres. This form of precipitation ●● In temperate regions, a zone of contact develops first as raindrops in a relatively between warm and cold air mass is called warm atmosphere where the temperature is the front. There may be warm or cold front. above freezing. Frontal precipitation occurs when the warm and moist air gradually rises over the cold air mass. The main cause of this precipitation is the mixing of air along the front. Frontal precipitation along the warm front is in the form of drizzle. It is always widespread and of long duration. ●● In case of cold front it is always in the form of intense thunder showers and is of very short duration. Frontal precipitation occurs in Europe and North. America. During winter season, cyclonic precipitation occurs in the northern parts of India. Forms of Precipitation ●● Rain: It is defined as liquid deposit that falls from the atmosphere to the surface, and Fig 1.38. ’The Ice Pellets’ Precipitation 34 Master Series : Geography These raindrops then descend into a colder ●● Hail: It is a type of frozen precipitation that is lower layer of the atmosphere where freezing more than 5 millimetres in diameter. Hailstones temperatures occur. In this layer, the cold often have concentric shells of ice alternating temperatures cause the raindrops to freeze between those with a white cloudy appearance into ice pellets during their transit to the and those that are clear. (Ref. Fig 1. 40) The ground surface. Similar to freezing rain, an cloudy white shells contain partially melted air temperature inversion is required for the snowflakes that freeze onto the surface of the formation of ice pellets. growing hailstone. The clear shells develop when ●● Snow: It is a type of precipitation common liquid water freezes to the hailstone surface. to the mid and high latitudes. Snow develops when water vapour deposits itself (skipping the liquid phase) directly on a six-sided (hexagon) deposition nuclei as a solid crystals, at temperatures below freezing. The unique form of snowflakes occurs because ice crystal growth is most rapid at the six points associated with geometric shape of the deposition nuclei. These points are more directly exposed to the atmosphere and consequently convert more water vapour Fig 1.40. The Hail into ice. Snow is usually generated by frontal lifting associated with mid-latitude cyclones. Fogs Snowfall can occur in the fall, winter, and Fog is simply a cloud of minute water droplets that spring months when atmospheric temperatures exists at the ground level. Fog develops when the air can drop below freezing. Much of the ground at ground level is cooled enough to cause saturation surface of North America can be covered with (relative humidity equals 100%). Meteorologist have snow for several months during a typical year a very specific definition to determine if fog exists. ●● Snow Pellets or Graupel: There are This definition suggests that fog is occurring when spherical white bits of ice that have a diameter the visibility of the atmosphere, near the Earth’s less than 5 millimetres. (Ref. Fig 1. 39) Snow surface, becomes less than 1 kilometre. Fog can be pellets develop when super cooled droplets created through a variety of processes: freeze onto the surface of falling snowflakes. ●● Radiation Fog (or Ground Fog): Snow pellets usually fall for only a brief Produced by near surface cooling of the period of time when a precipitation event atmosphere due to long wave radiation changes from ice pellets to snow. emissions. This particular type of fog is normally quite shallow and develops during the evening hours. Shortly after sunrise, the radiation fog disappears because of surface heating due to the absorption of the solar radiation. ●● Upslope Fog: Created when air flows over higher topography. When the air is forced to rise in altitude because of the topographic barrier, it is cooled by the adiabatic expansion. This type of fog is often found formed on the windward slopes

Fig 1.39. Precipitation in form of Snow Pellets of hills or mountains. Climatology 35 ●● Advection Fog: Generated when 2. Convectional Lifting: It is associated air flows over a surface with a different with surface heating of the air at the ground temperature. Warm air advection can surface. If enough heating occurs, the mass of produce fog if it flows over a cold air becomes warmer and lighter than the air in surface. The contact cooling associated with the surrounding environment, and just like a this process causes saturation to occur in a hot air balloon, it begins to rise, expand, and relatively thin layer of air immediately above cool. When sufficient cooling has taken place the ground surface. saturation occurs, forming clouds. This process ●● Evaporation Fog: A specific type of is active in the interior of continents and near advection fog. It occurs when one get cold the equator forming cumulus clouds and or air advancing over warm water or warm, cumulonimbus clouds (thunderstorms). The moist land surfaces. In this situation, fog rain that is associated with the development forms as water from the surface evapourates of thunderstorm clouds is delivered in large into the cold air and then saturates. This amounts over short periods of time in extremely type of fog can also be called steam fog localized areas. or sea smoke. It takes ●● Frontal Fog: it is a type of fog that is 3. Convergence or Frontal Lifting: associated with weather fronts, particularly place when two masses of air come together. warm fronts. In this situation, rain descending Generally, the two air masses have different into the colder air ahead of the warm front temperature and moisture characteristics. One can increase the quantity of water vapour in of the air masses is usually warm and moist, this atmosphere through evaporation. Fog while the other is cold and dry. The leading edge then forms when the quantity of water in of the latter air mass acts as an inclined wall or the atmosphere ahead of the front reaches front causing the moist warm air to be lifted. The a saturation (relative humidity equals to lifting causes the warm moist air mass to cool due 100%). to expansion, resulting in saturation. This cloud formation mechanism is common at the mid- Clouds latitudes where cyclones form along the polar front and near the equator where the trade winds Cloud Formation Process meet at the inter-tropical convergence zone. Condensation or deposition of water above the This situation occurs Earth’s surface creates clouds. In general, clouds 4. Radiative Cooling: when the Sun is no longer supplying the heat to develop in any air mass that becomes saturated (relative humidity becomes 100%). Saturation can the ground and overlying air with energy derived occur by way of atmospheric mechanisms that from solar insolation (e.g., night). Instead, the cause the temperature of an air mass to be cooled surface of the Earth now begins to lose energy in to its dew point or frost point. These mechanisms the form of longwave radiation which causes the or processes can achieve this outcome causing ground and air above it to cool. The clouds that clouds to develop. result from this type of cooling take the form of surface fog. 1. Orographic Uplift: It occurs when air is These causes of cloud development do not always forced to rise because of the physical presence act in a singular fashion. It is possible to get of elevated land. As the parcel of air rises it combinations of all four types, such as when cools due to adiabatic expansion at a rate of convection and orographic uplift cause summer approximately 10° Celsius per 1000 metres until afternoon cloud development and showers in the saturation. mountains. 36 Master Series : Geography Types of Clouds (Ref. Fig 1. 42) The types of clouds are described below:

Family Genus and Main Content Height (Mts.) Characteristics Cirrus (ice) 5000-13500 High altitude, white, silk like sheen, thin and wispy, often in streaks. Hight clouds Cirrostratus (ice) 5000-13500 High, whitish like a veil, may cover entire sky. Cirrocumulus 5000-13500 High, thick, white patches of clouds made up (usually ice, occasionally mixed) of small ripples. Altostratus 2000-7000 Middle altitude, greyish or bluish cloud (usually mixed, occasionally ice) sheets, thin, blanket layer, rain bearing, often distributed over the entire sky. Altocumulus (water) 2000-7000 Middle, white or gery, layer of individual Medium cloud masses in geometrical pattern or clouds rounded masses, “mackerel” sky. 900-3000 Middle, dark, dense cloud layer, rain or snow. Nimbostratus (water) (Nimbo means precipitation below 450 Low, grey cloud layer may produce drizzle coming from cloud) or snow grains. Low clouds Stratus (water) 450-2000 Low, whitish or grey with dark parts, in wave-like layers, usually no rain. Stratocumulus (water) 450-2000 Low, deta red clouds with white tufted tops, e.g. fog may bring showers. Cumulus (water) Clouds with vertical 450-2000 Usually low based, tall and towering white development have up- and black, bring thunder storms. currents within them Fig 1.41. Clouds Defined According to their Altitude

level of altitude. Such a mass has distinct boundaries and may extend hundreds or thousands of kilometres horizontally and sometimes as high as the top of the troposphere (About 10–18 km above the Earth’s surface). ●● An air mass forms whenever the atmosphere remains in contact with a large, relatively uniform land or sea surface for a time sufficiently long to acquire the temperature and moisture properties of that surface. The Fig 1.42. The Classification of Clouds Earth’s major air masses originate in polar or subtropical latitudes. The middle latitudes Air Masses constitute essentially a zone of modification, interaction, and mixing of the polar and ●● Air masses constitute a large body of tropical air masses. Air masses are named air having nearly uniform conditions of for the type of surface over which they are temperature and humidity at any given formed. Climatology 37

●● The nature of air masses is determined by 2. Maritime Polar (MP): Develop over the three factors: the source region, the age, and polar areas of both the Northern and the the modifications that may occur as they Southern hemispheres. They generally contain move away from their source region across considerably more moisture than the CP air the earth’s surface. masses. As they move inland in middle and ●● The primary classification of air masses is high latitudes, heavy precipitation may occur based on the characteristics of the source when the air is forced to ascend mountain region, giving Arctic (A), Polar (P) or slopes or is caught up in cyclonic activity (see Tropical air (T), and on the nature of the cyclone). surface in the source region: continental (c) 3. The Continental Tropical (CT): Originates or maritime (m). In addition, a large variety of in arid or desert regions in the middle or lower secondary types of air masses are defined. For latitudes, principally during the summer season. example, equatorial air (E) or Mediterranean It is strongly heated in general, but its moisture air. Sometimes there is a letter (k) or (w) content is so low that the intense dry convection attached to the two-letter initials indicating normally fails to reach the condensation level. whether the air is warmer or colder than the Of all the air masses, the cT is the most arid, surface. The former becomes more stable, and and it sustains the belt of subtropical deserts the latter more unstable. (Ref. Fig 1. 43) worldwide. 4. The Maritime Tropical (MT): These are Types of Air Masses the most important moisture-bearing and rain- 1. Continental Polar (CP): Usually formed producing air masses throughout the year. In during the cold period of the year over extensive winter it moves poleward and is cooled by the land areas such as central Asia and northern ground surface. Consequently, it is characterized by fog or low stratus or stratocumulus clouds, Canada. They are likely to be stable and with drizzle and poor visibility. A steep lapse characteristically free of condensation forms. rate aloft in regions of cyclonic activity ensures When heated or moistened from the ground with the occurrence of heavy frontal and convective strong turbulence, this type of air mass develops rains. In summers, the characteristics of the MT limited convective stratocumulus cloud with air mass over the oceans and in zones of cyclonic scattered light rain or snow showers. In summer, activity are basically the same as in winter. Over strong continental heating rapidly modifies the warm continental areas, however, the air mass coolness and dryness of the CP air mass as it is strongly heated, so that, instead of fog and moves to lower latitudes. low stratus clouds, widely scattered and locally heavy afternoon thunderstorms occur. Effect of Airmass on Local Weather and Climate ●● An air mass may sits over its source region for long periods of time, or it may migrate. An air mass on the move begins to transform as it passes over new landscapes, while at the same time retaining enough of its original conditions to alter the local weather. For example, a CP air mass originating from the tundra of northern Canada might push southwards during the winter. It brings frigid Fig 1.43. The Principal Air Masses of The World temperatures to the central United States, 38 Master Series : Geography even as it warms up somewhat on its journey ●● Maritime air masses also contribute to a across lower latitudes. moderating climatic influence on coastal ●● While dry in its source region (CP), air mass temperatures, as oceans heat up and cool often picks up substantial moisture during down more slowly and less dramatically than an early-winter transit of the Great Lakes, landmasses. allowing it to dump so-called lake effect snow ●● Where polar and tropical air masses meet in on leeward coasts. the mid-latitudes, prevailing westerly winds ●● The climates of most regions worldwide funnel along alternating low and high-pressure are affected by air masses. For example, centres called cyclones and anticyclones, maritime-tropical air sourced over warm respectively. Stormy cyclones form near the waters of the Atlantic Ocean, Caribbean air-mass fronts. Anticyclones represent stable, Sea and Gulf of Mexico, primarily between singular air masses, and are typically larger 10 and 30 degrees north of latitude, is and more sluggish than cyclones. These may the main contributor of precipitation be forces of weather, but their regularity gives for much of North America east of the them a climatic significance: The mixing of air Rocky Mountains. It’s also the cause of masses achieved along the alternating warm the persistent humidity typical of that big and cold fronts of a mid-latitude cyclone is region’s summer season. part of the process by which the heat of the lower latitudes is transferred polewards.

Frontogenesis, Cyclones and Anticyclones

Fronts another, along with the Coriolis force. There are three basic situations, which are conducive When two air masses with different physical to Frontogenesis and satisfy the two basic properties (temperature, humidity, density, and requirements. pressure and wind direction) meet, due to the effect (a) The wind flow is cross isothermal and of the converging atmospheric circulation, they flowing from cold air to warmer air. do not merge readily. The transition zone or the layer of discontinuity so formed between two air (b) The flow must be cross isothermal, masses is a three-dimensional surface, and is called resulting in a concentration of isotherms a front. The air masses of different densities don’t (increased temperature gradient). mix readily and tend to retain their identity as far (c) The flow does not have to be perpendicular; as the moisture is concerned. The front represents a however, the more perpendicular the cross transition zone between two air masses of different isothermal flow, the greater the intensity densities. of Frontogenesis. ●● Just as frontogenesis or front-formation Condition for Frontogenesis is caused by converging air (for instance, (Front Formation) along sub-polar low pressure belts), fron- ●● The ideal conditions for a front to occur tolysis or dissipation of front is caused by are temperature contrast, converging air of divergent air (for instance, fronts passing contrasting characteristics which should be through sub-tropical high pressure belt tend strong enough to move one air mass towards to dissipate). Climatology 39

Difference between Frontogenesis and Frontolysis

Frontogenesis Frontolysis

●● The process of forma- ●● The process of dissipa- tion of the fronts. tion of a front. ●● It involves conver- ●● Frontolysis is oppo- gence of two distinct site of frontogenesis, air masses involves divergence of ●● In northern hemisphere, the air masses. Frontogenesis (conver- ●● Frontolysis or dissipa- gence of air masses) tion of front is caused happens in anti-clock- by divergent air (for wise direction and in instance, fronts passing southern hemisphere, through sub-tropical in clockwise direction. high pressure belt tend This is due to Coriolis to dissipate). Effect. Fig 1.44. The Warm Air Fronts ●● Frontogenesis or front- formation is caused 2. Cold Fronts: It is formed when a cold air mass by converging air (for replaces a warm air mass by advancing into it, and instance, along sub- lifting it up, or when the pressure gradient is such polar low pressure belt). that the warm air mass retreats and cold air mass advances. Sudden upliftment of warm air due to this Types of Fronts the slope will be steep. The cold air mass remains close to the ground and sinks under the less dense 1. Warm Front: This is gently sloping frontal warm air mass. (Ref. Fig 1. 45) As the warm air surface, with a slope gradient between 1:100 rises above the cold air, the warm air cools. It soon and 1:200, along which active movement of chills to the temperature at which water vapour warm air over cold air takes place. As the in the air condenses into droplets of liquid water. warm air moves up the slope, it condenses and Towering cumulus and cumulonimbus clouds form causes precipitation, but, unlike a cold front, the drops of water produced by the rapid rise of the temperature and wind direction changes are warm air. Because cold fronts move quickly, they gradual. As the warm air moves up the slope, it can cause abrupt weather changes. Strong winds condenses and causes precipitation, but unlike a and severe thunderstorms are often brought about. cold front, the temperature and wind direction The weather along such a front depends on vertical changes are gradual. (Ref. Fig 1. 44) With the structure of the uplifted air mass, but is generally approach, the hierarchy of clouds is — cirrus, associated with a narrow band of cloudiness and stratus and nimbus.Cirrostratus clouds ahead precipitation. The approach of a cold front is of the warm front create a halo around sun and marked by increased wind activity in warm sectors moon. Such, fronts cause moderate to gentle and the appearance of cirrus clouds, followed precipitation over a large area, over several by lower, denser alto-cumulous and altostratus. hours. The passage of warm front is marked At actual front, dark nimbus clouds cause heavy by rise in temperature, pressure and change in showers. A cold front passes off rapidly, but the weather. weather along it is violent. 40 Master Series : Geography front and warm front. Warm front clouds and cold front clouds are on opposite side of the occlusion. Such fronts are common in western Europe.

(a)

(a)

(b) Fig 1.45 (a) & (b). The Cold Air Fronts 3. Occluded Front: At an occluded front, a warm air mass is caught between two cooler air masses. The denser cool air masses move underneath the less dense warm air mass and push it upwards. The two cooler air masses meet (b) in the middle and may mix. The temperature Fig 1.46 (a) & (b). The Occulded Fronts of the air masses meet in the middle and may 4. Stationary Front: When the surface mix. (Ref. Fig 1. 46) The temperature near the position of a front does not change, a ground becomes cooler. The warm air mass is stationary front is formed.Sometimes cold and cut off, or occluded, from the ground. As the warm air masses meet, but neither one has warm air cools and its water vapour condenses, enough force to move the other. The two air the weather may turn cloudy and rainy or masses face each other in a “standoff.” (Ref. snowy. Thus, a long and backward swinging Fig 1. 47) In this case, the front is called a occluded front is formed which could be a stationary front. Where the warm and cool air warm front type or cold front type occlusion. meet, water vapour in the warm air condenses Weather along an occluded front is complex into rain, snow, fog, or clouds. A stationary — a mixture of cold front type and warm front front can remain stalled over an area for type weather. The formation Mid-latitude many days. In this case, the wind motion on cyclones (temperate cyclones or extra-tropical both sides of the front is parallel to the front. cyclones) involve the formation of occluded Overrunning of warm air, along such a front front. A combination of clouds formed at cold causes frontal precipitation.Cumulonimbus Climatology 41 clouds are formed. Cyclones migrating along Fronts Around The Globe a stationary front can dump heavy amounts of precipitation, resulting in significant flooding The Atlantic Polar Front is formed when maritime along the front. tropical air masses meet continental polar air masses. Full development of this front takes place during winter. The Atlantic Arctic Front is formed when the maritime polar air masses meet the air masses developed along the boundary of Arctic source- region. The Mediterranean Front is formed when the cold polar air masses of Europe meet the winter air masses of Africa. Pacific Arctic Fronts are formed along the Rockies-Great Lakes region. These fronts change with seasons.

Cyclones Cyclones are centres of low pressure surrounded by the closed isobars having increasing pressure outward and closed air circulation from outside towards the central low pressure in such a way that air blows inwards in anticlockwise direction in the northern hemisphere and clockwise in the southern hemisphere. Cyclones are termed as atmospheric disturbances. Atmospheric and Oceanic Conditions Necessary for a Cyclonic Storm (i) A warm sea temperature in excess of 26 degrees centigrade to a depth of 60 m, which Fig 1.47 (a) & (b). The Stationary Fronts provides abundant water vapour in the air by evaporation. Importance of Fronts (ii) High relative humidity of the atmosphere to a height of above 7,000 m facilitates condensation ●● Initiation of severe weather conditions and of water vapour into water droplets and clouds, events. releases heat energy thereby inducing a drop in ●● Temperature and humidity forecasts. pressure. ●● Cloud and precipitation forecasts. (iii) Atmospheric instability encourages formation of ●● Precipitation type forecasts. massive vertical cumulus cloud convection with ●● Visibility and sky cover/cloud ceiling. condensation of rising air over ocean. ●● Wind speed and direction (particularly (iv) A location of at least 4-5 latitude degrees from important for aviation). the equator allows the influence of the forces due ●● Upper fronts are linked to clear-air turbulence, to the earth’s rotation to take effect in reducing which is a major hazard to aircraft. cyclonic wind circulation around low-pressure ●● Upper fronts and associated tropopause folds centres. are important to stratosphere. ●● Troposphere exchange (e.g., ozone depletion Tropical Cyclones and transport issues). ●● Tropical cyclones are compact, circular storms, generally some 320 km Cont...(200 42 Master Series : Geography miles) in diameter, whose winds swirl in a large pressure gradient force, which is around a central region of low atmospheric responsible for the strong winds present in pressure. The winds are driven by this the eyewall. low-pressure core and by the rotation of ●● Horizontal winds within the eye, on the the Earth, which deflects the path of the other hand, are light. In addition, there is wind through a phenomenon known as the a weak sinking motion, or subsidence, as air Coriolis force. is pulled into the eyewall at the surface. As ●● As a result, tropical cyclones rotate in a the air subsides, it compresses slightly and warms, so that temperatures at the center of counterclockwise (or cyclonic) direction in a tropical cyclone are some 5.5 °C higher the Northern Hemisphere and in a clockwise than in other regions of the storm. Because (or anticyclonic) direction in the Southern warmer air can hold more moisture before Hemisphere. condensation occurs, the eye of the cyclone ●● The wind field of a tropical cyclone may be is generally free of clouds. Reports of the divided into three regions. air inside the eye being “oppressive” or (a) First is a ring-shaped outer region, “sultry” are most likely a psychological typically having an outer radius of about response to the rapid change from high 160 km (100 miles) and an inner radius winds and rain in the eyewall to calm of about 30 to 50 km (20 to 30 miles). In conditions in the eye. this region the winds increase uniformly (b) The Eyewall in speed toward the center. ●● The most dangerous and destructive part of (b) Wind speeds attain their maximum value a tropical cyclone is the eyewall. Here winds at the second region, the eyewall, which are strongest, rainfall is heaviest, and deep is typically 15 to 30 km (10 to 20 miles) convective clouds rise from close to Earth’s from the center of the storm. surface to a height of 15,000 m. (c) The eyewall in turn surrounds the interior ●● As noted above, the high winds are driven by rapid changes in atmospheric pressure region, called the eye, where wind speeds near the eye, which creates a large pressure decreases rapidly and the air is often gradient force. Winds actually reach their calm. These main structural regions are greatest speed at an altitude of about 300 described in greater detail below. m above the surface. Closer to the surface they are slowed by friction, and higher than Structure of Cyclone 300 m they are weakened by a slackening of (a) The Eye the horizontal pressure gradient force. This ●● A characteristic feature of tropical cyclones is slackening is related to the temperature the eye, a central region of clear skies, warm structure of the storm. Air is warmer in the temperatures, and low atmospheric pressure. core of a tropical cyclone, and this higher Typically, atmospheric pressure at the surface temperature causes atmospheric pressure of Earth is about 1,000 milibar. in the center to decrease at a slower rate with height than occurs in the surrounding ●● At the center of a tropical cyclone, however, atmosphere. The lessened contrast in it is typically around 960 milibar, and in a atmospheric pressure with altitude causes very intense “super typhoon” of the western the horizontal pressure gradient to weaken Pacific it may be as low as 880 milibar. In with height, which in turn results in a addition to low pressure at the center, there decrease in wind speed. is also a rapid variation of pressure across the storm, with most of the variation occurring ●● Friction at the surface, in addition to lowering near the center. This rapid variation results wind speeds, causes the wind to turn inward Climatology 43 toward the area of lowest pressure. Air flowing Dissipation of Tropical into the low-pressure eye cools by expansion and in turn extracts heat and water vapor from Cyclones the sea surface. Areas of maximum heating ●● Tropical cyclones dissipate when they can no have the strongest updrafts, and the eyewall longer extract sufficient energy from warm exhibits the greatest vertical wind speeds in ocean water. As discussed above, a tropical the storm—up to 5 to 10 m per second, or cyclone can contribute to its own demise by 18 to 36 km per hour. While such velocities stirring up deeper, cooler ocean waters. Also, are much less than those of the horizontal a storm that moves over land will abruptly winds, updrafts are vital to the existence of lose its fuel source and quickly lose intensity. the towering convective clouds embedded Storm Categories in India in the eyewall. Much of the heavy rainfall ●● Worldwide, hurricanes are measured on the Saffir- associated with tropical cyclones comes from Simpson Hurricane Wind Scale, which runs from these clouds. Category 1 up to Category 5. (c) Rain bands of Tropical Cyclones ●● The Saffir-Simpson Hurricane Wind Scale is used to ●● Along with the deep convective cells estimate potential property damage, rates hurricanes (compact regions of vertical air movement) on a scale of 1 to 5 based on a storm’s wind speed. surrounding the eye, there are often secondary The scale tops out at Category 5 for any hurricane cells arranged in bands around the centre. with sustained winds of 157 mph or higher. But, These bands, commonly called rain bands, recently, Hurricane Irma, hit 185 mph, breaking the spiral into the center of the storm. In some Atlantic Ocean record, and other parts of the world cases the rain bands are stationary relative have seen even worse.Now, scientists thinking of a new classification of Category 6. to the center of the moving storm, and in other cases they seem to rotate around the ●● The Indian Meteorological Department (IMD) has six categories for tropical storm systems, based on center. The rotating cloud bands often are sustained wind speeds and the damage expected as associated with an apparent wobbling of the a result. storm track. 1. Deep Depression: Storms with wind speeds 52 ●● If this happens as the tropical cyclone to 61 kilometre per hour; can cause minor damage approaches a coastline, there may be large to unstable structures. Fishermen are advised not to differences between the forecast landfall go out to sea. positions and the actual landfall. As a 2. Cyclonic Storm: With winds of 62 to 87 tropical cyclone makes landfall, surface kilometres per hour, can cause damage to thatched friction increases, which in turn increases huts and minor damage to power and telecom lines, the convergence of airflow into the eyewall as well as flooding, fishing be completely suspended. and the vertical motion of air occurring 3. Severe Cyclonic: Storm A severe cyclonic there. storm has winds of 88 to 117 KMPH and can cause ●● The increased convergence and rising of widespread damage to huts, as well as the uprooting moisture-laden air is responsible for the of large trees; People in low areas are moved to higher torrential rains associated with tropical ground, all coastal people advised to remain indoors. cyclones, which may be in excess of 250 4. Very Severe: Cyclonic Storm Authorities start mm (10 inches) in a 24-hour period. At evacuating the coastal areas in preparation for winds times a storm may stall, allowing heavy of 118 to 167 KMPH; Could bring widespread damage to unstable structures and disruptions to rains to persist over an area for several days. power and communications. Roughly comparable In extreme cases, rainfall totals of 760 mm to a Category 1 Atlantic hurricane. (30 inches) in a five-day period have been Cont... reported. 44 Master Series : Geography

5. Super Cyclone: Winds of 222 KMPH ; “Large-scale” evacuation advised, along with “total suspension” of road and rail traffic in the areas likely to be affected. Category 4 and Category 5 Atlantic cyclones are of these types. ●● A tropical cyclone that remains over the ocean and moves into higher latitudes will change its structure and become extra tropical as it encounters the cooler water. The transformation from a tropical to an extra tropical cyclone is marked by an increase in the storm’s diameter and by a (a) change in shape from circular to comma- or v-shaped as its rain bands reorganize. An extra tropical cyclone typically has a higher central pressure and consequently has lower wind speeds. Extra tropical cyclones, which are fueled by a north-to-south variation of temperature, weaken and dissipate in a few days.

Cyclogenesis (b) Cyclogenesis is the process of extra tropical cyclone Fig 1.48 (a) & (b). The Cyclogenesis development and intensification. Cyclogenesis is initiated by a disturbance occurring along a stationary Naming a Cyclone or very slow-moving front between cold and warm air. ●● Tropical cyclones are classified into three This disturbance distorts the front into the wavelike main groups, based on intensity: tropical configuration. As the atmospheric pressure within depressions,tropical storms, and a third group of the disturbance continues to decrease, it assumes more intense storms, whose names depend upon the appearance of a cyclone and forces poleward the region. If a tropical storm in the North-western and equatorward movements of warm and cold air, Pacific reaches hurricane-strength winds on the respectively. (Ref. Fig 1. 48) Beaufort scale,it is referred to as a typhoon. If a As the cyclone continues to intensify, the cold tropical storm passes the same benchmark in the dense air streams rapidly equatorward, yielding a Northeast Pacific Basin, or in the Atlantic, it is called cold front with a propagation speed that is often a hurricane. Neither “hurricane” nor “typhoon” is 8 to 15 metres per second or more. At the same used in either the Southern Hemisphere or the Indian time, warm less-dense air moving in a northerly Ocean. In these basins, storms of tropical nature are direction blows up over the cold air east of the referred to simply as “cyclones”. cyclone to produce a warm front with a typically ●● Pacific and Indian basin storms are named according much slower propagation speed of about 2.5 to 8 to systems established by regional committees metres per second. This difference in propagation under the auspices of the World Meteorological speeds between the two fronts allows the cold Organization. Each region maintains its own list of front to overtake the warm front and produce yet names, and changes to the list (such as retiring a another, more-complicated frontal structure, known name) are ratified at formal meetings. Two or more as an occluded front. This occlusion process may be lists of names are alternated each year for several followed by further storm intensification; however, regions, including the central North Pacific (i.e., the the separation of the cyclone from the warm air Hawaii region), the western North Pacific and South towards the equator eventually leads to the storm’s China Sea, the southern Indian Ocean west of 90° decay and dissipation in a process called cyclolysis. E, the western South Pacific Ocean, and Australia’s eastern, central, and northern ocean regions. Climatology 45 Temperate Cyclones by the winds directly behind the cold front. If the winds are blowing 70 kilometres per hour, ●● Mid-latitude or frontal cyclones are large the cyclone can be projected to continue its traveling atmospheric cyclonic storms up to movement along the ground surface at this 2000 kilometres in diameter with centres of velocity. low atmospheric pressure. An intense mid- ●● Within the jet stream, localized areas of latitude cyclone may have a surface pressure air outflow can occur because of upper air as low as 970 milibar, compared to an average divergence. Outflow results in the development sea-level pressure of 1013 milibar. of an upper air vacuum. To compensate for the ●● Normally, individual frontal cyclones exist vacuum in the upper atmosphere, surface air for about 3 to 10 days moving in a generally flows cyclonically upwards into the outflow west to east direction. Frontal cyclones are the to replenish lost mass. The process stops and dominant weather events of the Earth’s mid- the mid-latitude cyclone dissipates when the latitudes forming along the polar front. Mid- upper air vacuum is filled with surface air. latitude cyclones are the result of the dynamic interaction of warm tropical and cold polar air masses at the polar front. This interaction causes the warm air to be cyclonically lifted vertically into the atmosphere where it combines with colder upper atmosphere air. This process also helps to transport excess energy from the lower latitudes to the higher latitudes. (Ref. Fig 1. 49) ●● The mid-latitude cyclone is rarely motionless and commonly travels about 1200 kilometres in one day. Its direction of movement is generally eastward. Precise movement of this weather system is controlled by the orientation of the polar jet stream in the upper troposphere. An estimate of future movement Fig 1.49. The Temperate Cyclone of the mid-latitude cyclone can be determined

Difference between Tropical and Temperate Cyclones Sl.No. Tropical Cyclones Temperate Cyclones 1. Origin Thermal Origin Dynamic Origin – Coriolis Force, Movement of air masses. 2. Latitude Confined to 10° - 30°N and S of equator. Confined to 35°-65°N and S of equator. More pronounced in Northern hemisphere due to greater temperature contrast. 3 Frontal system Absent The cyclone formation is due to frontogenesis. (Occluded Front) 4. Formation They form only on seas with temperature Can form both on land as well as seas. more than 26-27°C. They dissipate on reaching the land. 5. Season Seasonal: Late summers (Aug - Oct) Irregular. But few in summers and more in winters. 6. Size ●● Limited to small area. ●● They cover a larger area. ●● Typical size: 100 – 500 km in diameter. ●● Typical size: 300 – 2000 kms in diameter. ●● Varies with the strength of the cyclone. ●● Varies from region to region. Cont... 46 Master Series : Geography

7. Shape Elliptical Inverted ‘V’ 8. Rainfall Heavy but does not last beyond a few In a temperate cyclone, rainfall is slow and hours. If the cyclone stays at a place, the continues for many days, sometimes even rainfall may continue for many days. weeks. 9. Wind Much greater (100 – 250 kmph) (200 – ●● Comparatively low. Typical range: 30 – Velocity and 1200 kmph in upper troposphere) 150 kmph. Destruction Greater destruction due to winds, storm ●● Less destruction due to winds but more surges and torrential rains. destruction due to flooding. 10. Isobars Complete circles and the pressure gradient Isobars are usually ‘V’ shaped and the is steep pressure gradient is low. 11. Life time Doesn’t last for more than a week Last for 2-3 weeks. 12. Path ●● East – West. Turn North at 20° latitude West – East (Westerlies – Jet Streams). Move and west at 30° latitude. away from equator. ●● Move away from equator. ●● The movement of Cyclones in Arabian Sea and Bay of Bengal is a little different. ●● Here, these storms are superimposed upon the monsoon circulation of the summer months, and they move in northerly direction along with the monsoon currents. 13. Temperature The temperature at the center is almost No involvement of temperature. distribution equally distributed. 14. Calm region The center of a tropical cyclone is known In a temperate cyclone, there is not a single as the eye. The wind is calm at the center place where winds and rains are inactive. with no rainfall. 15. Driving force The tropical cyclone derives its energy from The energy of a temperate cyclone depends the latent heat of condensation, and the on the densities of air masses. difference in densities of the air masses does not contribute to the energy of the cyclone. 16. Influence of The relationship between tropical cyclones The temperate cyclones, in contrast, have a Jet streams and the upper level air-flow is not very distinct relationship with upper level air flow clear. (jet streams, Rossby waves etc). 17. Clouds The tropical cyclones exhibit fewer varieties The temperate cyclones show a variety of of clouds – cumulonimbus, nimbostratus, etc. cloud development at various elevations. 18. Surface anti- The tropical cyclones are not associated The temperate cyclones are associated with cyclones with surface anticyclones and they have a anticyclones which precede and succeed greater destructive capacity. a cyclone. These cyclones are not very destructive. 19. Influence on Both coasts a effected. But east coast is Bring rains to North–West India. The India the hot spot of activity. associated instability is called ‘Western Disturbances’.

Thunderstorms formation of thunderstorms are atmosphere instability, updraft of unstable wind, abundant ●● They are local storms with cumulonimbus supply of moisture and thick cloud presence. clouds with very strong updraft of wind. (Ref. Fig 1. 50) Similar conditions also leads to cloud bursts. ●● Surface happens, making thunderstorms Atmospheric condition which leads to to heating through intense solar insolation Climatology 47 hence mostly originate in summer months. Tornado Lightning due to electric discharge happens when electric potential difference ●● A tornado is “a violently rotating column of between two opposite charges become the air, in contact with the ground, either pendant maximum. from a cumulonimbus cloud or underneath cumulonimbus cloud, and often (but not always) visible as a funnel cloud”. ●● For a vortex to be classified as a tornado, it must be in contact with both the ground and the cloud base. They are often referred to as twisters, whirlwinds. ●● It has a low-pressure area at the centre around which winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern hemisphere. (Ref. Fig 1. 51) ●● Most tornadoes have wind speeds less than 180 km/h, are about 80 m across, and travel Fig 1.50. The Cross-section of a Thunderstorm several kilometres before dissipating. The ●● Thundering voice happens due to sudden most extreme tornadoes can attain wind expansion of air column, caused by intense speeds of more than 480 km/h are more than heat resulting from the lightning strike. 3 km in diameter, and stay on the ground for more than 100 km. Typhoon ●● A typhoon differs from a cyclone or hurricane only on the basis of location. A hurricane is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a cyclone occurs in the south Pacific or Indian Ocean. There are six main requirements for tropical Cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to

develop a low pressure centre, a pre-existing Most tornadoes travel at 16-32 kmph, wind speeds low level focus or disturbance, and low can reach up to 400 kmph vertical wind shear.

Fig 1.51. Tornado Formation 48 Master Series : Geography Geography Master Exercise 1

MCQs for Preliminary Examination

1. Which of the following factors affect the Choose the correct conditions using the codes distribution of insolation on the Earth? given below. 1. Angle of sun’s rays (a) 1 and 3 only 2. Length of the day (b) 2, 3 and 4 only 3. Sunspot activity (c) 2 and 4 only 4. Land and sea surface temperature (d) 1, 3 and 4 only Choose the correct answer using the codes given 4. Which of the following statements related to below. water vapour is/are correct? (a) 1, 2 and 4 only 1. Water vapour contains huge numbers of (b) All of the above solid and liquid particles. (c) 2, 3 and 4 only 2. Water vapour in atmosphere increases with (d) 1 and 2 only altitude. 2. Which of the following factors affect the Select the correct answer using the codes given horizontal distribution of temperature on the below. Earth? (a) Only 1 1. Ocean currents (b) Only 2 2. Latitude (c) Both 3. Relief and altitude (d) None 4. Slope and aspect 5. Consider the following statements regarding Choose the correct answer using the codes given temperate cyclones. below. 1. They cover a larger area in comparision to (a) All of the above tropical cyclones. (b) 1, 3 and 4 only 2. They have their origin in frontogenesis. (c) 1, 2 and 4 only 3. They last for few days only. (d) 1 and 4 only Choose the correct answer using the codes given 3. Which of the following conditions are ideal for below. the formation of Temperature inversions? (a) 1 only 1. Short nights 2. Cloudless and clear skies (b) 1 and 2 only 3. Vertical mixing of air at ground level (c) 2 and 3 only 4. Snow covered ground surface (d) 1 and 3 only Climatology 49 Subjective type questions for explained as an outcome of El-Nino effect, do you agree? Main Examination 6. Tropical cyclones are largely confined to South 1. How does the cryosphere affect the global climate? China Sea, Bay of Bengal and Gulf of Mexico. 2. What characteristics condan be assigned to Why? monsoon climate that succeeds in feeding more 7. The recent Cyclone on east coast of India was than 50 percent of the world population residing in called phailin. How are the tropical cyclones Monsoon Asia? named across the world? Elaborate. 3. Discuss the concept of air mass and explain its role 8. What do you understand by the phenomena of in macro-climatic changes. temperature inversion in meteorology, how does it 4. How far do you agree that the behaviour of affect weather and the habitants of the place? the Indian monsoon has been changing due to humanizing landscapes? Discuss. Answers to MCQs 5. Most of the unusual climatic happenings are 1. (b) 2. (a) 3. (c) 4. (a) 5. (b)