CHAPTER 3.3 ATMOSPHERIC CIRCULATION & SYSTEMS

 The weight of a column of air contained in a unit area Isobar is a line connecting points that from the mean sea level to the top of the atmosphere have equal values of pressure. Isobars are is called the atmospheric pressure. The atmospheric analogous to the contour lines on a relief pressure is expressed in units of milibar. At sea level the map. The spacing of isobars expresses average atmospheric pressure is 1,013.2 milibar. Due the rate and direction of change in air to gravity the air at the surface is denser and hence has pressure. This change in air pressure is higher pressure. referred as pressure gradient.  The distribution of atmospheric pressure over the globe is known as horizontal distribution of pressure. It is shown on maps with the help of isobars. The horizontal distribution of atmospheric pressure is not uniform in the world. It varies from time to time at a given place; it varies from place to place over short distances.

The factors responsible for variation in the horizontal Air Pressure : The fundamental rule about distribution of pressure are as follows: gases is that when they are heated, they  Air Temperature: The earth is not heated uniformly become less dense and expand in volume because of unequal distribution of insolation, and rise. Hence, air pressure is low in diff erential heating and cooling of land and water equatorial regions and it is higher in polar surfaces. regions. Along the equator lies a belt of  Generally there is an inverse relationship between low pressure known as the “equatorial low air temperature and air pressure. The higher the or doldrums”. Low air pressure in equatorial air temperature, the lower is the air pressure. regions is due to the fact that hot air ascends there with gradual decrease in temperature  In polar region, cold air is very dense hence it causing thinness of air on the surface. descends and pressure increases. From this we might expect, a gradual increase in average temperature towards equator. Areas of convergence experience low  The Earth’s Rotation: The earth’s rotation generates pressure while those of divergence have centrifugal force. This results in the defl ection of high pressure. air from its original place, causing decrease of pressure. It is believed that the low pressure belts of the sub polar regions and the high pressure belts of the sub-tropical regions are created as a result of the earth’s rotation. The earth’s rotation also causes convergence and divergence of moving air.  Pressure of Water Vapour: In winter the continents are relatively cool and tend to develop high pressure Air with higher quantity of water vapour centres; in summer they stay warmer than the oceans has lower pressure and that with lower and tend to be dominated by low pressure, conversely, quantity of water vapour has higher the oceans are associated with low pressure in winter pressure. and high pressure in summer.

100 Pressure Belts of the World  Equatorial Low Pressure Belt : At the Equator heated air rises leaving a low- pressure area at the surface. This low pressure area is known as equatorial low pressure. This area extends between 50N and 50S latitudes. The zone shifts along with the northward or southward movement of sun during summer solstice and winter solstice respectively. The pressure belt is thermally induced because the ground surface gets heated during the day. Thus warm air expands, rises up and creates low pressure.  Sub-tropical High Pressure Belts: The warm air risen up at the equator due to heating reaches the troposphere and bend towards the pole. Due to Coriolis force the air descends at 30-35º latitude thus creates the belt of sub-tropical high pressure. The pressure belt is dynamically induced as it owes its origin to the rotation of the earth and sinking and settling of winds. This zone is characterized by anticyclonic conditions which cause atmospheric stability and aridity. Thus, the hot deserts of the world are present in this region extending between 25-35 degree in both the hemisphere.  Sub- Pressure Belt: This belt is located between 60-65 degree latitudes in both the hemisphere. This pressure belt is also dynamically induced. As shown in the fi gure the surface air spreads outward from this zone due to rotation of the earth thus produces low pressure. The belt is more developed and regular in the southern hemisphere than the northern due to over dominance of water in the former.  Pressure Belt: High pressure persists at the pole due to low temperature. Thus the Polar High Pressure Belt is thermally induced as well as dynamically induced as the rotation of earth also plays a minor role.

 Winds

The air is set in motion due to the diff erences in atmospheric pressure. The air in motion is called wind. The wind blows from high pressure to low pressure. The wind at the surface experiences friction. In addition, rotation of the earth also aff ects the wind movement. The force exerted by the rotation of the earth is known as the Coriolis force. Thus, the horizontal winds near the earth surface respond to the combined eff ect of three forces – the pressure gradient force, the frictional force and the Coriolis force. In addition, the gravitational The Coriolis force : It defl ects the wind force acts downward. to the right direction in the northern Factors Aff ecting Wind Motion hemisphere and to the left in the southern hemisphere. The defl ection is more when  Pressure Gradient Force: The diff erences in the wind velocity is high. The Coriolis force atmospheric pressure produces a force. The rate of is directly proportional to the angle of change of pressure with respect to distance is the latitude. It is maximum at the poles and is pressure gradient. The pressure gradient is strong absent at the equator. The Coriolis force where the isobars are close to each other and is weak acts perpendicular to the pressure gradient where the isobars are apart. force.

101  Frictional Force: It aff ects the speed of the wind. It is greatest at the surface and its infl uence generally extends upto an elevation of 1 - 3 km. Over the sea surface the friction is minimal.  Coriolis Force: The rotation of the earth about its axis aff ects the direction of the wind. This force is called the Coriolis force after the French physicist Gustave-Gaspard Coriolis who described it in 1835.  The pressure gradient force is perpendicular to an isobar. The higher the pressure gradient force, the more is the velocity of the wind and the larger is the defl ection in the direction of wind. As a result of these two forces operating perpendicular to each other, in the low-pressure areas the wind blows around it.  At the equator, the Coriolis force is zero and the wind blows perpendicular to the isobars. The low pressure gets fi lled instead of getting intensifi ed. That is the reason why tropical are not formed near the equator.

Planetary Winds Planetary winds are major component of the general global circulation of air. These are known as planetary winds because of their prevalence in the global scale throughout the year. Planetary winds occur due to temperature and pressure variance throughout the world.

 Trade Wind  Winds blowing from the subtropical high pressure belt or towards the equatorial low pressure belt of the ITCZ are the trade winds. In the Northern Hemisphere, the trade winds blow from the northeast and are known as the Northeast Trade Winds; in the Southern Hemisphere, the winds blow from the southeast and are called the Southeast Trade Winds.  The weather conditions throughout the tropical zone remain more or less uniform. This belt is subjected to seasonal variation due to northward and southward movement of sun.  The equator ward part of the trade wind is humid because they are characterized by atmospheric instability thus causing heavy precipitation.

 Westerly Wind  The Westerlies are the prevailing winds in the middle latitudes between 35º and 65º blowing from the high pressure area in the sub tropical high pressure belt, i.e., horse latitudes towards the sub polar low pressure belt.  The winds are predominantly from the south-west to north-east in the Northern Hemisphere and from the north- west to south- east in the Southern Hemisphere.  The Westerlies are strongest in the winter season at times when the pressure is lower over the poles, while they are weakest in the summer season when pressures are higher over the poles.  The Westerlies are particularly strong, especially in the southern hemisphere, as there is less land in the middle latitudes to obstruct the fl ow.

102  The Westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse.

 Polar Wind:  The winds blowing in the Arctic and the Antarctic latitudes are known as the Polar Winds. They have been termed as the ‘Polar Easterlies’, as they blow from the Polar High Pressure belt towards the Sub- Polar Low- Pressure Belts. In the Northern Hemisphere, they blow in general from the north-east, and are called the North-East Polar Winds; and in the Southern Hemisphere, they blow from the south- east and are called the South-East Polar Winds. As these winds blow from the ice- capped landmass, they are extremely cold. They are more regular in the Southern Hemisphere than in the Northern Hemisphere Periodic/Seasonal Winds The pattern of wind circulation is modifi ed indiff erent seasons due to the shifting of regions of maximum heating, pressure and wind belts. This results in seasonal winds. The most pronounced eff ect of such a shift is noticed in the monsoons, especially over southeast Asia.  The word ‘Monsoon’ has been derived from the Arabic word ‘Mausim’ meaning season. The winds that reverse their direction with the change of seasons are called monsoon winds. During summer the monsoon winds blow from sea towards land and during winter from land towards sea.  Traditionally these winds were explained as land and sea breezes on a large scale. But this explanation does not hold good now. Now a days the monsoon is generally accepted as seasonal modifi cation of the general planetary wind system.  The Asiatic monsoon is the result of interaction of both planetary wind system and regional factors, both at the surface and in the upper troposphere. India, Pakistan, Bangladesh, Myanmar (Burma), Sri Lanka, the , the , South-east Asia, North Australia, China and Japan are important regions where monsoon winds are prevalent.

Summer Monsoon  During the summer, monsoon winds blow from the cooler ocean surfaces onto the warmer continents.  In the summer, the continents become much warmer than the oceans because of a number of factors. These factors include:  Specifi c heat diff erences between land and water.  Greater evaporation over water surfaces.  Sub-surface mixing in ocean basins, which redistributes heat energy through a deeper layer.  Precipitation is normally associated with the summer monsoons. Onshore winds blowing inland from the warm ocean are very high in humidity, and slight cooling of these air masses causes condensation and . In some cases, this precipitation can be greatly intensifi ed by orographic uplift. Some highland areas in Asia receive more than 10 meters of rain during the summer months.

Winter Monsoon  In the winter, the wind patterns reverses, as the ocean surfaces are now warmer. With little solar energy available, the continents begin cooling rapidly as longwave radiation is emitted to space. The ocean surface retains its heat energy longer because of water’s high specifi c heat and sub-surface mixing. The winter monsoons bring clear dry weather and winds that fl ow from land to sea.

World Monsoonal Climate  The Asiatic monsoon is the result of a complex climatic interaction between the distribution of land and water, topography, and tropical and mid- latitudinal circulation. In the summer, a low-pressure center forms over northern India and northern Southeast Asia because of higher levels of received solar insolation.

103  Warm moist air is drawn into the thermal lows from air masses over the Indian Ocean. Summer heating also causes the development of a strong latitudinal pressure gradient and the development of an easterly jet stream at an altitude of about 15 kilometers at the latitude of 25° north. The jet stream enhances rainfall in Southeast Asia, in the Arabian Sea, and in South Africa. When autumn returns to Asia the thermal extremes between land and ocean decrease and the westerlies of the mid-latitudes move in.  The easterly jet stream is replaced with strong westerly winds in the upper atmosphere. Subsidence from an upper atmosphere cold layer above the Himalayas produces outfl ow that creates a surface high- pressure system that dominates the weather in India and Southeast Asia.  Besides, Asian continent, monsoon wind systems also exist in Australia, Africa, South America, and North America.

Local Winds Local winds occur on a small spatial scale, their horizontal dimensions typically several tens to a few hundreds of kilometres. They also tend to beshort-lived lasting typically several hours to a day. There are many such winds around the world, some of them cold, some warm, some wet, some dry. There are many hazards associated with the winds.

Land and Sea Breezes  A land breeze is created when the land is cooler than the water such as at night and the surface winds have Hot Winds to be very light. When this happens the air over the Sirocco Sahara Desert water slowly begins to rise, as the air begins to rise, the air over the surface of the ocean has to be replaced, Leveche Spain this is done by drawing the air from the land over the Khamsin Egypt water, thus creating a sea breeze. Harmattan Sahara Desert  A sea breeze is created when the surface of the land Santa Ana USA is heated suffi ciently to start air rising. As air rises, it is replaced by air from the sea and creates a sea Zonda Argentina breeze. Sea breezes tend to be much stronger and can Brick fi elder Australia produce gusty winds as the sun can heat the land to very warm temperatures, thereby creating a signifi cant Cold Winds temperature contrast to the water. Mistral Spain and France Mountain and Valley Winds Bora Adriatic coast  Mountain-valley breezes are formed by the daily Pampero Argentina diff erence of the thermo eff ects between peaks and Buran Siberia valleys. In daytime, the mountainside is directly heated by the sun, the temperature is higher, air expands, air Descending Winds pressure reduces, and therefore air will rise up the Chinook USA mountainside from the valley and generate a valley breeze. Fohn Switzerland  The valley breeze reaches its maximum force in day Berg Germany time (mostly in afternoon). After this time, the breeze Norwester New Zealand decreases in power and come to a complete stop by sunset. By Sunset, the mountain side region is able to Samun Persia (Iran) dissipate heat more quickly, due to its higher altitude Nevados Ecuador and therefore temperature drops rapidly. Cold air will then travel down the mountain side from the top and fl ow into the valley, forming a mountain breeze.

104  In daytime, valley breezes carry water vapor to the peak, which will often condense into clouds; these are the commonly seen peak and fl ag clouds. When the mountain breeze travels down and gathers in the valleys, water vapor will condense. Therefore in valley or basin regions, there are usually clouds and fog before sunrise. During late spring or early autumn, the cold air trapped in the valley and basin will often generate frost.

Regional winds  Hot winds - Loo, Foehn and Chinook are important hot winds of local category.  The Foehn is a warm, dry, gusty wind which occurs over the lower slopes on the lee side (the side which is not directly exposed to wind and weather) of a mountain barrier. It is a result of forcing stable air over a mountain barrier. The onset of a Foehn is generally sudden. For example, the temperature may rise more than 10°C in fi ve minutes and the wind strength increase from almost calm to gale force just as quickly. Foehn winds occur quite often in the Alps (where the name foehn originated) and in the Rockies (where the name Chinook is used). The local cold winds originate in the snow-capped mountains during winter and move down from the slopes towards the valleys. They are known by diff erent names in diff erent areas.

 Jet-Streams

The term was introduced in 1947 by Carl Gustaf Rossby. Average speed is very high with a lower limit of about 120 km in winter and 50 km per hours in summer. The Jet Streams located in the upper troposphere (9 - 14 km) are bands of high speed winds (95-190 km/hr). The two most important types of jet streams are the Polar Jet Streams and the Subtropical Jet Streams. They are found in both the Northern and Southern Hemispheres.  Polar Jet Stream: These jet streams are created when cold air from the Polar Regions meets warmer air from the equator. This temperature gradient as a result forms a pressure gradient that increases wind speed. During the winter, these jet streams bring winter and blizzards to the United States and in summer, they become weaker and move towards high latitudes. They are found in both Northern and Southern Hemispheres.  Sub-Tropical Jet Stream: These jet streams are formed as warm air from the equator moves towards the poles that form a steep temperature incline along a subtropical front that like the polar jet streams produce strong winds. In Southeast Asia, India, and Africa, these jet streams help bring about the region’s monsoon or rainy climate. As these jet streams warm, the air above the Tibetan highlands, a temperature and pressure gradient is formed as the air from the ocean is cooler than that above the continental high lands. This as a result forms on- shore winds that produces the monsoon.

Formation of Jet Stream  Warm air masses in the south meet cool air masses from the north and create temperature and air pressure gradients. The pressure diff erence between a high and low pressure zone can be very large, thereby creating high winds. Pressure and temperature diff erences in the jet stream can be large as a global warm front from the south and a cold front from the north meet.

 Air Mass

An air mass is an extensive portion of the atmosphere having uniform characteristics of temperature, pressure and moisture which are relatively homogeneous horizontally.

105  Air masses form over large surfaces with uniform temperatures and humidity, called source regions. Low wind speeds let air remain stationary long enough to take on the features of the source region, such as heat or cold. When winds move air masses, they carry their weather conditions (heat or cold, dry or moist) from the source region to a new region. When the air mass reaches a new region, it might clash with another air mass that has a diff erent temperature and humidity.  The major source regions of the air masses are the high latitude polar or low latitude tropical regions having such homogeneous conditions. Air masses, therefore, are of two kinds-polar and tropical air masses. Polar air mass is cold and tropical air mass is warm.  When cold air mass and warm air mass blow against each other, the boundary line of convergence separating the two air masses is termed as front. When the warm air mass, moves upward over the cold air mass the front formed in such a situation is called warm front. On the contrary, when the cold air mass advances faster and undercuts the warm air mass and forces the warm air upwards, the front so formed is called cold front.  The frontal surface of cold front is steeper than that of a warm front. A prevailing air mass in any region - polar, tropical, maritime or continental largely controls the regions general weather.

Air Mass Symbol Characteristics/Comments

Form exclusively in the Arctic and Antarctic regions and descend Continental cA toward Arctic the equator. Bitterly cold and extremely dry in the winter, cool and dry during the summer.

Form over dry lands, Cold and dry during the winter, mild and dry Continental Polar cP during the - winter.

Continental Form over deserts and plains. In the United States, a fl ow into a the US cT out of Mexico often sends a cTair mass northward. Typically hot and Tropical dry during the summer and mild dry during the winter. Marine type humidities with cool or cool weather. Typically provide for miserable, damp, gray days. Mild to cold and humid with low Martine Polar rnP status clouds and precipitation is often the rule with Marine Polar air masses. Hot, humid, sticky weather. A good example of when ml air masses aff ect the United States is during the summer with the Bermuda High Maritime Tropical mT phenomena. A southerly flow of hot, humid, Sticky weather is circulated northward into the US. Rarely will mTair masses aff ect the US during the winter.

 Air masses are named according to their source region. Polar (P) air masses form at high latitudes toward Earth’s poles. Air masses that form at low latitudes are Tropical (T) air masses. The terms Polar and tropical describe the temperature characteristics of an air mass. Polar air masses are cold, while tropical air masses are warm. In addition to their overall temperature, air masses are classifi ed according to the surface over which they form. Continental (c) air masses form over land. Maritime  (m) air masses form over water. The terms continental and maritime describe the moisture characteristics of the air mass. Continental air masses are likely to be dry. Maritime air masses are humid. Using this classifi cation scheme, there are four basic types of air masses. A Continental Polar (cP) air mass is dry and cool. A Continental Tropical (cT) air mass is dry and warm or hot. Maritime Polar (mP) and Maritime Tropical (mT) air masses both form over water. But a Maritime Polar air mass is much colder than a Maritime Tropical air mass.

106  Fronts

 When cold air mass and warm air mass blow against each other, the boundary line of convergence separating the two air masses is termed as front. At the boundary, or front, there is a marked drop in temperature, increase in humidity, sudden wind change and a marked pressure rise. This marked discontinuity lends support to the theory of two separate air masses with the front being the boundary between the two.  When the warm air mass, moves upward over the cold air mass the front formed in such a situation is called warm front. On the contrary, when the cold air mass advances faster and undercuts the warm air mass and forces the warm air upwards, the front so formed is called cold front. The frontal surface of cold front is steeper than that of a warm front.

High Pressure and Low Pressure Systems The pressure diff erences cause air to move in ways that may make a high or low become the centre of a whole system of weather. At a high-pressure centre, air sinks slowly down. As the air nears the ground, it spreads out toward areas of lower pressure.  In the Northern Hemisphere, the Coriolis eff ect makes the air turn clockwise as it moves outward. Most high-pressure systems are large and change slowly. When a high-pressure system stays in one location for a long time, an air mass may form. The air-and resulting air mass-can be warm or cold, moist or dry.  A high-pressure system generally brings clear skies and calm air or gentle breezes. This is because as air sinks to lower altitudes, it warms up a little bit. Water droplets evaporate, so clouds often disappear.  A small area of low pressure can also develop into a larger system. It begins as air moves around and inward toward the lowest pressure and then up to higher altitudes. The upward motion of the air lowers the air pressure further, and so the air moves faster.

107  The pattern of motion strengthens into a low- pressure weather system. The rising air produces stormy weather. In the Northern Hemisphere, the air in a low-pressure system circles in a counter clockwise direction.  A low-pressure system can develop wherever there is a centre of low pressure. One place this often happens is along a boundary between a warm air mass and a cold air mass.

 Cyclones

Typically, cyclones are elliptical arrangement of isobars having low pressure at the centre with a Atlantic Ocean and North West Europe are convergence of winds within them. The wind direction major regions of temperate cyclones. They in the cyclones is anti-clockwise in the northern are generally extensive having a thickness of 9 hemisphere and clockwise in the southern hemisphere. to11 kilometers and with 1040-1920 km short Cyclones are of two types - the temperate or mid and long diameters respectively. The weather latitude cyclones and the tropical or low latitude associated with the is drizzling rain cyclones and of cloudy nature for number of days.

 Temperate Cyclones

Temperate cyclones are formed along a front in mid-latitudes between 35°and 65° N and S. They blow from west to east and are more pronounced in winter season.

Polar Front Theory- Development and Evolution of a Wave Cyclone, the wave cyclone (often called a frontal wave) develops along the polar front- when a large temperature gradient exists across the polar front - the atmosphere contains a large amount of Available Potential Energy. An instability (kink) forms in the polar front. This instability is the incipient cyclone. A fully-developed “wave cyclone” is seen 12- 24 hours from its inception. It consists of:  a warm front moving to the northeast.  a cold front moving to the southeast.  region between warm and cold fronts is the “warm sector”.  the central low pressure (low, which is deepening with time).  overrunning of warm air over the warm front.  cold air surging southward behind the cold front.  wide-spread of precipitation ahead of the warm front.  narrow band of precip along the cold front.

108  Wind speeds continue to get stronger as the low deepens - the Available Potential Energy (APE) is being converted to Kinetic Energy (KE).  The production of clouds and precip also generates energy for the as Latent Heat is released. As the cold front moves swiftly eastward, the systems starts to occlude.  Storm is most intense at this stage.  have an occluded front trailing out from the surface low.  triple point/occlusion - is where the cold, warm and occluded fronts all intersect. The warm sector diminishes in size as the systems further occludes. The storm has used most all of its energy and dissipates.  All of the APE has been utilized and the KE has dissipated into turbulence- cloud/ precip production has diminished.  The warm sector air has been lifted upward.  The cold air is at the surface-stable situation.

Seasonal Occurrence of Temperate Cyclones  The temperate cyclones occur mostly in winter, late autumn and spring. They are generally associated with rainstorms and cloudy weather.  During summer, all the paths of temperate cyclones shift northwards and there are only few temperate cyclone over sub-tropics and the warm temperate zone, although a high concentration of storms occurs over Bering Strait, USA and Russian Arctic and sub-Arctic zone.

Distribution of Temperate Cyclones  USA and Canada – extend over Sierra Nevada, Colorado, Eastern Canadian Rockies and the Great Lakes region,  the belt extending from Iceland to Barents Sea and continuing over Russia and Siberia,  winter storms over Baltic Sea,  Mediterranean basin extending up to Russia and even up to India in winters (called western disturbances) and the Antarctic frontal zone.

Associated Weather  The approach of a temperate cyclone is marked by fall in temperature, fall in the mercury level, wind shifts and a halo around the sun and the moon, and a thin veil of cirrus clouds.  A light drizzle follows which turns into a heavy downpour. These conditions change with the arrival of the warm front which halts the fall in mercury level and the rising temperature.  Rainfall stops and clear weather prevails until the cold front of an anticyclonic character arrives which causes a fall in temperature, brings cloudiness and rainfall with thunder. After this, once again clear weather is established.  The temperate cyclones experience more rainfall when there is slower movement and a marked diff erence in rainfall and temperature between the front and rear of the cyclone. These cyclones are generally accompanied by .

 Tropical Cyclones

Tropical cyclones are intense cyclonic storms that develop over the warm oceans of the tropics. Surface atmospheric pressure in the centre of tropical cyclones tends to be extremely low. Most storms have an

109 average pressure of 950 millibars. To be classifi ed as Tropical cyclones, sustained wind speeds must be greater than 118 kilometers per hour at the storm’s centre. The main characteristics of tropical cyclones are:-  Have winds that exceed 34 knots (39 mi/hr),  Blow clockwise in the Southern Hemisphere, and  Counter-clockwise about their centres in the Northern Hemisphere.

Condition for formation of Tropical Cyclones:  For a cyclone to form several preconditions must be met:  Warm ocean waters (of at least 26.5°C) throughout a suffi cient depth (unknown how deep, but at least on the order of 50 m). Warm waters are necessary to fuel the heat engine of the .  An atmosphere which cools fast enough with height (is “unstable” enough) such that it encourages activity. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.  Relatively moist layers near the mid- troposphere (5 km). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity.  A minimum distance of around 500 km from the equator. Some of the earth’s spin (Coriolis force) is needed to maintain the low pressure of the system. (Systems can form closer to the equator but it’s a rare event)  A pre-existing disturbance near the surface with suffi cient spin (vorticity) and infl ow (convergence). Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organised system with sizeable spin and low level infl ow.  Little change in the wind with height (low vertical , i.e. less than 40 km/h from surface to tropopause). Large values of wind shear tend to disrupt the organisation of the that are important to the inner part of a cyclone.  Connection between tropical cyclones and wind speed.

Depending on the speed, tropical cyclones will be designated as follows:  It is a tropical depression when the maximum sustained wind speed is less than 63 km/h.  It is a tropical storm when the maximum sustained wind speed is more than 63 km/h. It is then also given a name.  Depending on the ocean basins, it is designated either a hurricane, , severe tropical cyclone, severe cyclonic storm or tropical cyclone when the maximum sustained wind speed is more than 119 km/h.  Tropical cyclones can be hundreds of kilometers wide and can bring destructive high winds, torrential rain, and occasionally tornadoes.  The impact of a tropical cyclone and the expected damage depend not just on wind speed, but also on factors such as the moving speed, duration of strong wind and accumulated rainfall during and after landfall, sudden change of moving direction and intensity, the structure (e.g. size and intensity) of the tropical cyclone, as well as human response to tropical cyclone disasters.

Structure of the Tropical Cyclone  Tropical cyclones have no fronts associated with them like the mid-latitude cyclones of the polar front. They are also smaller than the mid-latitude cyclone, measuring on average 550 kilometers in diameter. Mature tropical cyclones usually develop a cloud-free at their centre.

110  In the eye, air is descending creating clear skies. The eye of the tropical cyclones may be 20 to 50 kilometers in diameter. Surrounding the eye are bands of organized thunderstorm clouds formed as warm air move in and up into the storm. The strongest winds and heaviest precipitation are found in the area next to the eye where a vertical wall of thunderstorm clouds develops from the Earth’s surface to the top of the troposphere.  The circular eye or centre of a tropical cyclone is an area characterised by light winds, fi ne weather and often clear skies. The eye is the region of lowest surface pressure.  The eye is surrounded by a dense ring of cloud known as the eye wall. This marks the most dangerous part of the cyclone having the strongest winds and heaviest rainfall.  Radar and satellite imagery often show that the eye wall clouds are the inner-most coil of a series of spiral rain-band cloudsthat extend hundreds of kilometres from the centre and typically produce very strong wind squalls. The eye-wall is not always symmetrical, in fact at any one time it is common for the strongest and surface winds to be in one part of the eye wall.

Life Cycle of Tropical Cyclone Tropical cyclones have a distinct life cycle. For cyclones that reach at least severe (category 3 or higher having wind gusts of at least 165 km/h) the life-cycle may be divided into four stages. For non- severe cyclones, their development is constrained by one or more of a number of factors such as being located in an unfavourable atmospheric environment, movement over cooler water or making landfall.

The Life Cycle Processes are Explained as Follows

 The Formative Stage  On satellite images the disturbance appears as an unusually active, but poorly organised, area of convection (thunderstorms). The circulation centre is usually ill-defi ned but sometimes curved cumulus cloud bands spiralling towards an active area of thunderstorms indicate the location of the centre.  Initially the amount of convection near the centre is dependent upon the normal diurnal cycle of tropical convection, increasing overnight and subsiding during the day. As development occurs the convection persists throughout the day. Apart from local squalls the maximum wind is usually less than gale force. When formative stage tropical cyclones move inland they produce little or no damage on landfall but are often associated with heavy rain and sometimes fl ooding.

111  The Immature Stage  In this stage the area of convection persists and becomes more organised. Intensifi cation occurs simultaneously. The minimum surface pressure rapidly drops below 1000 hPa and convection becomes organized into long bands spiralling inwards. In satellite images several well organised curved bands of active convection may be seen spiralling in towards a central dense mass of clouds covering the focal point of the banding, or surrounding the centre.  The eye (if it exists) may be masked by a canopy of , which itself may contain curved striations associated with the outfl ow at the top of the tropical cyclone. The immature tropical cyclone can cause devastating wind and storm surge eff ects upon landfall, although damage is usually confi ned to a relatively small area. In this stage of development very rapid intensifi cation can occur and the associated structural changes observed when the cyclone is under radar surveillance can sometimes be confusing.

 The Mature Stage  During this stage the tropical cyclone acquires a quasi-steady state with only random fl uctuations in central pressure and maximum wind speed. However, the cyclonic circulation and extent of the gales increase markedly. Asymmetries in the wind fi eld may also become more pronounced. In satellite images the cloud fi eld is highly organized and becomes more symmetrical.  The more intense cyclones are characterized by a round central dense overcast containing a well- centred, distinct round eye. The surrounding convective bands are tightly coiled and quasi-circular. Typically a cyclone spends just a day or so at maximum intensity until it begins to weaken, unless the cyclone remains in a highly favourable environment.

 The Decay Stage  The warm core is destroyed during this stage, the central pressure rises, and the belt of maximum wind expands away from near the centre. Decay may occur very rapidly if the system moves into an unfavourable atmospheric or geographic environment, but sometimes only the tropical characteristics are modifi ed while the cyclonic circulation moves on to higher latitudes.  In satellite images the decaying stage is characterised by the weakening of organised convection near the centre and disappearance of major curved convective bands. The low- level circulation centre may still be very well defi ned by narrow bands of low clouds. Those cyclones that cross the coast and weaken over land may continue to produce heavy rain a considerable distance inland.

 Tropical Cyclone Formation Basin  Given that sea surface temperatures need to be at least 80°F (27°C) for tropical cyclones form, it is natural that they form near the equator. However, with only the rarest of occasions, these storms do not form within 5° latitude of the equator. This is due to the lack of suffi cient Coriolis Force, the force that causes the cyclone to spin.

 Tropical Cyclones Damage and Destruction  Cyclones are characterized by their destructive potential to damage structures such as houses, lifeline infrastructure such as power and communication towers, hospitals, food storage facilities, roads, bridges, culverts, crops, etc., due to high velocity winds. Exceptionally heavy rainfall causes fl ooding. Storm surge inundates low-lying areas in the coastal areas resulting in loss of life and destruction of property, besides eroding beaches and embankments, destroying vegetation and reducing soil fertility.  Besides the loss of lives and livestock, cyclones have high destructive potential due to the strong winds that damage structures, and heavy rainfall which causes fl oods and storm surge that inundates low- lying coastal areas.  Storm surge and large waves produced by hurricanes pose the greatest threat to life and property along the coast.

112  STORM SURGE is an abnormal rise of water generated by a storm’s winds. Storm surge can reach heights well over 20 feet and can span hundreds of miles of coastline. In the northern hemisphere, the highest surge values typically occur in the right front quadrant of a hurricane coincident with onshore fl ow; in the southern hemisphere, the left front quadrant. More intense and larger hurricanes produce higher surge.  STORM TIDE is the water level rise during a storm due to the combination of storm surge and the astronomical tide. For example, if a hurricane moves ashore at a high tide of 2 feet, a 15 foot surge would be added to the high tide, creating a storm tide of 17 feet. The combination of high winds and storm tide topped with battering waves can be deadly and cause tremendous property damage along an area of coastline hundreds of miles wide.  The destructive power of storm surge and large battering waves can result in loss of life, buildings destroyed, beach and dune erosion and road and bridge damage along the coast. Storm surge can travel several miles inland. In estuaries and bayous, salt water intrusion endangers public health and the environment.

 Thunderstorms

Thunderstorms form when moist, unstable air is lifted vertically into the atmosphere. Lifting of this air results in condensation and the release of latent heat. The process to initiate vertical lifting can be caused by:  Unequal warming of the surface of the Earth.  Orographic lifting due to topographic obstruction of air fl ow.  Dynamic lifting because of the presence of a frontal zone.

Formation of the Thunderstorm  Immediately after lifting begins, the rising parcel of warm moist air begins to cool because of adiabatic expansion. At a certain elevation the dew point is reached resulting in condensation and the formation of a cumulus cloud.  For the cumulus cloud to form into thunderstorm, continued uplift must occur in an unstable atmosphere. With the vertical extension of the air parcel, the cumulus cloud grows into a cumulonimbus cloud. Cumulonimbus clouds can reach heights of 20 kilometers above the Earth’s surface. Severe weather associated with some these clouds includes hail, strong winds, thunder, , intense rain, and tornadoes. Generally, two types of thunderstorms are common:  Air mass storm  Thunderstorms associated with mid- latitude cyclone cold fronts or dry lines.

 Air Mass Storm The most common type of thunderstorm is the air mass storm. Air mass thunderstorms normally develop in late afternoon hours when surface heating produces the maximum number of convection currents in the atmosphere. The life cycle of these weather events has three distinct stages.

113  1st Stage- Cumulus Stage: The fi rst stage of air mass thunderstorm development is called the cumulus stage. In this stage, parcels of warm humid air rise and cool to form clusters of puff y white cumulus clouds. The clouds are the result of condensation and deposition which, releases large quantities of latent heat. The added heat energy keeps the air inside the cloud warmer than the air around it. The cloud continues to develop as long as more humid air is added to it from below. Updrafts dominate the circulation patterns within the cloud.  2nd Stage- Mature Stage: When the updrafts reach their maximum altitude in the developing cloud, usually 12 to 14 kilometers, they change their direction 180° and become downdrafts. This marks the mature stage. With the downdrafts, precipitation begins to form through collision and coalescence. The storm is also at its most intense stage of development and is now a cumulonimbus cloud. The top of the cloud takes on the familiar anvil shape, as strong stratospheric upper-level winds spread ice crystals in the top of the cloud horizontally. At its base, the thunderstorm is several kilometers in diameter. The mature air mass thunderstorm contains heavy rain, thunder, lightning, and produces wind gusts at the surface.  3rd Stage- Dissipating Stage: The mature thunderstorm begins to decrease in intensity and enters the dissipating stage after about half an hour. Air currents within the convective storm are now mainly downdrafts as the supply of warm moist air from the lower atmosphere is depleted. Within about 1 hour, the storm is fi nished and precipitation has stopped. Thunderstorms form from the equator to as far north as Alaska. They occur most commonly in the tropics were convectional heating of moist surface air occurs year round. Many tropical land based locations experience over 100 thunderstorm days per year. Thunderstorm formation over tropical oceans is less frequent because these surfaces do not warm rapidly. Outside the tropics, thunderstorm formation is more seasonal occurring in those months where heating is most intense.

Indian Context  The Indian subcontinent is one of the worst aff ected regions in the world. The subcontinent with a long coastline of 8041 kilometres is exposed to nearly 10 per cent of the world’s tropical cyclones. Of these, the majority of them have their initial genesis over the Bay of Bengal and strike the East coast of India.  On an average, fi ve to six tropical cyclones form every year, of which two or three could be severe. More cyclones occur in the Bay of Bengal than the Arabian Sea and the ratio is approximately 4:1. Cyclones occur frequently on both the coasts (the West coast - Arabian Sea; and the East coast-Bay of Bengal).  Cyclone over Arabian Sea is weak in comparison to that of Bay of Bengal because: Cyclones that form over the Bay of Bengal are either those develop insitu over southeast Bay of Bengal and adjoining Andaman Sea or remnants of over Northwest Pacifi c and move across south China sea to Indian Seas.  As the frequency of typhoons over Northwest Pacifi c is quite high (about35 % of the global annual average), the Bay of Bengal also gets its increased quota. The cyclones over the Arabian Sea either originate insitu over southeast Arabian Sea (which includes Lakshadweep area also) or remnants of cyclones from the Bay of Bengal that move across south peninsula.  As the majority of Cyclones over the Bay of Bengal weaken over land after landfall, the frequency of migration into Arabian Sea is low. In addition the Arabian Sea is relatively colder than Bay of Bengal and hence inhibits the formation and intensifi cation of the system.  Tropical cyclones occur in the months of May- June and October-November. Cyclones of severe intensity and frequency in the North Indian Ocean are bi-modal in character, with their primary peak in November and secondary peak in May.  The disaster potential is particularly high during landfall in the North Indian Ocean (Bay of Bengal and the Arabian Sea) due to the accompanying destructive wind, storm surges and torrential rainfall. Of these, storm surges cause the most damage as sea water inundates low lying areas of coastal regions and causes heavy fl oods, erodes beaches and embankments, destroys vegetation and reduces soil fertility.

114  RECENT DEVELOPMENTS

 Bomb Cyclone

A large, rapidly intensifying winter storm hit east coast of United States midnest this winter. This type of cyclone is called Bomb Cyclone. About:  Bomb Cyclone: Bombogenesis, a popular term used by meteorologists, occurs when a midlatitude cyclone rapidly intensifi es, dropping at least 24 millibars over 24 hours. A millibar measures atmospheric pressure.  This can happen when a cold air mass collides with a warm air mass, such as air over warm ocean waters. The formation of this rapidly strengthening weather system is a process called bombogenesis, which creates what is known as a bomb cyclone.  This event is also called “explosive ” and even “meteorological bombs”.  The four most active regions where extratropical occurs in the world are the Northwest Pacifi c, the North Atlantic, the Southwest Pacifi c, and the South Atlantic.

Impacts:  As it occurs during winter in northern hemisphere, it is accompanied by snow and frost which can cause exposed skin to freeze within 30 minutes. Temperatures usually falls as low as -40 degrees Celsius which causes disruption in normal lives and weather related sickness such as frostbite.

 How the 2015-16 El Nino aff ected disease outbreaks

According to a study published in the journal Nature global climatic disruptions due to the strong and extended positive phase of the ENSO conditions, or simply El Nino in 2015-16 increased the outbreak of diseases in the regions of its infl uence. About:

Disease outbreak from El-Nino event:  Major diseases like chikungunya, dengue, malaria, Hantavirus, rift valley fever, cholera, plague and zika are aff ected by the weather events induced by El Nino.  The scientists analyzed certain disease outbreaks in the 2015-16 period and tried to correlate them with higher temperatures and erratic rainfall patterns characteristic of the El Nino.

Outcomes of the study:  Researchers found that in regions like Southeast Asia, Tanzania, western United States and Brazil — which are generally aff ected by the El Nino — the spread of diseases came after shifts in rainfall, temperature and vegetation.  There was either excess of droughts or fl oods in this period which created the environmental conditions that favored the growth and propagation of disease causing microorganisms and their carriers.

115 El-Nino Southern Oscillation (ENSO)  The El Niño-Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacifi c Ocean. On periods ranging from about three to seven years, the surface waters across a large swath of the tropical Pacifi c Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal.

 This oscillating warming and cooling pattern, referred to as the ENSO cycle, directly aff ects rainfall distribution in the tropics and can have a strong infl uence on weather across the United States and other parts of the world  El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral. El Niño:

116  A warming of the ocean surface, or above-average Sea surface temperatures (SST), in the central and eastern tropical Pacifi c Ocean. Over Indonesia, rainfall tends to become reduced while rainfall increases over the central and eastern tropical Pacifi c Ocean.  The low-level surface winds, which normally blow from east to west along the equator (“easterly winds”), instead weaken or, in some cases, start blowing the other direction (from west to east or “westerly winds”). In general, the warmer the ocean temperature anomalies, the stronger the El Niño (and vice-versa).  La Niña: A cooling of the ocean surface, or below-average Sea surface temperatures (SST), in the central and eastern tropical Pacifi c Ocean. Over Indonesia, rainfall tends to increase while rainfall decreases over the central and eastern tropical Pacifi c Ocean. The normal easterly winds along the equator become even stronger. In general, the cooler the ocean temperature anomalies, the stronger the La Niña (and vice-versa).  Neutral: Neither El Niño or La Niña. Often tropical Pacifi c SSTs are generally close to average. However, there are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not playing along (or vice versa).

Signifi cance:  If correlation between El-Nino and disease outbreak is justifi ed by further research then countries aff ected by would be ready after El-Nino predictions.  Early Warning of possible changes in weather conditions after El-Nino prediction can help countries to launch awareness campaign to make their citizens cautious about negative health eff ects of the event.

: Type of Tropical Cyclone

Hurricane Bud hit parts of North America especially in parts of Mexico and United States.

About:  Hurricane Bud was a powerful tropical cyclone that produced heavy rainfall and fl ash fl ooding across Northwestern Mexico and the Southwestern United States.  Bud originated from a tropical wave that departed from Africa on May 29,2018.  It then travelled across the Atlantic Ocean before crossing over South America and entering the Northeast Pacifi c Ocean late on June 6.  Bud ultimately peaked with maximum sustained winds of 140 mph (220 km/h) and a minimum barometric pressure of 943 mbar.  It made landfall on Baja California Sur as a minimal tropical storm early on June 15, 2018.

 Tropical

A tropical cyclone named Sagar formed in Indian Ocean and hit eastern coast of .

About:  Cyclonic Storm Sagar was the strongest tropical cyclone to make landfall in Somalia in recorded history of the country.

117  It formed on May 16 east of the , Sagar intensifi ed into a cyclonic storm the next day.  The storm turned to the west-southwest and traversed the entirety of the , making landfall over northwestern Somalia farther west than any other storm on record in the North Indian Ocean.

Impact:  In Somalia, Sagar dropped a years’ worth of rainfall, or around 200 mm (7.9 in).  The caused deadly fl ash fl ooding that washed away bridges, homes, and thousands of farm animals.  Other countries like , and also faced heavy storm followed by rainfall causing damage to life and property.

 How is naming of cyclone done?

A number of cyclonic storms such as Ockhi, Titli, Hudhud etc. has hit Indian coast.

About:

 History of naming of Cyclone  Traditionally cyclones were given general names such as Hurricane in the Atlantic, Typhoon in the Pacifi c and Cyclone in the Indian Ocean.  If the storm’s wind speed reaches or crosses 74 mph, it is then classifi ed into a hurricane/cyclone/ typhoon.

 Naming with people’s name  Initially, people living in the Caribbean Islands would name the storms after the saint of the day from the Roman Catholic liturgical calendar for the day on which the hurricane/cyclone occurred.  In 1953, the US weather service offi cially adopted the idea and created a new phonetic alphabet (international) of women’s names from A to W, leaving out Q, U, X, Y and Z.  The year’s fi rst tropical storm was given the name beginning with the letter “A”, the second with the letter “B” and so on through the alphabet.  In even-numbered years, odd-numbered storms got men’s names and in odd-numbered years, odd- numbered storms got women’s names.  The process of naming cyclones involves several countries in the region and is done under the aegis of the World Meteorological Organization.

 Naming of cyclones in Indian Ocean  For the Indian Ocean region, deliberations for naming cyclones began in 2000 and a formula was agreed upon in 2004.  Eight countries in the region - Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand - all contributed a set of names which are assigned sequentially whenever a cyclonic storm develops.  For example, the name Nilam was contributed by Pakistan, Murjan was named by Oman. Similarly, Phailin was named by Thailand.

118 Signifi cance:

 The naming of cyclones with local names are signifi cant as they bear the area of their origin with them.  It also helps in better understanding about cyclones and their study by metrologists.

 Titli cyclone : RIMES declared it rarest of rare

The Regional Integrated Multi-Hazard Early Warning System (RIMES) for Africa and Asia, a 45-nation international organisation on disaster warning, has termed ‘Titli’, the severe cyclonic storm that devastated in October,2018, as ‘rarest cyclone’.

About:

 How is Titli rarest of rare?

 Titli cyclone is the rarest of rare in terms of its characteristics such as recurvature after landfall and retaining its destructive potential after landfall and recurvature away from the coastal areas for more than two days.  Titli didn’t follow the pre-assumed track as other cyclones follow predicted paths.  Due to this factor it became more devastating than others and caused heavy landslides in Gajapati district of Odisha.  after landfall had changed the path in an erratic way.

Regional Integrated Multi-Hazard Early Warning System (RIMES)  The Regional Integrated Multi-Hazard Early Warning System for Africa and Asia (RIMES) is an international and intergovernmental institution, owned and managed by its Member States, for the generation and application of early warning information.  RIMES evolved from the eff orts of countries in Africa and Asia, in the aftermath of the 2004 Indian Ocean tsunami, to establish a regional early warning system within a multi-hazard framework for the generation and communication of early warning information, and capacity building for preparedness and response to trans-boundary hazards.  RIMES operates from its regional early warning center located at the campus of the Asian Institute of Technology in Pathumthani, Thailand.

Signifi cance:

 Study of unique cyclones like ‘Titli’ will help to reduce the eff orts of disaster mitigation and lessen the loss of life and property.  As these cyclones have unpredictable path after landfall, whole area must be alerted in this type of situation.  58 per cent of the storms that form in the Bay of Bengal (BOB) hit the eastern coast of India. Hence, a high level monitoring is needed to mitigate risks related to these unique type of cyclones with the help of international agencies like RIMES.

119  Cyclone Pabuk

A yellow alert was initiated by Indian Metrological Department (IMD) when Cyclone Pabuk originated at Gulf of Thailand & Neighbourbood

About:  Cyclone Pabuk originated over Gulf of Thailand in the western part of South China Sea.  The storm made landfall in Thailand. Rain, winds and stormy waves hit villages and popular tourist resorts on Thailand’s southeastern coast.  Further its eff ect were felt at Andaman and Nicobar Islands which witnessed heavy rainfall.  Cyclone Alert System of IMD (Indian Metrological Department)  Pre Cyclone Watch: The cyclone warnings are issued to state government offi cials in four stages. The First Stage warning known as “PRE CYCLONE WATCH” issued 72 hours in advance contains early warning about the development of a cyclonic disturbance in the north Indian Ocean, its likely intensifi cation into a tropical cyclone and the coastal belt likely to experience adverse weather.  Cyclone Alert: The Second Stage warning known as “CYCLONE ALERT” is issued at least 48 hrs. in advance of the expected commencement of adverse weather over the coastal areas. It contains information on the location and intensity of the storm likely direction of its movement, intensifi cation, coastal districts likely to experience adverse weather and advice to fi shermen, general public, media and disaster managers.  Cyclone Warning: The Third Stage warning known as “CYCLONE WARNING” issued at least 24 hours in advance of the expected commencement of adverse weather over the coastal areas. Landfall point is forecast at this stage. These warnings are issued by ACWCs/CWCs/and CWD at HQ at 3 hourly interval giving the latest position of cyclone and its intensity, likely point and time of landfall, associated heavy rainfall, strong wind and storm surge along with their impact and advice to general public, media, fi shermen and disaster managers.  Post Landfall Outlook :The Fourth Stage of warning known as “POST LANDFALL OUTLOOK” is issued by the concerned ACWCs/CWCs/and CWD at HQ at least 12 hours in advance of expected time of landfall. It gives likely direction of movement of the cyclone after its landfall and adverse weather likely to be experienced in the interior areas.

Colour Coding of alerts:

Cyclone Alert Yellow

Cyclone Warning Orange

Post landfall outlook Red

 Cyclone Fani which has been classifi ed as on extranely severe cyclone hit the Odisha coast.  India Meteorological Department (IMD) issued yellow warning for Fani, which shows severely bad weather in the region, and warned people who are at risk to take preventive action.

120  With sustained winds of 240 kmph, the storm was the equivalent of a Category 4 hurricane on the Saffi r- Simpson Hurricane Wind Scale.

Saffi r-Simpson Hurricane Wind Scale  It gives ratings to hurricane from 1 to 5 based on its sustained wind speed.  This scale estimates potential property damage.

About Fani  Fani is not just a severe cyclone but an “extremely severe cyclone”. Tropical cyclones in the Bay of Bengal are graded according to maximum wind speeds at their centre. At the lower end are depressions that generate wind speeds of 30 to 60 km per hour, followed by cyclonic storms (61 to 88 kph), severe cyclonic storms (89 to 117 kph) and very severe cyclonic storms (118 to 166 kph). At the top are extremely severe cyclonic storms (167 to 221 kph) and super cyclones (222 kph or higher).  Fani is, thus, unusual, and that is mainly because of the place it originated, very close to the Equator, and the long route it has taken to reach the landmass.

How the cyclones formed at sea?  Cyclones are formed over slightly warm ocean waters. The temperature of the top layer of the sea, up to a depth of about 60 metres, need to be at least 28°C to support the formation of a cyclone. This explains why the April-May and October-December periods are conducive for cyclones. Then, the low level of air above the waters needs to have an ‘anti-clockwise’ rotation (in the northern hemisphere; clockwise in the southern hemisphere). During these periods, there is a zone in the Bay of Bengal region (called the inter- tropical convergence zone that shifts with seasons) whose southern boundary experiences winds from west to east, while the northern boundary has winds fl owing east to west. This induces the anti-clockwise rotation of air.  Once formed, cyclones in this area usually move northwest. As it travels over the sea, the cyclone gathers more moist air from the warm sea, and adds to its heft.  A thumb rule for cyclones (or hurricanes and typhoons as they are called in the US and Japan) is that the more time they spend over the seas, the stronger they become. Hurricanes around the US, which originate in the vast open Pacifi c Ocean, are usually much more stronger than the tropical cyclones in the Bay of Bengal, a relatively narrow and enclosed region. The cyclones originating here, after hitting the landmass, decay rapidly due to friction and absence of moisture.

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