CHAPTER 3.3 ATMOSPHERIC CIRCULATION & WEATHER SYSTEMS Atmospheric Pressure 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-Polar Low 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. Polar High 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 cyclones 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 horse latitudes 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.