NCERT Solutions for Class 11 Geography Chapter 2 - Atmospheric Circulation and Weather Systems

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(c). 900 mb

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(a). near the Equator

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(c). anti-clockwise

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(c). the Siberian Plain

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Atmospheric pressure is measured in terms of the height of a column of mercury in the Barometer.

Three systems of units are used–

1. Inches
2. Centimetres
3. Millibars

The normal pressure is about 29.92 inches or 76 cms or 760 m.m. The common unit of measuring pressure is millibar. One millibar is a force equal to 1000 dynes per sq. cm. Normal pressure is equal to 1013 mb.
$\therefore$ 30 inches = 76 cms = 1013mb.
1 inch = 34 mb. nearly.
1 cm = 13 mb. nearly.
The pressure decreases at the rate of 12 mb for every 300 metres. So to eliminate the effect of height, it is reduced to sea level.

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Change in direction. Winds do not blow in a North-South direction. Earth’ rotation, and the coriolis Force deflect the direction of winds. This is known as Ferral’s law of deflection. According to this, winds are deflected to their right in the northern hemisphere and to their left in the southern hemisphere.

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The speed and direction of the wind are the net result of the wind generating forces. The winds in the upper atmosphere, 2 to 3 kilometre above the surface, are free from frictional effect of the surface and are controlled by pressure gradient and the Coriolis force. When isobars are straight, as there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar. These winds are known as the geotropic winds.

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Land and sea breezes.

1. Sea Breeze. During the day time, the land gets rapidly heated up than the sea. The warm air rises forming a low pressure locally on land. The sea being cooler develops a high pressure. So a cool sea breeze blows from sea to land. Sea breeze moderates the temperature of the coastal areas resulting in an equable or maritime climate.
2. Land Breeze. During the night the land becomes cooler than the sea. The cold and heavy air produces a local high pressure on land. The sea remains comparatively warmer with a low pressure. Therefore, a land breeze blows from land to sea.

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Forces affecting the speed and direction of the wind. The air is set in motion due to the differences in atmospheric pressure. The air in motion is called wind. The wind blows from high pressure to low pressure. The wind at the surface expriences friction. In addition rotation of the earth also affects 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 effect of three forces :
(a) the pressure gradient force,
(b) the frictional force and
(c) Coriolis force. In addition, gravitational force is present, which acts downward.
(1) Pressure gradient force. The differences in atmospheric pressure produce a force. The rate of change of pressure with respect to distance is the pressure gradient. The presure gradient is strong where the isobars are close to each other and is weak where the isobars are apart. This is the driving force and wind moves from high pressure to low pressure.
(2) Frictional force. The frictional force is presnet near the Earth’s surface. It affects the speed of the wind. It is greatest at the surface and its influence generally extends up to an elevation of 1 km. On clear day when the air is turbulent, the frictional effect may extend even up to 3 km. The friction may have effect on the direction of surface winds. Over the sea surface the friction is minimal.
(3) Coriolis force. The rotation of Earth about its axis affects the direction of the wind. This force is called the Coriolis force after the French physicist who described it in 1844. It deflects the wind to the right in the northern hemisphere and to the left in the southern hemisphere. The deflection is more when the speed of the wind is high. The coriolis force is proportional to the angle of latitude. It is maximum at the poles and is absent at the equator.

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Distribution of Pressure Belts. The distribution of pressure on the earth is not simple and continuous. Its pattern is interrupted due to many disturbances. Even then, in some parts there are pressure systems which persist for most of the year. A zonal pattern of pressure belts is found. “A pressure belt is a zone of uniform width, around the earth extending in an East-West direction parallel to the lines of latitude.” On the earth’s surface, there are in all seven pressure belts. Four belts of high pressure separate three low pressure belts.
1. The equatorial low pressure belt. The equatorial belt of low pressure extends from the equator to 5°N and 5°S latitudes. There is intense heating due to constant high temperatures. The air expands and rises upwards as convection currents. It is a region of calms called the ‘Doldrums’.
2. Sub-tropical high pressure belts. Two subtropical high pressure belts occur near the tropics and extend between 30°—35° North and South latitudes. The rising air from the equator moves towards the poles. It results in the piling up of the air in the sub-tropics. These winds are forced to descend near 30°—35° latitudes. Due to descending air currents, the pressure is high. It is a belt of calms and is frequently known as ‘Horse latitudes’.
3. Sub-polar low pressure belts. Two sub-polar low pressure belts are located between 60°— 65° North and South latitudes. These are also known as ‘Arctic low’and ‘Antarctic low’. On the basis of their location, these are also called ‘Temperate lows’. The origin of these lows is mostly dynamic.
Causes :
(a) As a rule, the pressure decreases from tropics towards poles, therefore, midway these, low pressure is found.
(b) The centrifugal force and the frictional drag resulting from rotation of the earth, displace air between 60^°— 65^° latitudes and move it towards poles. So low pressure is developed near the poles.

(c) It is a zone of convergence and cyclonic activity. These temperate lows are due to passage of cyclones or depressions.
(d) Due to warm ocean currents, and rarefied atmosphere, low pressure is found in these latitudes.
(e) Due to contrasts between land and water a series of low pressure cells are found in this region. 4. Polar high pressure belts. Two high pressure belts are found around North Pole and South Pole. This is a region of permanently low temperature. Due to cold dense air, high pressure surrounds the Arctic Ocean and the Antarctic ice-cap.

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Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourable for the formation and intensification of tropical storms are:
(i) Large sea surface with temperature higher than 27^°C;
(ii) Presence of the Coriolis force;
(iii) Small variations in the vertical wind speed;
(iv) A pre-existing weak low-pressure area or low-level-cyclonic circulation;
(v) Upper divergence above the sea level system.
Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges. This is one of the most devastating natural calamities. They are known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea and Willy-willies in the Western Australia.