Atmospheric Circulation Weather Systems – Chapter 10 Class 11 NCERT Geography

1. Multiple choice questions.

(i) If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be:

(a) 700 mb (b) 1,100 mb (c) 900 mb (d) 1,300 mb

(ii) The Inter Tropical Convergence Zone normally occurs:

(a) near the Equator  (b) near the Tropic of Cancer (c) near the Tropic of Capricorn (d) near the Arctic Circle

(iii) The direction of wind around a low pressure in northern hemisphere is:

(a) clockwise (b) perpendicular to isobars (c) anti-clock wise (d) parallel to isobars

(iv) Which one of the following is the source region for the formation of air masses?

(a) the Equatorial forest (b) the Himalayas  (c) the Siberian Plain  (d) the Deccan Plateau

Related: Subtropical High, Subpolar Low, Easterlies, Westerlies | Pressure Belts of World

2. Answer the following questions in about 30 words.

(i) What is the unit used in measuring pressure? Why is the pressure measured at station level reduced to the sea level in preparation of weather maps?

Ans. Pressure is expressed in units of millibar (mb) and Pascals. In order to eliminate the effect of altitude while comparing different isobars, pressure measured at station level is reduced to sea level in preparation of weather maps.

(ii) While the pressure gradient force is from north to south, i.e. from the subtropical high pressure to the equator in the northern hemisphere, why are the winds north easterlies in the tropics.

Ans. Winds always move from High Pressure to Low Pressure zone, that’s why pressure gradient force is from north to south. And the north easterlies winds move towards south western region.

(iii) What are the geostrophic winds?

Ans. Winds in the upper atmosphere i.e. 2 – 3 km above the surface are free from earth’s frictional effect. They are totally controlled by pressure gradient and Coriolis force. These winds which are  straight and parallel to the isobar are known as geostrophic wind.

(iv) Explain the land and sea breezes.

Ans. Land and sea has different heating patterns. During the day the land heats up faster and becomes warmer than the sea. Because of the warm temperature over the land, the air rises giving rise to a low pressure area. On the other hand, the sea is relatively cool and the pressure over sea is relatively high. Thus, pressure gradient from sea to land is created and the wind blows from the sea to the land as sea breeze. In the night the reverse happens. The land loses heat faster and is cooler than the sea. The pressure gradient at nigh is from the land to the sea and the wind blows from the land to the sea as land breeze.

3. Answer the following questions in about 150 words.

(i) Discuss the factors affecting the speed and direction of wind.

Ans. Factors affecting the speed and direction of wind are:-

1. Pressure Gradient Force – The differences in atmospheric pressure produces a force. The rate of change of pressure with respect to distance is the pressure gradient. The pressure gradient is strong where the isobars are close to each other and is weak where the isobars are apart.
2. Frictional Force – 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. Over the sea surface the friction is minimal.
3. Coriolis Force – The rotation of the earth in its axis affects the direction of the wind. This force is called the Coriolis force. It deflects the wind to the right direction in the northern hemisphere and to the left in the southern hemisphere. The deflection is more when the wind velocity is high. Coriolis forces is maximum at the poles and is zero or absent at the equator.

(ii) Draw a simplified diagram to show the general circulation of the atmosphere over the globe. What are the possible reasons for the formation of subtropical high pressure over 30o N and S latitudes?

Ans. Along 30° N and 30° S exists high-pressure areas known as the subtropical highs. This pressure belts is not permanent in nature, it oscillate with the movement of the sun. Equator is the region of high insulation it receives ample of sun rays all throughout the year. As a result, equator is a low pressure zone. Wind in this region gets warm, and warm air rises. It goes up to a height of 14 km from the surface of the earth. The warm air moves towards 30° N and 30° S and sinks to the ground and forms subtropical high.

Possible reasons for the formation of subtropical high pressure are:-

• latitudinal variation of atmospheric heating (from equator to poles)
• emergence of pressure belts (HP and LP system)
• the migration of belts following the path of the sun (warm temperature cause air to rise)
• the distribution of continents and oceans (land heats faster than water)
• the rotation of earth (Coriolis force)

(iii) Why does tropical cyclone originate over the seas? In which part of the tropical cyclone do torrential rains and high velocity winds blow and why?

Ans. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately re-condenses into clouds and rain when moist air rises and cools to saturation.

Tropical cyclones forming between 5 and 30 degrees North latitude typically move toward the west. Sometimes the winds in the middle and upper levels of the atmosphere change and steer the cyclone toward the north and northwest. When tropical cyclones reach latitudes near 30 degrees North, they often move northeast.

The cyclones, which cross 20 degrees North latitude generally are more destructive. Torrential rains in high velocity winds are found in area around the eye of the cyclone. Wind velocity often reaches 250 km per hour. The diameter of the storm over the Bay of Bengal, Arabian sea and Indian ocean is between 600 – 1200 km. The cyclone creates storm surges and they inundate the coastal lowlands.