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Class 11 Geography NCERT Solutions Chapter 10 Atmospheric Circulation and Weather Systems

Class 11 Geography NCERT Solutions Chapter 10

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

Explanation: The standard atmospheric pressure lapse rate is around 100 mb per 1 km increase in altitude. Therefore, at 1 km above the surface, the air pressure would be around 900 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

Explanation: The Inter Tropical Convergence Zone (ITCZ) is a region near the equator where the trade winds from both hemispheres meet. It is characterized by a band of clouds, showers, and thunderstorms. The ITCZ shifts north and south with the changing position of the Sun during the year, leading to the wet and dry seasons in tropical regions.

(iii) The direction of wind around a low pressure in the northern hemisphere is:
(a) clockwise
(b) perpendicular to isobars
(c) anti-clockwise
(d) parallel to isobars

Explanation: In the northern hemisphere, wind flows around a low-pressure area in an anti-clockwise direction. This phenomenon is due to the Coriolis effect, which results from the Earth’s rotation and causes moving air to be deflected to the right in the northern hemisphere. As a result, air converges towards a low-pressure center and spirals in an anti-clockwise manner.

(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

Explanation: The Siberian Plain is a vast continental region with cold and dry conditions, making it an important source region for the formation of polar air masses. Air masses acquire their characteristics (temperature and moisture) from the surface over which they form. The Siberian Plain, with its cold temperatures and limited moisture, is an ideal source region for polar air masses that influence weather patterns in various parts of the world.

(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?

Answer: The unit used in measuring pressure is the millibar (mb). Pressure is measured at the station level and reduced to sea level to account for altitude differences. This is done to standardize pressure readings and facilitate accurate comparisons on weather maps. The reduction corrects for the influence of altitude, ensuring that pressure variations are attributed to weather systems rather than elevation.

(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.

Answer: The Coriolis effect deflects winds in the northern hemisphere to the right. The pressure gradient force, combined with the Coriolis effect, results in the northeast trade winds between the subtropical high pressure and the equator. These north easterlies are established due to the balance between the pressure gradient force and the Coriolis effect.

(iii) What are the geotrophic winds?

Answer: Geotrophic winds are horizontal winds that result from the balance between the pressure gradient force and the Coriolis effect. They flow parallel to the isobars (lines of equal pressure) at a certain height above the Earth’s surface. Geotrophic winds are prevalent at higher altitudes and play a crucial role in shaping large-scale wind patterns.

(iv) Explain the land and sea breezes.

Answer: Land and sea breezes are local wind patterns that occur due to differential heating and cooling of land and water. During the day, land heats up faster than the sea, causing air to rise over the warmer land. Cooler air from the sea then flows inland to replace the rising air, creating a sea breeze. At night, land cools faster, and the process reverses, resulting in a land breeze. These breezes provide localized shifts in wind direction and temperature near coastal areas.

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

Answer: The speed and direction of wind are influenced by various factors:

1. Pressure Gradient Force: Wind flows from high-pressure to low-pressure areas due to differences in atmospheric pressure. The steeper the pressure gradient, the faster the wind.
2. Coriolis Effect: Earth’s rotation deflects winds to the right in the northern hemisphere and to the left in the southern hemisphere. This effect determines wind direction.
3. Friction: Surface friction slows down wind near the Earth’s surface, affecting its speed and direction. Friction is more pronounced over land than over oceans.
4. Centripetal Force: Near curved isobars (lines of equal pressure), an inward force results in wind spiraling towards low-pressure centers.
5. Gravity: Gravity influences vertical motion of air and can impact wind direction, especially in mountainous regions.
6. Temperature Gradient: Air moves from colder to warmer regions, creating local wind patterns like land and sea breezes.
7. Land-Water Contrasts: Differential heating of land and water generates breezes. During the day, cooler air from water moves inland (sea breeze), while at night, land breezes blow seaward.
8. Mountainous Terrain: Mountains can channel and divert wind flow, creating unique wind patterns such as valley breezes.

(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?

The subtropical high-pressure belts at around 30° N and S latitudes form due to several factors:

1. Hadley Cell Circulation: Warm air near the equator rises, cools, and diverges poleward in the upper atmosphere. As it subsides around 30° N and S, it creates areas of high pressure.
2. Coriolis Effect: The Coriolis effect deflects air masses descending from higher latitudes towards the subtropical belts, leading to accumulation and high pressure.
3. Trade Winds: The converging trade winds from both hemispheres meet near the subtropics, causing air to sink and pressure to rise.

(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?

Answer: Tropical cyclones form over warm ocean waters where sea surface temperatures are above 26.5°C. Warm, moist air rises from the ocean surface, creating low pressure. The Coriolis effect imparts rotation to the developing system.

Torrential rains and high-velocity winds are concentrated in the eyewall of the tropical cyclone. The eyewall is the region surrounding the calm eye, where the strongest updrafts occur. Moist air rapidly ascends, cools, and condenses, releasing latent heat and intensifying the storm. The descending air in the eye, due to subsidence, creates calm conditions. The eyewall’s intense convection and updrafts result in the heaviest rainfall and strongest winds, making it the most destructive part of the cyclone.

Thanks for reading article on Class 11 Geography NCERT Solutions Chapter 10.