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NCERT Solutions Class 11 Geography Chapter 9 Atmospheric Circulation and Weather Systems: You may get the right answers to the practice problems from the textbook in the NCERT Solutions for Cass 11 Geography Chapter 9.
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NCERT Solutions Class-11 Geography Chapter-9: Atmospheric Circulation and Weather Systems
Exercises Page No: 84
Que 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
Answer:
(c) 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
Answer:
(a) near the Equator
(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
Answer:
(c) anti-clock wise
(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
Answer:
(c) the Siberian Plain
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?
Answer:
The unit used in measuring pressure is millibar or Pascal. Pressure measured at station level is reduced to sea level on weather maps to account for differences in altitude and provide consistent comparisons.
(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 force and the pressure gradient force work together in the atmosphere. The pressure gradient force acts across isobars, pushing air from areas of high pressure to low pressure. Meanwhile, the Coriolis force acts perpendicular to the pressure gradient force, deflecting wind to the right in the Northern Hemisphere. When these forces interact, winds blow around low-pressure areas. Since the pressure gradient force typically points from north to south, winds move from northeast to southwest, creating the northeast trade winds.
(iii) What are the geotrophic winds?
Answer:
When isobars, which are lines connecting points of equal atmospheric pressure, are straight and there is minimal friction, the pressure gradient force is counteracted by the Coriolis force. As a result, the resultant wind blows parallel to the isobars. This type of wind is called the geostrophic wind.
(iv) Explain the land and sea breezes.
Answer:
During the day, the land heats up more quickly than the sea, causing the air over the land to rise and creating a low-pressure area. Meanwhile, the cooler sea maintains higher pressure. This pressure difference results in a gradient from sea to land, leading to the formation of a sea breeze, with wind blowing from the sea to the land.
At night, the conditions reverse. The land loses heat rapidly and becomes cooler than the sea, causing higher pressure over the land and lower pressure over the sea. This sets up a pressure gradient from land to sea, resulting in the formation of a land breeze, with wind blowing from the land to the sea.
3. Answer the following questions in about 150 words.
(i) Discuss the factors affecting the speed and direction of wind.
Answer:
The speed and direction of wind are influenced by three main forces:
Pressure Gradient Force: This force arises from differences in atmospheric pressure, creating a push or pull on the air. It is stronger when there’s a steep change in pressure over a short distance (high pressure gradient) and weaker when pressure changes more gradually (low pressure gradient).
Frictional Force: Friction between the air and Earth’s surface slows down the wind, particularly near the ground. This effect diminishes with height, and over the sea, where surfaces are smoother, friction is minimal.
Coriolis Force: This force results from the Earth’s rotation and causes the deflection of moving objects, including air masses. In the Northern Hemisphere, the Coriolis force deflects winds to the right, while in the Southern Hemisphere, it deflects them to the left. This phenomenon was first described by the French physicist Gaspard-Gustave de Coriolis in 1844.
(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?
Answer:
The general circulation of the atmosphere influences ocean water circulation, impacting Earth’s climate. At the Inter-Tropical Convergence Zone (ITCZ), intense sunlight heats the air, causing it to rise due to convection and creating a low-pressure area. Winds from the tropics converge at the ITCZ and rise with the convective cell, reaching the top of the troposphere at around 14 km altitude before moving towards the poles.
This movement results in air accumulation around 30°N and S latitudes. Some of this accumulated air sinks back down to the surface, forming subtropical highs. The sinking is also prompted by the cooling of air as it reaches these latitudes. This circulation pattern plays a important role in shaping global climate patterns.
(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, also known as hurricanes or typhoons, originate and strengthen over warm tropical oceans. Favorable conditions for their formation and intensification include:
(i) Warm Sea Surface: Sea surface temperatures above 27°C provide the energy needed for tropical cyclones to develop and strengthen.
(ii) Coriolis Force: This force, caused by the Earth’s rotation, is essential for the cyclonic rotation of tropical storms.
(iii) Stable Vertical Wind: Minimal variation in vertical wind speed allows the storm to maintain its structure and intensify.
(iv) Weak Low-Pressure Area: A pre-existing area of low pressure or cyclonic circulation provides the initial conditions necessary for tropical cyclone development.
(v) Upper-Level Divergence: Upper-level divergence supports the upward movement of air, aiding in the storm’s intensification.
The most intense rainfall and strongest winds occur in the “eye-wall” region surrounding the calm “eye” of the storm. The eye is characterized by sinking air, while the eye-wall features strong, spiraling ascent of air reaching high into the troposphere, with wind speeds reaching up to 250 km per hour.
NCERT Solutions Class 11 Geography Chapter 9: Atmospheric Circulation and Weather Systems Summary
Here is the summary of Chapter 9 Atmospheric Circulation and Weather Systems
- Atmospheric pressure: It is the weight of air in a certain area from sea level to the top of the atmosphere, measured in millibars.
- Wind: Air in motion is called wind.
- Pressure gradient force: Differences in air pressure create a force. It’s the rate of pressure change over distance.
- Frictional force: It slows down wind speed, especially near the Earth’s surface.
- Coriolis force: Earth’s rotation affects wind direction, deflecting it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
- Geostrophic wind: When pressure and Coriolis forces balance out, wind flows parallel to isobars.
- General circulation of the atmosphere: It’s the global pattern of movement of winds.
- Cell: Circulations of air in the atmosphere, such as the Hadley, Ferrel, and Polar cells.
- Valley breeze: During the day, warm air rises up mountain slopes, pulling in cool air from the valley below.
- El Nino: A climate event where warm water from the central Pacific moves towards South America, altering weather patterns.
- Southern oscillation: Fluctuations in air pressure over the Pacific Ocean.
- Katabatic wind: Cold, dense air flows downhill at night from high elevations.
- Air mass: A large body of air with consistent temperature and humidity characteristics.
- Source regions: Areas where air masses form.
- Fronts: Boundaries between different air masses, like cold fronts, warm fronts, and occluded fronts.
- Extratropical cyclone: Cyclones formed outside the tropics, often in middle latitudes.
- Tropical cyclones: Violent storms originating over warm tropical oceans.
- Tornado: Intense, spiraling winds that descend from severe thunderstorms.
- Water spouts: Tornadoes over water bodies are called water spouts.
Related Links
NCERT Solutions Class 11 Geography Chapter 9 Atmospheric Circulation and Weather Systems FAQs
What is atmospheric pressure?
Atmospheric pressure is the weight of air in a certain area from sea level to the top of the atmosphere, measured in millibars.
How does the Coriolis force affect wind direction?
The Coriolis force, caused by Earth’s rotation, deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
What are the different types of air masses?
Air masses are large bodies of air with consistent temperature and humidity characteristics. They form over source regions and can be categorized based on their temperature and humidity properties.
What is a front?
A front is a boundary between two air masses with different temperature and humidity characteristics. There are different types of fronts, such as cold fronts, warm fronts, and occluded fronts.
How do extratropical cyclones differ from tropical cyclones?
Extratropical cyclones develop outside the tropics, often in middle latitudes, while tropical cyclones originate over warm tropical oceans. They have different characteristics and patterns of movement.
What causes tornadoes?
Tornadoes are intense, spiraling winds that descend from severe thunderstorms. They can cause significant damage and are often associated with severe weather events.
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