Atmospheric Circulation and Weather SystemsClass 11 Fundamentals of Physical Geography NCERT Solutions
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Q1EXERCISES
Multiple choice questions. (i) If the surface air pressure is , the air pressure at 1 km above the surface will be:
(a)
700 mb
(b)
(c)
900 mb
(d)
Solution
(c) 900 mb
As stated in the chapter, in the lower atmosphere, the atmospheric pressure decreases with height at a rate of about 1 mb for each 10 m increase in elevation.
Therefore, for an increase of 1 km (which is equal to 1,000 m), the pressure decrease would be:
(1,000 m / 10 m) * 1 mb = 100 mb.
If the surface pressure is 1,000 mb, the pressure at 1 km above the surface will be:
1,000 mb - 100 mb = 900 mb.
Q1EXERCISES
(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
Solution
(c) the Siberian Plain
An air mass forms when air remains over a homogenous area for a long time, acquiring the characteristics of that surface. The chapter lists "The very cold snow covered continents in high latitudes" as a major source region. The Siberian Plain is a vast, homogenous, snow-covered plain in the high latitudes, making it an ideal source region for the formation of a Continental Polar (cP) air mass.
Q1EXERCISES
(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
Solution
(a) near the Equator
The chapter mentions that near the equator, the sea level pressure is low, and this area is known as the equatorial low. The air at the Inter Tropical Convergence Zone (ITCZ) rises due to high solar insolation, creating this low-pressure zone. The winds from the tropics converge in this zone, which is located near the equator.
Q1EXERCISES
(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
Solution
(c) anti-clock wise
According to the chapter, the circulation of wind around a low-pressure system is called cyclonic circulation. Due to the effect of the Coriolis force, which deflects winds to the right in the Northern Hemisphere, the wind circulates in an anti-clockwise direction around a low-pressure center.
Q2EXERCISES
(iii) What are the geotrophic winds?
Solution
Geostrophic winds are winds in the upper atmosphere, typically 2-3 km above the surface, where the effect of friction is negligible. They occur when the pressure gradient force is balanced by the Coriolis force. As a result, the geostrophic wind blows parallel to the isobars.
Q2EXERCISES
(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.
Solution
Winds do not blow directly from high to low pressure due to the Earth's rotation. The Coriolis force deflects moving air to the right in the Northern Hemisphere. Therefore, the wind moving south from the subtropical high towards the equatorial low is deflected to its right, turning it into a northeasterly wind.
Q2EXERCISES
(iv) Explain the land and sea breezes.
Solution
Land and sea breezes are local winds caused by the differential heating of land and sea.
- Sea Breeze: During the day, land heats up faster than the sea, creating a low-pressure area over the land. The cooler, denser air from the high-pressure area over the sea blows towards the land. This is called a sea breeze.
- Land Breeze: At night, the land cools faster than the sea, creating a high-pressure area over the land. The air then blows from the land towards the warmer, low-pressure sea. This is called a land breeze.
Q2EXERCISES
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?
Solution
The unit used for measuring atmospheric pressure is the milibar (mb).
Pressure measured at a station is reduced to sea level for weather maps to eliminate the effect of altitude on pressure readings. This allows for an accurate comparison of pressure between different locations at a standard level, making it possible to identify high and low-pressure systems correctly.
Q3EXERCISES
Answer the following questions in about 150 words. (i) Discuss the factors affecting the speed and direction of wind.
Solution
The speed and direction of wind near the Earth's surface are controlled by the combined effect of three primary forces: the Pressure Gradient Force, the Frictional Force, and the Coriolis Force.
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Pressure Gradient Force: This is the initial force that sets air in motion. It is produced by differences in atmospheric pressure between two locations. Air moves from high-pressure areas to low-pressure areas. The speed of the wind is determined by the pressure gradient; when isobars are close together, the gradient is steep, and the wind is strong. When isobars are far apart, the wind is weak.
-
Frictional Force: This force opposes the motion of the wind and affects its speed. It is greatest at the Earth's surface due to terrain features like mountains, forests, and buildings, and its influence extends up to about 1-3 km. Over smooth surfaces like the sea, friction is minimal. This force slows down the wind.
-
Coriolis Force: This is an apparent force caused by the Earth's rotation. It does not affect wind speed but significantly alters its direction. It deflects the wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is greatest at the poles and absent at the equator. The strength of the Coriolis force increases with wind velocity.
Q3EXERCISES
(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 and S latitudes?
Solution
The general circulation of the atmosphere involves three main cells in each hemisphere: the Hadley Cell, the Ferrel Cell, and the Polar Cell. (A diagram would show these three cells, with rising air at the equator and 60° latitudes, and sinking air at 30° latitudes and the poles, along with surface winds like the trade winds, westerlies, and polar easterlies.)
The formation of subtropical high-pressure belts over 30° N and S latitudes is primarily due to the dynamics of the Hadley Cell. The reasons are as follows:
-
Sinking of Air: At the Inter Tropical Convergence Zone (ITCZ) near the equator, intense heating causes moist air to rise to the top of the troposphere. This air then moves poleward. As it travels, it cools and becomes denser.
-
Accumulation of Air: Around 30° N and S latitudes, this poleward-moving air begins to pile up or accumulate. The Coriolis force also contributes to this convergence of air aloft.
-
Subsidence: The combination of cooling and accumulation forces the air to sink towards the surface. This process of large-scale sinking of air is called subsidence. Sinking air gets compressed and warmed, leading to dry conditions and the formation of a persistent high-pressure zone at the surface. This is why most of the world's major hot deserts are located in these subtropical latitudes.
Q3EXERCISES
(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?
Solution
Tropical cyclones originate over warm tropical oceans because they require a continuous supply of heat and moisture, which only large bodies of warm water can provide. The key conditions for their formation are:
- A large sea surface with a temperature higher than 27°C.
- The presence of the Coriolis force to initiate rotation.
- A pre-existing weak low-pressure area.
The energy that powers and intensifies the storm is the latent heat of condensation, which is released when the abundant water vapor from the warm ocean surface rises, cools, and condenses into clouds. Once a cyclone moves over land, its moisture supply is cut off, and it rapidly dissipates.
The most destructive part of a tropical cyclone, with the most torrential rains and highest velocity winds, is the eye wall. This is the dense ring of towering cumulonimbus clouds immediately surrounding the calm, central eye of the storm. The reason for this intensity is that the eye wall is where the most rapid and forceful upward movement of moist air occurs. This strong updraft leads to massive condensation, releasing enormous amounts of latent heat energy, which in turn fuels the storm and drives the spiraling winds to their maximum speed, often exceeding 250 km per hour.