Class 11 Geography Chapter 9 Atmospheric Circulation and Weather NCERT Exercise Solution (English Medium)
NCERT Exercises
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
Ans. (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
Ans. (a) Near the Equator
(iii) The direction of wind around a
low pressure in northern hemisphere is:
(a) Clockwise (c) Anti-clock wise
(b) Perpendicular to isobars (d) Parallel to isobars
Ans. (c) Anti-clock wise
(iv) Which one of the following is the source region for the formation of air masses?
(a) The Equatorial forest (c) The
Siberian Plain
(b) The Himalayas (d) The Deccan Plateau
Ans. (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?
Ans. The weight of a column of air contained in a unit
area from the mean sea level to the top of the atmosphere is called the atmospheric
pressure. The atmospheric pressure is expressed in units of milibar (mb).
At sea level the average atmospheric pressure is 1,013.2 milibar. Air pressure
is measured with the help of a mercury barometer or the aneroid
barometer.
Small differences in pressure are highly significant in terms of the wind direction and velocity. Horizontal distribution of pressure is studied by drawing isobars at constant levels. Isobars are lines connecting places having equal pressure. In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level for purposes of comparison.
(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. Wind moves from high pressure areas to low pressure areas. Small differences in pressure are highly significant in terms of the wind direction and velocity. Near the equator the sea level pressure remains low and this area is known as equatorial low. The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created. Along 30° N and 30° S are found the high-pressure areas known as the subtropical highs. The winds from the tropics converge at this low pressure zone and flows towards the equator as the easterlies.
(iii) What are the geotrophic winds?
Ans. The velocity and direction of the wind are the net
result of the wind generating forces. The winds in the upper atmosphere, 2 - 3
km above the surface, are free from frictional effect of the surface and are
controlled mainly by the pressure gradient and the Coriolis force.
When isobars are straight and when there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar. This wind is known as the geostrophic wind.
(iv) Explain the land and sea
breezes.
Ans. The land and sea absorb and transfer heat
differently. During the day the land heats up faster and becomes warmer than
the sea. Therefore, over the land the rising air create a low pressure area,
whereas 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 the sea breeze.
In the night the reversal of condition
takes place. The land loses heat faster and is cooler than the sea. The
pressure gradient is from the land to the sea and the wind blows from the
land to the sea as the land breeze.
3. Answer the following questions in
about 150 words.
(i) Discuss the factors affecting the
speed and direction of wind.
Ans. You already know that 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
experiences 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 – the pressure gradient force, the
frictional force and the Coriolis force. In addition, the gravitational force
acts downward.
Major factors OR Forces Affecting the Velocity and Direction of Wind:
(a) 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.
(b) 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.
(c) Coriolis Force - The rotation of the 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 direction in
the northern hemisphere and to the left in the southern hemisphere. The
deflection is more when the wind velocity is high. The Coriolis force is
directly proportional to the angle of latitude. It is maximum at the poles and
is absent at the equator.
The Coriolis force acts perpendicular to the pressure gradient force. The pressure gradient force is perpendicular to an isobar. The higher the pressure gradient force, the more is the velocity of the wind and the larger is the deflection in the direction of wind. As a result of these two forces operating perpendicular to each other, in the low-pressure areas the wind blows around it. At the equator, the Coriolis force is zero and the wind blows perpendicular to the isobars. The low pressure gets filled instead of getting intensified. That is the reason why tropical cyclones are not formed near 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 30° N and
S latitudes?
Ans. The air at the Inter Tropical Convergence Zone (ITCZ)
rises because of convection caused by high insolation and a low pressure is
created. The winds from the tropics converge at this low pressure zone. The
converged air rises along with the convective cell. It reaches the top of the
troposphere up to an altitude of 14 km and moves towards the poles.
This causes accumulation of air at about
30° N and S. Part of the accumulated air sinks to the ground and forms a subtropical
high. Another reason for sinking is the cooling of air when it reaches 30° N
and S latitudes. Down below near the land surface the air flows towards the
equator as the easterlies. The easterlies from either side of the
equator converge in the Inter Tropical Convergence Zone (ITCZ).
Such circulations from the surface
upwards and vice-versa are called cells. Such a cell in the tropics is
called Hadley Cell. In the middle latitudes the circulation is that of
sinking cold air that comes from the poles and the rising warm air that blows
from the subtropical high. At the surface these winds are called westerlies
and the cell is known as the Ferrel cell.
At polar latitudes the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the polar cell. These three cells set the pattern for the general circulation of the atmosphere. The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.
(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 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. 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.
The energy that intensifies the storm
comes from the condensation process in the towering cumulonimbus clouds,
surrounding the centre of the storm. With continuous supply of moisture from
the sea, the storm is further strengthened. On reaching the land the
moisture supply is cut off and the storm dissipates. The place where a
tropical cyclone crosses the coast is called the landfall of the cyclone. The
cyclones, which cross 20° N latitude generally, recurve and they are more
destructive.
A mature tropical cyclone is
characterised by the strong spirally circulating wind around the centre, called
the eye. The eye is a region of calm with subsiding air. Around
the eye is the eye wall, where there is a strong spiralling ascent of air to
greater height reaching the tropopause. The wind reaches maximum velocity in
this region, reaching as high as 250 km per hour. Torrential rain occurs
here. From the eye wall rain bands may radiate and trains of cumulus and
cumulonimbus clouds may drift into the outer region. The diameter of the tropical
cyclone can vary between 150 and 250 km. The diameter of the storm over the Bay
of Bengal, Arabian Sea and Indian Ocean remains 600 to 1200 km. The system
moves slowly about 300 - 500 km per day.
