NCERT Solutions for Class 11 Geography Chapter 9 - Atmospheric Circulations and Weather Systems

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    78. 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 0occurs:

    (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

    (b) perpendicular to isobars

    (c) anti-clock wise

    (d) parallel to isobars

    Ans. (c) anti clockwise

    (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

    Ans. (c) The Siberian plains.

    79. 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. Pascal or millibar is the unit used to measure pressure and is the most frequently used unit. The horizontal distribution of pressure is evaluated by drawing isobars at constant levels. Isobars are lines that connect areas with the same pressure. Pressure is recorded at any station after being lowered to sea level for comparison in order to minimise the impact of height on pressure. In order to create weather maps, the pressure measured at station level is lowered to sea level.

    (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. The rotation of the earth also affects wind movement. The force exerted by the rotation of the earth is known as the Coriolis force. Due to this effect, wind move to the right from their original direction in the northem hemisphere and to the left in the southern hemisphere. The deflection is more the wind velocity is high. The Coriolis force is directly is directly proportional to the angle of latitude. It is maximum at the poles and is absent at the equator. The Coriolis 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 the wind. As a result of these two forces operating perpendicular to each other, in the low-pressure areas the wind blows around it. Therefore, when the pressure gradient force is from south to north then winds move from south to north easterlies.

    (iii) What are the geotrophic winds?

    Ans. 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. During the day, the land heats up faster and becomes warmer than the sea. Therefore, the air rises over the land, giving rise to a low-pressure area, whereas the sea is relatively cool and the pressure over the sea is relatively high. Thus, a pressure gradient from sea to land is created, and the wind blows from the sea to the land, known as a sea breeze.

    During the night, the reversal of the condition takes place. The land loses heat faster and is cooler than the sea. The pressure gradient is from the land to the sea. This breeze is known as a land breeze.

    80. Answer the following questions in about 150 words.

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

    Ans. The variations in atmospheric pressure cause air to move around. The wind is the term for the moving air that blows from high to low pressure. At the surface, the wind encounters resistance. The earth’s rotation also impacts the  movement of the wind. The Coriolis force is the  force generated by the rotation of the earth. As a result, the pressure gradient force, the frictional force, and the Coriolis force interact to produce horizontal winds close to the earth’s surface. The gravitational force also pulls downward.

    (a) Pressure gradient force: The differences in atmospheric pressure produce a force. The pressure gradient is the rate of change of pressure with respect to distance. The pressure gradient is strong when the isobars are close to each other and weak when 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. It deflects the wind in the right direction in the northern hemisphere and nature. They oscillate with the apparent movement of the sun. In the northern hemisphere, they move southwards and in the summer northwards.

    (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 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 an 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 air cooling when it reaches 30°N and S latitudes.


    (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. The wind blows perpendicular to the isobars, and the Coriolis force is zero near the equator. Instead of being intensified, the low pressure is filled. Because of this, tropical cyclones do not develop close to the equator. In the cyclone’s eye, torrential rain falls. The eye is the name for the powerful wind that is spiralling around the centre. The circulating system’s diameter can range from 150 to 250 kilometres. The eye is a calm area with subsiding air. The eye wall surrounds the eye, where air ascends strongly in a spiralling motion to increasing heights until it reaches the tropopause. In this area, the wind may blow at its fastest, up to 250 kilometres per hour. From the eye wall, rain bands may radiate, and trains of cumulus and cumulonimbus clouds may drift into the outer region. Due to torrential rain, the wind blowing from those regions is humid. It brings precipitation to oceanic regions. Due to torrential rains, heavy rain takes place on the eastern coast of India and the northeast coast of China.

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