NCERT Solutions for Class 11 Geography Fundamentals of Physical Geography Chapter 9: Atmospheric Circulation and Weather Systems

Chapter at a glance

Exercise Questions

Question 1 (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

In the lower atmosphere pressure decreases by about 1 mb for every 10 m of height, i.e. about 100 mb per km: 1000 − 100 = 900 mb.

3. 900 mb

Question 1 (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

1. near the Equator

Question 1 (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

3. anti-clockwise

Question 1 (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

Air masses form over extensive, homogeneous surfaces such as the Siberian plain (continental polar air).

3. the Siberian Plain

Question 2 (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

Pressure is measured in millibars (mb) or hectopascals (hPa). Because pressure decreases with altitude, stations at different heights would record different pressures even under the same weather conditions. To remove the effect of altitude and make the readings comparable, station pressures are reduced to sea level before drawing isobars on weather maps.

Millibar or hectopascal; station pressure is reduced to sea level to remove the effect of altitude so that pressures can be compared on maps.

Question 2 (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

Because of the earth’s rotation, the Coriolis force deflects moving winds to the right in the northern hemisphere. Winds blowing from the subtropical high towards the equator (north to south) are therefore deflected to the right (westward), so they blow from the north-east — the north-east trade winds.

The Coriolis force deflects the north-to-south winds to the right in the northern hemisphere, making them blow from the north-east.

Question 2 (iii)

What are the geostrophic winds?

Answer

Geostrophic winds blow in the upper atmosphere (2–3 km above the surface), free from friction. Here the pressure gradient force is balanced by the Coriolis force, so the winds blow parallel to the isobars.

Upper-air winds, free of friction, where the pressure gradient force balances the Coriolis force so they blow parallel to the isobars.

Question 2 (iv)

Explain the land and sea breezes.

Answer

• Sea breeze (day): land heats faster than the sea; a low pressure forms over land and a relatively high pressure over the sea, so cool air blows from the sea to the land.
• Land breeze (night): land cools faster than the sea; pressure becomes higher over land, so air blows from the land to the sea.

These breezes moderate temperatures in coastal areas.

By day, land heats faster and air blows from sea to land (sea breeze); by night, land cools faster and air blows from land to sea (land breeze).

Question 3 (i)

Discuss the factors affecting the speed and direction of wind.

Answer

1. Pressure gradient force: the rate of change of pressure with distance. Where isobars are close, the gradient is steep and winds are strong; where they are far apart, winds are weak. Wind blows from high to low pressure.
2. Frictional force: the surface of the earth slows down winds; friction is greatest at the surface (especially over land with rough terrain) and decreases with height. It is least over the sea.
3. Coriolis force: caused by the earth’s rotation; it deflects winds to the right in the northern hemisphere and to the left in the southern. It is zero at the equator and maximum at the poles, and increases with wind speed. It acts perpendicular to the pressure gradient force.
4. Combined effect: near the surface, wind direction depends on all three forces, and winds cross isobars at an angle. In the upper atmosphere, without friction, the pressure gradient and Coriolis forces balance, giving geostrophic winds parallel to isobars. Around low-pressure centres winds blow anticlockwise (cyclones) in the northern hemisphere, and clockwise around highs (anticyclones).

Pressure gradient force (sets speed and drives wind from high to low pressure), friction (slows surface winds) and Coriolis force (deflects winds right in the north, left in the south); their combination determines direction, giving geostrophic winds aloft.

Question 3 (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?

Answer

Reasons for subtropical highs at 30°N and S

1. Descending air: at the equator, intense heating makes air rise (ITCZ). At high altitudes this air moves poleward, cools, and becomes denser. Around 30° latitude it sinks, piling up and creating high pressure at the surface (the descending limb of the Hadley cell).
2. Coriolis force: the poleward-moving upper air is deflected and becomes westerly; it accumulates around 30°, adding to the subsidence.
3. Air that converges in the upper atmosphere at these latitudes sinks and is compressed and warmed, making these belts dry (hot deserts lie here).

The diagram shows the three-cell circulation with pressure belts and planetary winds. Subtropical highs form because air rising at the equator moves poleward aloft, cools and is deflected by the Coriolis force, and sinks around 30°, piling up as high pressure.

Question 3 (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

Why over the seas: tropical cyclones need:

• a large, continuous supply of warm and moist air — sea surface temperature above 27°C;
• presence of the Coriolis force;
• small differences in vertical wind speed;
• a pre-existing low or circulation; and
• upper-level divergence.

The energy of a cyclone comes from the latent heat of condensation released by moist air rising over warm seas. Over land, this supply of moisture is cut off, so cyclones weaken after landfall.

Torrential rain and strongest winds: in the eye wall — the ring of towering clouds surrounding the calm eye at the centre. Here, warm moist air spirals in and rises violently; massive condensation releases latent heat, producing cumulonimbus clouds, torrential rain, and the highest wind speeds (because the pressure gradient is steepest close to the centre). The eye itself is calm, with descending air.

They need warm seas (over 27°C) to supply moist air and latent heat, plus Coriolis force; the heaviest rain and strongest winds occur in the eye wall, where moist air rises violently around the calm eye and the pressure gradient is steepest.

Topic checklist

• Atmospheric pressure, its measurement and distribution
• Forces affecting winds
• General circulation; seasonal winds; local winds
• Air masses and fronts
• Extra-tropical and tropical cyclones; thunderstorms and tornadoes

Total questions answered: 11 of 11