Chapter 4 - Air Pressure
Q. 1. Give reasons:
(1) Air pressure decreases with increasing altitude.
Ans:
(i) The proportion of dust in the air, water vapour, heavy gases, etc. is higher in the air closer to the surface of the earth.
(ii) This proportion decreases with increasing altitude.
(iii) As one moves higher and higher from the surface of the earth,the air becomes thinner and thinner.
(iv) As a result, the air pressure decreases with increasing altitude.
(2) Pressure belts oscillate.
Ans:
(i) In both hemispheres, the duration and intensity of the sun’s rays vary during particular periods of the year.
(ii) As a result, the locations of the temperature zones and the pressure belts dependent on the sun’s heat also vary.
(iii) This change is of the order of 5° to 7° toward the north in Uttarayan and 5° to 7° South in Dakshinayan. This is called the oscillation of pressure belts.
Q.2. Give short answers to the following questions.
(1) What effect does temperature have on air pressure?
Ans:
(i) Temperature and air pressure are closely related.
(ii) Wherever the temperature is high, the air pressure is low.
(iii) As the temperature rises, the air gets heated, expands, and becomes lighter.
(iv) This lighter air in the vicinity of the earth’s surface starts moving up towards the sky.
(v) As a result, the air pressure in such areas decreases.
(2) Why is the subpolar low pressure belt formed?
Ans:
(i) Due to the earth’s curvature, the areas between two parallels get smaller as we move towards the pole.
(ii) This results in less friction between the air and the earth’s surface.
(iii) Air in this region is thrown out because of this reduced friction and also because of the earth’s rotational motion.
(iv) This leads to the development of a low-pressure belt.
Q. 3. Write notes on:
(1) Mid-latitudinal high pressure belts
Ans:
(i) Due to low temperatures at higher altitudes, the air cools down and becomes heavier.
(ii) This heavier air descends in both hemispheres in the region between 25° and 35° parallels and forms high-pressure belts.
(iii) This air is dry, hence the region does not get rainfall.
(iv) As a result, most of the hot deserts on earth are found in these regions.
(v) This high pressure belt is called the mid-latitudinal high pressure belt.
(2) Horizontal distribution of air pressure.
Ans:
(i) There is an uneven distribution of air pressure on all the places on the earth’s surface.
(ii) The air pressure is found to be high in some regions and low in some other regions on the earth’s surface.
(iii) Such a distribution of air pressure is known as the horizontal distribution of air pressure.
Q.4. Fill in the gaps with the appropriate option.
(1) At higher altitudes air becomes ______.
(thicker, thinner, hotter, more humid)
Ans: thinner
(2) Air pressure is expressed in ______.
(millibars, millimetres, millilitres, milligrams)
Ans: millibars
(3) On the earth, air pressure is ______.
(uniform, uneven, high, low)
Ans: uneven
(4) The ______ pressure belt spreads between 5° North and 5°South parallel.
(Equatorial low, Polar high, Subpolar low, Mid-latitudinal high)
Ans: Equatorial low
Q.5. How does a high pressure belt get formed near 30° parallel? Why does this region have hot deserts?
Ans:
(i) The heated air becomes lighter, starts ascending, and after reaching higher altitudes, moves towards the polar region, i.e., towards the North and South Poles.
(ii) Due to low temperatures at higher altitudes, the air cools down and becomes heavier.
(iii) This heavier air descends in both hemispheres in the region between 25° and 35°parallels.
(iv) This leads to the formation of high-pressure belts in these parallel latitudes in both hemispheres.
(v) This air is dry, hence the region does not get rainfall.
(vi) As a result, most of the hot deserts on earth are found in these regions.
6. Draw a neat diagram showing pressure belts. Label the diagram.
Ans:
