Atmospheric Pressure and Winds — Question 3

(i) The Westerlies in the Southern Hemisphere blow with greater force than those in the Northern Hemisphere because there is absence of landmass in the southern hemisphere.

(ii) The causes of shifting of pressure belts are inclination of Earth's axis and change in relative position of Earth with respect to the Sun -

1. At the time of Summer solstice the Sun is vertical over the Tropic of Cancer and therefore, all the pressure belts except the northern polar high pressure belt shift northward.
2. After Autumn Equinox, there is southward migration of the Sun which becomes vertically over the Tropic of Capricorn at the time of Winter Solstice and therefore the pressure belt shifts southward except in the southern polar high pressure belt.
3. The Sun is overhead over the Equator at the time of Spring Equinox and Autumn Equinox and hence all the pressure belts occupy their normal positions.

(iii) As we go higher upwards, the atmospheric pressure decreases at an average rate of 34 millibars per 300 metres of latitude. It is because the air in the lower layer of the atmosphere is denser than at higher level. The air is denser in lower layer because of gravitational pull of earth and also because the lower layers are compressed under upper layers of atmosphere.