Question

Give brief explanation of the pressure belts of the world. Why do they shift slightly?

Answer 1

Pressure belts are created due to the heating and cooling of the air at different places and in different magnitude due to the difference in heating of the surface of the earth from the sun rays, due to the spherical shape of the earth. The latitudes of the earth can be divided into seven pressure belts, which are as follows:
1. Equatorial Low-Pressure Belt: It lies between 5 degrees north and 5 degrees south latitude around the equator. Now as direct sunlight falls on the equator, this is a region of high heat, and hence the heated air ascends upward, creating a region of low pressure. It is also called doldrums because it is a region of extreme calmness not so much as even a breeze.
2. Sub-tropical High-Pressure Belts: There are two such belts, one each in north and south hemisphere at 30-degrees latitudes from the equator, in the respective hemispheres. This is a region where the light air from the equator descends and cools down, and hence this is a region of high pressure. It is called the horse latitude.
Winds move from high to low-pressure regions and hence, the wind from here diverges, one part moves towards the equator called the Tradewinds and on to the circumpolar low-pressure region called the Westerlies.
3. Circum-Polar Low-Pressure Belts: Again there are two of these, between 60 degrees and 70 degrees in each hemisphere. Now due to the rotation of the earth, the centrifugal force due to the gravity cause the winds to go towards the equator and towards the poles, in the region near the poles, thus creating a low pressure. It is characterised by high storms in winter.
4. Polar High-pressure Belts: Two of them, between 70 degrees and 90 degrees latitude in the northern and southern hemisphere. The temperature at the poles is extremely low, characteristics of the ice caps in the region and hence the air here is dense, creating a high pressure in the region.
Shifting: The above-mentioned zones of latitudes would have remained the same if the earth were not tilted to the plane of revolution around the Sun. But as it is not so and the earth is tilted at 23 and 1/2 degrees, when the sun is overhead the Tropic of Cancer(the July month), the belts shift 5 degrees to the north of the equator; when the sun is overhead the Tropic of Capricorn(the January month), the belts shift 5 degrees to the south of the equator.