The distribution of atmospheric pressure across the latitudes is known as the global horizontal distribution of pressure. Differences in pressure conditions over the Earth result in the creation of various pressure belts. Because the Earth is spherical, different parts of the Earth get heated differently. Because the equatorial regions receive maximum sunlight, warm air rises to create low-pressure conditions. Similarly, at the Poles, cold air sinks (because cold air is heavy) creating a high-pressure area.
Major pressure belts of the Earth
Equatorial Low Pressure Belt
- This belt lies between 0°N to 5°N and 0°S to 5°S.
- Because the vertical rays of the Sun fall on the Earth, the heat is intense and the temperature is high. The air becomes warm and expands.
- The convection currents help spread the heat to the upper layers of the atmosphere also.
- This leads to the creation of a low-pressure belt in the region. Winds in this pressure belt do not blow at a fast pace, and hence, this belt is known as the Belt of Calms or the Belt of Doldrums.
Sub-Tropical High Pressure Belt
- This belt is located between 30°N to 35°N and 30°S to 35°S.
- As the air at the Equator moves towards the Poles, it begins to cool and sink at about 30° North and South of the Equator. This creates areas of high air pressure.
- Because of the rotation of the Earth, air from 60°N and 60°S also descends in the Sub-Tropical Belts.
- This pressure belt is also a Belt of Calms and is called Horse Latitudes.
Circum Polar Low Pressure Belt
- This belt is located between 60°N to 65°N and 60°S to 65°S.
- It is a zone of two winds—the warm Westerly winds meet the cold Polar Easterlies. As the Westerlies are lighter, they rise over the cold polar winds creating a low pressure area.
- These belts experience stormy weather and cyclonic activity, especially during winters.
Polar High Pressure Belts
- This belt is located between 85°N to 90°N and 85°S to 90°S.
- The Polar Regions experience cold climatic conditions as the rays of the Sun are extremely slanting.
- The cold air sinks giving rise to high pressure areas.
- At places where the temperature is permanently low, air is dense and heavy. These Polar Regions have permanent ice caps.
Shifting of Pressure Belts
- The inclination of the Earth at 23 1/2° creates differences in heating of oceans and continents. The pressure conditions vary in the Northern and Southern Hemispheres.
- On 21 June, when the Sun is overhead on the Tropic of Cancer, the pressure belts shift 5° northwards. On 22 December, when the Sun shines vertically overhead on the Tropic of Capricorn, they shift 5° southwards from their original position.
- On 21 March and 23 September, when the Sun shines vertically over the Equator, the pressure belts remain balanced in both hemispheres.
- This shifting of pressure belts results in seasonal changes in climate.
World Distribution of Pressure
- The land and oceans get heated differently. Hence, there is a marked difference in pressure conditions over the land and the oceans.
- In January, the places in the Northern Hemisphere experience winter conditions. In places such as Siberia and the North Indian Plains, the temperature is low and the pressure is high.
- On the contrary, low-pressure conditions exist over the Indian Ocean, North Atlantic Ocean, and North Pacific Ocean. These oceans are warmer.
- The places in the Southern Hemisphere experience summer in January as there is high temperature and low-pressure conditions over the continents.
- During July in the Northern Hemisphere, high pressure occurs over the oceans, and the continental landmasses experience high temperature and low-pressure conditions. This is summer in the Northern Hemisphere.
- We find major seasonal fluctuations occurring during this period because of the overheating of landmasses.
Also, Read Pressure belts and types of winds