Temperature and Pressure Belts of World

The unequal heating of the earth and its atmosphere by the sun, because of revolution of the earth on its tilted axis causes difference in pressure.

Air is a mixture of several gases present in atmosphere and it exerts pressure on the earth’s surface.

The pressure got its own weight and weight of air on a unit area is called air pressure.As we go up the layer of atmosphere,the pressure falls rapidly.The air pressure is highest at sea level.

Horizontally the distribution of air pressure is influenced by temperature of air at a given place.The areas where temperature is high,the air gets heated and rise and creates a low pressure areas.The areas having low temperature,the air gets cold and the heavy air get sinks and creates a high pressure area.

Low pressure area is associated with cloudy skies and wet weather and high pressure area is associated with clear and sunny skies.

There are three low pressure belts with alternate belts of high pressure.

Equatorial low pressure belts:

This belt is  extends from 0 to 5 deg North and South of Equator.Due to the vertical rays of the sun the tempera­ture here is high,the heated air is light and hence rises forming an area of low pressure.This low pressure belt is also called DOLDRUMS,because it is a zone of total calm without any breeze.

Sub Tropical High Pressure belts:

At about 30°North and South of Equator lies the area where the ascending equatorial air currents descend. This area is thus an area of high pressure. It is also called as the Horse latitude.

Winds always blow from high pressure to low pressure. So the winds from sub tropical region blow towards Equator as Trade winds and another wind blows towards Sub-Polar Low-Pressure as Westerlies.

Circum-polar Low Pressure Belts:

These belts located between 60° and 70° in each hemisphere are known as Circum-polar Low Pressure Belts. In the Sub-tropical region the descending air gets divided into two parts. One part blows towards the Equatorial Low Pressure Belt. The other part blows towards the Circum- polar Low Pressure Belt.

 This zone is marked by ascent of warm Sub-tropical air over cold polar air blowing from poles. Due to earth's rotation, the winds surrounding the Polar region blow towards the Equator.

Centrifugal forces operating in this region create the low pressure belt appropriately called Circum-polar Low Pressure Belt. This region is marked by violent storms in winter.

Polar High pressure belts :
At the North and South Poles, between 70° to 90° North and South, the temperatures are always extremely low.
The cold descending air gives rise to high pressures over the Poles. These areas of   Polar high pressure are known as the Polar Highs. These regions are characterised by permanent Ice Caps.

Trade winds:

The winds blowing from the sub tropical high pressure area(30 deg N and S)toward the equatorial low pressure belt are the extremely steady winds known as the trade winds.These trade winds lying in the zone of 5 deg -30 deg North and South.

In the Northern Hemisphere the wind is moving toward the equator is deflected by the earth’s rotation to flow South-West ward.Thus,the prevailing wind there is from the “North-East trade”.

In the Southern Hemisphere ,deflection of the wind is towards the left,this causes the “South-East Trade”

Westerlies:

The Westerlies are prevailing winds from the west toward the east in the middle latitude between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and tend towards the poles and steer extratropical cyclones in this general manner. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.

The Westerlies are strongest in the western hemisphere and at times when the pressure is lower over the poles, while they are weakest in the southern hemisphere and when pressures are higher over the poles. The Westerlies are particularly strong in areas where land is absent, because land amplifies the flow pattern, making the current more north-south oriented, slowing the Westerlies. 

The strongest westerly winds in the middle latitudes can come in the Roaring Forties, between 40 and 50 degrees latitude. The Westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse

Easterlies:

The polar easterlies are the dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the north and south poles towards low-pressure areas within the Westerlies at high latitudes.

Cold air subsides at the pole creating the high pressure, forcing a southerly (northward in the southern hemisphere) outflow of air towards the equator.

This outflow is then deflected westward by the Coriolis effect, therefore these prevailing winds blow from the east to the west.

Since the winds orriginate in the east, they are then known as easterlies. Unlike the westerlies in the middle latitudes, the polar easterlies are often weak and irregular.

SHIFTING OF PRESSURE BELTS:

If the earth had not been inclined towards the sun, the pressure belts, as described above, would have been as they are. But it is not so, because the earth is inclined 23 1/2° towards the sun. On account of this inclination, differences in heating of the continents, oceans and pressure conditions in January and July vary greatly. January represents winter season and July, summer season in the Northern Hemisphere. Opposite conditions prevail in the Southern Hemisphere.

When the sun is overhead on the Tropic of Cancer (21 June) the pressure belts shift 5° northward and when it shines vertically overhead on Tropic of Capricorn (22 December), they shift 5° southward from their original position.

The shifting of the pressure belts cause seasonal changes in the climate, especially between latitudes 30° and 40° in both hemispheres. In this region the Mediterranean type of climate is experienced because of shifting of permanent belts southwards and northwards with the overhead position of the sun. During winters Westerlies prevail and cause rain. During summers dry Trade Winds blow offshore and are unable to give rainfall in these regions.

When the sun shines vertically over the Equator on 21st March and 23rd September (the Equinoxes), the pressure belts remain balanced in both the hemispheres.