Atmospheric Pressure and Winds

Atmospheric Pressure

The air present in the atmosphere has its own weight and it exerts pressure on the surface of the earth. The pressure exerted by atmosphere is known as atmospheric pressure. It is the weight of column of air at any given place and time. It is measured with the help of a barometer. Under normal conditions, it is equal to the weight of a column of mercury 76 cm or 760 mm high at sea-level. Meteorologists measure atmospheric pressure in milli bars (mb). One millibar is equal to the force of one gram on a square centimetre. The normal pressure of 76 cm at sea-level is equal to 1013.25 millibars. Atmospheric pressure is measured with the help of instruments like Fortins barometer, aneroid barometer, and barograph. Distribution of pressure on the map is shown by isobars.

An isobar is an imaginary line drawn through places having equal atmospheric pressure reduced to sea-level. Spacing of isobars expresses pressure gradient. It is defined as the rate of decrease in pressure per unit distance in the direction in which the pressure decreases. Close spacing of isobars indicates a strong pressure gradient, while wide spacing suggests a weak gradient.

The atmospheric pressure is a very important factor in meteorology. Variations in the atmospheric pressure are responsible for horizontal movements of air called winds. Winds transport heat and moisture from one place to another and help in the occurrence of precipitation and influence both temperature and humidity. In addition, different climate types and regions are characterized by distinctive pressure and wind conditions.

Factors affecting atmospheric pressure include temperature of the air, altitude, water vapour, gravitational pull and rotation of the earth. Rise in temperature leads to fall in pressure and pressure also decreases with increasing altitude. e pressure decreases with increase in water vapour because dry air is heavier than moist air. Gravitational pull of the earth also affects the atmospheric pressure because gravitational pull of the earth at a given place is inversely proportional to the square of its distance from the centre of the earth. Rotation of the earth causes centrifugal force which affects atmospheric pressure in its own way.

 Vertical Distribution of Atmospheric Pressure

Atmospheric pressure is maximum at the sea-level under normal conditions; and it decreases with altitude. There is no simple relationship between altitude and pressure. However, in general, we can say that the atmospheric pressure decreases on an average at the rate of about 1 millibar per every 10 metres of height. The atmospheric pressure is 1013.25 mb at the sea-level which is reduced to half (from 1013.2 mb to 540 mb) at a height of 5 km and only 265 mb at a height of 10 km.

Horizontal Distribution of Atmospheric Pressure

Horizontal distribution of pressure is expressed in terms of pressure belts which run in east-west direction along certain special latitudes. There are seven pressure belts over the globe along certain specific latitudes. They are the equatorial low, the sub-tropical highs, the sub-polar lows, and the polar highs. Except the equatorial low, all others form matching pairs in the northern and southern hemispheres. The equatorial low pressure belt and the polar highs are thermally induced belts while the sub-tropical highs and sub-polar lows are dynamically induced belts. Thus, the seven pressure belts are put into following four groups.

Equatorial Low Pressure Belt

This is a low pressure belt which extends upto 10° N and S of equator. Low pressure prevails there due to high temperature, high water vapour content in the air and maximum speed of the earth’s rotation at the equator. Surface winds are generally absent since winds approaching this belt begin to rise near its margin. Thus, only vertical currents are found. is belt is also called the doldrums because of the extremely calm air movements.

Sub-Tropical High Pressure Belt

These belts extend from near the tropics to about 35° N and 35° S. These belts in the northern and the southern hemispheres occur due to descent of cold air from high altitude. The descending air creates a calm condition with variable and feeble winds in these high pressure belts. They are, therefore, called ‘Belts of Calm’ or ‘Horse Latitude’. In early days, the sailing vessels with the cargo of horse found it very difficult to sail under such calm conditions. They used to throw horses in the sea to make their vessels light.

Sub-Polar Low Pressure Belts

The sub-polar lows are located between 45° N and S to the Arctic and Antarctic circles respectively. These belts are formed due to rotation of the earth. The axial rotation of the earth should blow away air from the poles and should cause low pressure at the poles but the poles are very cold and the effect of axial rotation in displacing air is felt near the Arctic and Antarctic circles and low pressure belts are formed in the sub-polar areas.

Polar High Pressure Belts

From 80° N and S to N and S poles, there exist high pressure belts known as Polar High Pressure Belts. These high pressure belts are caused by extremely low temperate at the poles which remain below freezing point even during the summer season. The air remains cold and dense and results in high pressure there parts of the oceans where the cool ocean currents are effective. In the northern hemisphere, a well-developed sub-tropical high pressure area extends over the continents. The sub-polar low of the southern hemisphere extends as a trough whereas, in the northern hemisphere, there are two cells of low pressure extending over North Atlantic and North Pacific.

These are known as Icelandic low and the Aleutian low, respectively. The above mentioned system of pressure distribution is changed in the month of July. At this time of the year the sun shines almost vertically on Tropic of Cancer in the northern hemisphere which results in the northward shift of the equatorial low pressure belt. This shift is maximum in Asia. The land masses of the northern hemisphere become excessively hot and low pressure areas develop over them. The sub-tropical high pressure belt of the southern hemisphere extends continuously. On contrary, in the northern hemisphere, it is broken over the continents and remains conned to the North Atlantic and North Pacific Oceans. Sub-polar low is deep and continuous trough in the southern hemisphere, while in the northern hemisphere, there is only a faint oceanic low.

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