Air expands when heated and gets compressed when cooled. This results in variations in the atmospheric pressure. Atmospheric pressure also determines when the air will rise or sink.

●  The result is the movement of air from high pressure to low pressure, this horizontal motion of air is called wind. The wind redistributes the heat and moisture across the planet, thereby, maintaining a constant temperature for the planet as a whole.

Atmospheric Pressure

●  The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure.

●  Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface.

●  The atmospheric pressure is expressed in units of a millibar.

●  At sea level, the average atmospheric pressure is 1,013.2 millibar.

●  Air pressure is measured with the help of a mercury barometer or the aneroid barometer.

●  Due to gravity, the air at the surface is denser and hence has higher pressure.

●  The air pressure is highest at sea level and decreases with height.

●  As the temperature increases the molecules in the air start moving vigorously. The air, therefore, expands and its density decreases. Therefore, pressure decreases with an increase in temperature.

●  As a result, the air becomes lighter and moves upward. Cold Air is dense and heavy, it, therefore, moves downwards while hot air rises up. Therefore, pressure increases with a decrease in temperature.

Vertical Variation of Pressure

●  In the lower atmosphere, the pressure decreases rapidly with height.

●  The atmospheric pressure decreases at a rate of about 1 mb for each 10 m increase in elevation. It does not always decrease at the same rate.

●  The vertical pressure gradient force is much larger than that of the horizontal pressure gradient.

●  The pressure gradient is defined as the decrease in pressure per unit distance in the direction in which the pressure decreases most rapidly,

Horizontal Distribution of Pressure

●  As the earth’s land and water surfaces warm-up and cool down, the air near them also does the same. The air starts moving from places of high pressure to places of low pressure. The horizontal movement of Air near the earth’s surface is known as Wind.

●  The horizontal distribution of pressure is studied by drawing isobars at constant levels.

●  Isobars are lines connecting places having equal pressure. In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level for purposes of comparison.

●  The Low-Pressure system is enclosed by one or more isobars with the lowest pressure in the centre. A high-pressure system is also enclosed by one or more isobars with the highest pressure in the centre

World Distribution of Sea Level Pressure

●  The main feature of the distribution of pressure is its zonal or belted character. Each zone or belt comprises several cells of high or low pressures.

●  The cells can be either circular or elongated.

●  The zonal or belted character is strikingly developed in the Southern Hemisphere which has a more homogeneous, water-covered surface.

●  Low pressure is experienced near the equator and the area is known as equatorial low.

●  Along 30° N and 30° S are found the high-pressure areas known as the subtropical highs.

●  Further pole wards along 60° N and 60° S, the low-pressure belts are termed as the sub polar lows.

●  Near the poles the pressure is high and it is known as the polar high.

●  These pressure belts are not permanent. They oscillate with the apparent movement of the sun.

●  In the northern hemisphere in winter, they move southwards and in the summer northwards.