Atmospheric pressure is the force exerted by any overlying air at a given location. It is the weight of a column of air exerted on a unit area from the mean sea level to the top of the atmosphere.
Atmospheric pressure is measured with barometer. Two types of barometer are mercury barometer, and aneroid barometer.
Standard Sea level atmospheric pressure is 76 cm or 29.92 inches of Mercury.
Atmospheric pressure is expressed in units of millibars and Pascal.
Distribution of pressure is shown by isobars. An isobar is an imaginary line drawn through places having equal atmospheric pressure reduced to sea level.
Atmospheric pressure is responsible for horizontal movement of air called winds. It helps in weather prediction by measuring the changes in atmospheric elements.
Atmospheric pressure is not the same everywhere.
Several factors affect the distribution of atmospheric pressure, like altitude, temperature, water vapour, gravitational pull and rotation of the earth.
Air Pressure changes with altitude. It decreases rapidly with increasing altitude.
Pressure decreases with increasing temperature.
Water vapour is lighter than air of the atmosphere. Thus, moist air exerts less pressure.
Earth’s gravity pulls the molecules of air to the ground. More gravitational pull results in high pressure and less gravitational pull results in low pressure.
Rotation of the earth generates centrifugal force and throws the air outwards.
Atmospheric pressure changes vertically, horizontally and seasonally. Atmospheric pressure decreases with increasing height. It decreases at the rate of 34 millibars per 300 metres of height.
Horizontal distribution of atmospheric pressure is the distribution of atmospheric pressure across the latitudes.
Main feature of horizontal distribution of atmospheric pressure is zonal character known as pressure belts.
Earth has a pattern of high and low pressure belts. These belts are not continuous.
Pressure belts are caused due to unequal heating of the earth and its atmosphere.
The major pressure belts on earth are Equatorial Low Pressure Belt, Sub-Tropical High Pressure Belt, Sub-Polar Low Pressure Belt and, Polar High Pressure Belt.
Equatorial low pressure belt extends up to 5° North and South of the equator. This belt receives a great amount of heat throughout the year and makes the air warm.
Sub-tropical high pressure belt is located between 30° and 35° North and South latitudes. These latitudes are also known as Horse latitudes.
Sub-polar low pressure belt is located between 60° and 65° North and South latitudes. It is an area of stormy weather, particularly in winters.
Polar high pressure belt is located in the polar region. It is the permanent centre of high pressure.
Seasonal shifting of pressure belts occurs due to the annual revolution of the earth. Pressure belts shift Northwards in July and southwards in January. Shifting of pressure belts affects the direction in which wind is blowing.
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