Air Pressure and Pressure Belts
The force per unit area exerted on the surface of earth by weight of the air above that surface is known as atmospheric pressure. The upper layers of atmosphere exert pressure on the lower layers.
Atmospheric pressure is measured by an instrument known as barometer. Standard sea level pressure is 76 cm or 29.92 inches of Hg
Mercury barometer is the standard instrument for measuring atmospheric pressure.
The main factors, which affect atmospheric pressure are altitude, temperature and, water vapour.
An increase in elevation results in a decrease in pressure.
Atmospheric Pressure decreases with increasing temperature.
Moist air is lighter than dry air. Hence, moist air exerts less pressure than dry air.
Atmospheric pressure affects weather conditions.
Fluctuation in atmospheric pressure indicates change in weather conditions.
Differences in air pressure generate wind. Air moves from high pressure area to low pressure area.
The imaginary lines, drawn on a map joining the places having same atmospheric pressure are known as isobars.
Meteorologists use millibars as a pressure unit. 1013 millibars represents the normal atmospheric pressure.
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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