Atmospheric Pressure
1.Air pressure or atmospheric pressure is defined as total weight of a mass of column of air above per unit area at sea level. The air pressure equivalent to 76 cm high mercury column i.e.1013.25 mb is the atmospheric pressure at the sea level.
2. The horizontal distribution of mean sea level pressure is shown by means of isobars, which are lines connecting places with equal pressure. When isobars are above normal value, form clustering around in a circular manner and are known as high pressure rigon are simply high. When there is clustering of isobars belonging to less than the normal value, they are known as low pressure region.
3.An outward extension of high pressure into a region of predominantly low pressure is known as ridge, while an extension of low pressure into a regon of high pressure is Known as trough.
4.An area of almost uniform pressure between two highs and two lows is known as Col. Lines joining places with equal barometric tendency, where the pressure is rising or falling by the same amount in a given period of time are known as isallobars.
5.Wind moves from high pressure areas to low pressure areas. The distribution of air pressure is controlled by altitude, temperature, rotation of the Earth, water vapour, air circulation etc. The rate of change of pressure per unit horizontal distance is called pressure gradient.
6.The amounts decrease to about 1 mb for each 10 m increase in elevation, but it does not always decreases at the same rate. The vertical gradient force is much larger than that of the horizontal pressure gradient.
Pressure Belts
1 The pressure in the atmosphere has a combined effect of latitudinal variation, effect of termperature and effect of rotation and shows a general pattern of pressure throughout the Earth.
2. Earth can be divided into regions of broader pressure belts,even though have regional anomalies or deviations.
3.There are seven pressure belts on the globe. There high and low alternate pressure in Northern and Southern hemisphere and one common equatorial low pressure belt.
Equatorial Low Pressure Belt (Thermal in Origin)
• It is located on either side of the geographical equator in a zone extending between 5"N and 5'S latitudes and may shift to higher latitudes during summers in respective hemisphere.
• The equatorial low pressure belt represents the convergence zone of North-East and South-East trade winds. This belt is also called doldrums because of frequent calm conditions. This belt is caused due to intense heat leading to low pressure.
Sub-Tropical High Pressure Belt (Dynamic in Origin)
It extends between the latitudes of 25° to 35° in both the hemisphere. The convergence of winds at higher altitude above this zone results in the subsidence of air from higher altitudes This zone, is characterised by anti-cyclonic conditions, which cause atmospheric stability and aridity because of result of descent winds in the contraction of their volume and ultimately, causes high pressure over surtace. This zone of high pressure is called horse latitude because of prevalence of frequent calms. The belt is dynamic in origin.
Sub-Polar Low Pressure Belt (Dynamic in Origin)
It extends between the latitudes of 60° to 65° in both the hemisphere. The sub-polar low pressure belt is more developed and regular in the Southern hemisphere, while it is broken in the Northern hemisphere because of over dominance of water in the former. There are well defined low pressure centre or cells over the oceans in the Northern hemisphere eg, in the neighbour of Aleutian Islands in the Pacific Ocean and between Greenland and iceland in the Atlantic Ocean.
Polar High Pressure Belt.
Because of prevalence of very low temperature high pressure persists at the poles throughout the year. Both the factors thermal and dynamic, operate at the poles. Here, high pressure is produced due to very low temperature.
Important Note
Shifting of Pressure Belts Pressure belts is normally shifted along with the position of overhead Sun. In Northern hemisphere, pressure belts shifted to North in summer and shifted towards South in winter season.
Diurnal change of thermal condition alters the pressure condition and, so wind shifts its course regionally. Sea breezes and land breezes are the examples of local shifting of pressure.
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