Variation in Atmospheric Pressure
1) Variation with height or vertical variation:
The pressure depends on the density or mass of the air. The density of air depends on its temperature. Its composition and force of gravity. I t is observed that the density of air decreases with increase in height so the pressure also decreases with increase in eight.
The pressure at sea level is 1013.25 mb at 50 km height it becomes 0.93 mb and 80 km it is only 0.03 mb. This indicates how rapidly the atmospheric gas becomes thinner to decrease density and so the pressure. The pressure decreases on an average at the rate of about 34 mb per every 300 meters height.
2) Horizontal variation of pressure:
The horizontal variation of atmospheric pressure depends on temperature, extent of water vapor, latitude and land and water relationship.
i)The equatorial low pressure belt :
Along the equator lies a belt of low pressure known as the equatorial low or doldrums or calm. This low pressure belt lies between 50 North and 50 South latitudes.
ii) Sub – tropical high pressure belt:
The high pressure belt are found between 24 – 300C latitudes in both the hemispheres.
iii) Low pressure belts near 600 latitudes:
The airs from this area get thrown outwards on account of the rotation of the earth and this is how the low pressure belts are created.
iv) Polar high pressure belts:
The temperature is extremely low in the Polar Regions. The air being cold and heavy throughout the year a high pressure belt is created in both Polar Regions.
3. Diurnal variation:
At a given station the pressure show the two high and two lows. On normal pressure day two maxima i.e. one at 10 a.m. and another at 10 p.m. and two minimasi.e. one at 4 a.m. and another at 4 p.m. are observed. Thus there is double oscillation caused by alternate heating and cooling of atmosphere.
Factors affecting atmospheric pressure:
1. Temperature of air
3. Water vapour in air
4. Revolution and gravitation of the earth.