The air, that is a mixture of several gases, exerts pressure through its weight. Air pressure or atmospheric pressure is defined as total weight of a mass of column of air above per unit area at sea level.
Vertical distribution : The density ,of air and the atmospheric pressure is high at the lower layer of the atmosphere. Atmospheric pressure decreases with increasing height. Though there is no direct relation between the increasing height and the rate of decrease in atmospheric pressures, yet in the troposphere the rate of decrease in air pressure is 34 mb for every 300 m of height.
This horizontally moving air is called wind. Blowing winds are an effort on the part of nature to balance the pressure differences at various places. The air blowing almost vertically is called air current.
The direction of surface winds is usually controlled by the pressure gradient and rotation of the earth. Because of the rotation of the earth along its axis, the winds are deflected does not blow at right angle of the isobars. The force which deflects the direction of winds is called Coriolis force. Because of Coriolis force, all winds are deflected to the right clockwise in the Northern Hemisphere, while they are deflected to the left anti-clockwise in the Southern Hemisphere with respect to the rotating earth. Since this phenomenon was firstly proved by a French scientist Ferrel, it is called Ferrel's Law.
Based on their characteristic, winds are classified as:
The winds blowing almost in the same direction throughout the year are called permanent or planetary winds. Trade winds, Westerlies and Polar winds are included under it.
Trade winds : These are the permanent winds blowing in both the hemispheres from the subtropical high-pressure belts to the equatorial low pressure belt. The word "Trade' is derived from a German word which means 'a fixed path' or 'track'. Hence, trade winds are the winds having fixed paths. These winds have north-eastely direction in the Northern Hemisphere, while they have south-easterly direction in the Southern Hemisphere. Near the equator, these converge and rise causing convectional rainfall in the equatorial region.
Westerlies : These are the permanent winds blowing from the sub-tropical high pressure, belts to the sub-polar low pressure belts in both hemispheres. The general direction of Westelies is South West to North East in the N. Hemisphere and North West to South East Southern Hemisphere. These winds are developed in the 40°-65°latitudes. In this Southern Hemisphere, because of lack masses and dominance of ocean, their velocity so high that they are called Roaring Forties (40°S), Furious Fifties (50°S) and Shriekig Sixties (60°S). These names are given by the sailors who were being effected by those westerlies.
Polar winds: Winds blowing from the high-pressure belts to the sub-polar low belts, in both the hemispheres, are called polar winds. These winds are north-easterly in the Hemisphere and south-easterly in the Hemisphere. Due to very low temperature humidity bearing capacity of polar winds is very low. When these winds meet the Westerlies sub-polar regions, polar fronts are develop, temperate cyclones are generated.
The winds which change their direction of blowing with the changing seasons are called seasonal winds. They are also called temporary winds, Monsoon winds, sea breeze, land breeze, mountain breeze and valley breeze are included under it.
Monsoon winds : The entire surface winds, which change their directions with changing seasons are called monsoon winds. These winds blow from sea to land in summer and from land to sea in winter.
Land and Sea Breezes : Land is heated more quickly than'the adjacent sea during the day time. As a result, warm air rises over the land creating low pressure area there. However, there is prevailing high pressure over the adjacent sea. As a result, the pressure gradient causes the air to blow from the high pressure to the low pressure areas i.e. from the sea to the land. This is called sea breeze.
Mountain and Valley Breezes : The slope:, and valley floors in the mountainous regions are more heated through insolation during daytime. "Consequently, the warm air moves upslope. This upward moving breeze during daytime is called Valley Breeze. Valley breezes reach mountain peaks and many a time yield precipitation. In the night, the upper part of the mountain cools quickly and starts falling down along the slope of the mountain. This is called Mountain Breeze.
These winds blow due to local variation in the temperature and pressure, and influence a very small area. Some important local winds are :
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