The unequal distribution of heat on the surface of the Earth causes the formation of high and low pressure belts over the Earth. Lighter air ascends to form low pressure belts while heavier air descends to create the high pressure belts. This vertical movement of air is called ‘air current’.
Moreover, air also moves from high pressure to low pressure areas. This horizontal movement of air is called ’wind’. In other words, wind is horizontal movement of air, which blows in every corner of the earth’s surface from a region of high pressure to a region of lower pressure. They are named according to the direction from which they blow, like the south wind or the north wind. The rotation of the Earth deflects winds to the right of their flow in the Northern Hemisphere and to the left to their flow in the southern Hemisphere.
Winds are classified into three categories. They are – the Planetary or the Prevailing Winds, the Secondary or the Periodic Winds, which includes the Seasonal Winds like the Monsoon Winds, and the Local Winds.
The general distribution of winds throughout the lower atmosphere is termed as the Planetary or the Prevailing Winds. They are also called the Primary or the Permanent Winds. The direction of the Planetary Winds is guided by the Earth’s rotation and their distribution is closely related to the position of the Pressure Belts. Due to the rotation of the Earth, the planetary winds do not blow due north and due south from high pressure belts to the low pressure belts, but are deflected by the Coriolis force. The Coriolis, named after a French Physicist, is the force generated by Earth’s rotation, which causes wind deflection.
This is explained in terms of Ferrel’s Law of Deflection. This law states that all freely moving bodies are deflected to their right in the northern hemisphere and to their left in the southern hemisphere. Thus, a south wind is deflected as a south-west wind in the northern hemisphere and as a south-east wind in the southern hemisphere.
There are three main planetary winds that constantly blow in the same direction all around the world, the Trade Winds or the Tropical Easterlies, the Westerlies and the Polar Winds or the Polar Easterlies.