Description of wind: Wind is simply air in motion, moving from high pressure to low pressure.
Mechanism of air movements: Winds are described by the direction they blow from - an easterly wind lows from the east, a westerly from the west. Prevailing winds (winds which occur frequently in a particular direction) are arranged in a series of belts around the globe. This pattern is the result of differences in the speed the Earth rotates and the different amounts of solar heating it gets, depending on where a place is between the poles and the equator.
Atmosphere: The atmosphere is composed of air which, in turn, is made up of tiny particles of different gases like nitrogen, hydrogen and oxygen. The Sun shines on our atmosphere all of the time. But, it heats the surface of the Earth unevenly, so that in some places it is warm while in other places it is cold. As air gets warmer, its particles spread out. This makes the air lighter, or less dense, so it rises. As air cools, it becomes heavier, or denser, and sinks. As warm air rises, air from cooler areas flows in to take the place of the heated air. This process is called convection and causes air to move. The differential heating of the Earth's surface and the resulting convection is what causes wind on this planet. Wind circles the Earth and plays an important role in determining weather conditions.