When a place gets heated by the sun, the air above it also gets heated, when air is heated, it expands and becomes lighter. 
    The warmer, lighter air rises and cooler, denser air blows in from the neighbouring areas. The movement of air is what we call wind. When air moves gently (low speed). it is called a breeze. 
    When it moves violently (high speed), it is called a storm. Storms are classified in many ways and given different names. But they have one thing in common-high-speed winds.

Air pressure :
    The air surrounding the earth exerts a pressure on its surface. Like temperature and humidity, air pressure also determines the weather of a place on any particular day. It is measured by an instrument called a barometer and it determines whether a day is           going to be calm or stormy.
    There is a close link between (air) pressure and wind. We can say that winds blow from regions of high pressure to regions of low pressure. 
    The greater the difference in pressure, the greater is the speed of the wind.
    Note : 
    (i) High speed wind is accompanied by a reduced air pressure.
    (ii) The instrument used to measure atomspheric pressure is called as barometre.