We know that air is a mixture of oxygen, nitrogen, carbon dioxide, water vapour, dust particles and numerous other gases in trace amounts. Air in motion is called the wind. Have you wondered why the wind blows? When the Sun’s rays heat any place on the Earth, the air of that place also gets heated. Due to this heating, the air expands and hence its density decreases and it becomes lighter. The hot air, due to this lightness, goes up in the atmosphere. And this causes a decrease in atmospheric pressure in that area. Under such a condition, air from high-pressure cold regions rushes to the lowpressure belts thus creating equilibrium. This movement of air is called the blowing of wind. In the areas adjoining the sea, the Earth becomes hot during the day. Due to this, air becomes light and goes up in the atmosphere. To restore balance, cold air from the sea blows towards the land. During the night, a reverse movement takes place, that is, the Earth becomes colder than the seawater and as such the air moves from the land towards the sea. Air is very hot in the regions surrounding the equator. It creates an equatorial belt of low pressure. So there is constant upward movement of hot air from these regions. This hot air flows out to the north and south. The rotation of the Earth at its axis considerably affects the direction of the winds. The westerly winds are the direct result of the Earth’s rotation from the west to the east. The spinning of the Earth makes air in the northern hemisphere deflect towards the right and in the southern hemisphere towards the left. Superimposed on the general wind systems are local winds. Winds are also caused by temperature differentials associated with topographical features such as mountains and coastal belts. The presence of mountains also influences the wind direction. The mountains obstruct the winds and change their directions. An instrument called an anemometer is used to measure the speed and direction of wind.