Wind currents : 
    Wind currents are formed due to uneven heating  of the earth. Two situations may arise :
1.    Uneven heating between the equator and the pole
      The equator, which receives the direct rays of the sun throughout the year, is the hottest part of the earth. The air above the equator gets heated (hence, becomes lighter) and rises. 
      This makes cooler, denser air from the tropical belts blow in towards the equator from the north to south. These permanent winds are called the north-east and south-east trade winds. 
      The rotation of the earth makes these winds blow in an eastern direction rather than straight north and south. Wind flow pattern due to uneven heating between the equator and the pole.

2.    Uneven heating of land and water : 
      Originally, the seasonal winds that bring rain to India and the other countries of South Asia between June and September are called monsoon. 
      The word monsoon was derived from the Arabic word ‘mausam’ which means weather. In summer, the landmass of northern India becomes much hotter than the surrounding seas. 
      The hot air rises and moist air blows in from the Arabian Sea and Bay of Bengal. These moisture-laden winds bring heavy rainfall in most parts of India, are called the summer monsoon.
      In winter, the direction of the wind flow gets reversed, it flows from the land to the ocean. These winds are called the winter monsoon. They pick up moisture as they blow over the Bay of Bengal and bring rainfall to coastal Tamil Nadu.