Due to the rotation of the earth winds curve as they blow. This curving motion of wind is called coriolis effect. According to this any freely moving bodies get deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Winds that blows from sub-tropical high pressure belt to equatorial low pressure belt are called trade winds. In Northern Hemisphere trade wind blows from North East direction due to deflection. So it is called North East trade winds.
In southern Hemisphere trade winds blows from South East direction hence it is called South East trade winds. Westerlies winds blow from the sub-tropical high pres sure to sub-polar low pressure. Due to coriolis effect in Northern Hemisphere it is from south west direction and in Southern Hemisphere North west direction. Westerlies are just opposite of trade winds in terms of their direction so westerlies are also called an ti trade winds.
