Wind flows outward down the pressure gradient away from the subtropical highs. As it does so, it encounters the coriolis effect caused by the rotation of the Earth. This force causes the winds in the Northern Hemisphere to move from the east towards the west below the subtropical high, and from the west towards the east above the subtropical high. The opposite is true in the Southern Hemisphere. Above the subtropical high winds move from east to west, and below the subtropical high winds move from west to the east.
Thus, in the Northern Hemisphere below the subtropical high, which is located just above the equator, we find winds blowing from east towards the west. At the same time, in the Southern Hemisphere above the subtropical high, which is located just below the equator, we also find that winds are blowing from the east towards the west. The result is that between 30 ° North latitude and 30 ° South latitude winds usually blow from the east towards the west. Winds are named for the direction from which they come. Thus, this wind is referred to as an easterly wind. These easterly winds are known as the trade winds.