The cooler air that replaces the warm rising air is denser. Changes in atmospheric pressure vary from place to place, hour to hour, and season to season. These changes are due to movements of air masses. The movements of our atmosphere are prompted by the rotation of the Earth and by differences in temperature which creat differences in density and, thus, in pressure. If an area of low pressure is close to an area of high pressure, there is a tendency for the air in the high pressure system to move into the area of low pressure in an effort to produce equilibrium.
Air moving in a horizontal plane from high pressure to low pressure regions is called wind.
The strength of a wind depends upon the difference in pressure between the two regions. This is air that moves parallel to the Earth's surface. Wind direction is the direction from which the wind is blowing, thus a south wind comes from the south point on the compass. The air in this case is traveling northward. Therefore, winds are named according to the direction from which they come.