Wind is the movement of air across the earth’s surface. The differences in air density causes wind, which results in horizontal differences in air pressure. These pressure systems are both the result and the cause of atmospheric circulation.
There are different types of winds such as gusts, which are short bursts of high speed wind; squalls are strong winds of intermediate duration; a breeze is of long-duration of weaker strength; and there are strong winds that are of hurricane or typhoon strength.
The main factors that affect wind direction and speed are: the pressure-gradient force, the Coriolis force and friction. These factors working together cause the wind to blow in different directions and at different speeds.
The pressure-gradient force
Air flows from areas of higher pressure to areas of lower pressure. This is the pressure gradient force that sets the air in motion and causes it to move with increasing speed down the gradient. It follows the law of gravitation and is as natural as water running downhill. The heating of the earth’s surface is uneven which causes the continual generation of these pressure differences. The greater the pressure difference over a certain horizontal distance, the greater the force and therefore, the stronger the wind.
On a surface weather map the variations in air pressure over the earth’s surface are shown by isobars. These isobar lines connect places of equal air pressure. The spacing of the isobars indicate the amount of pressure change over a given distance. This is expressed as the pressure gradient. If the isobars are closely spaced there is a steep pressure gradient giving strong winds. Widely spaced isobars indicate a weak pressure gradient and light winds.
The Coriolis force
The second force that affects the direction of the winds is the deflecting force of the rotation of earth, known as the Coriolis force. Winds are deflected to the right of the gradient in the Northern Hemisphere and to the left in the Southern.
The Coriolis force is directed at right angles to the direction of air flow. It does not affect the wind speed, only the wind direction. However, the stronger the wind, the greater the deflecting force. There is no deflection of winds at the equator, but it increases to its maximum at the poles.
Friction is the third force that affects both speed and direction of winds. Friction is operative only to about 2,000 feet above the earth’s surface. This force greatly slows the speed of surface air and reduces the Coriolis force. This alters the force balance which causes the pressure-gradient force to move the air at right angles across the isobars toward the area of lower pressure. The angles that surface winds make with isobars vary over land from 20 to 40 degrees. Over the ocean where there is less frictional drag the isobars are reduced to as little as 10 degrees.
Wind is an air movement that occurs in addition to the movement associated with rotation of the earth. The earth’s atmosphere is fixed to the earth and moves with it in its west-to-east rotation. Wind is nature’s way of trying to correct air pressure inequalities which are the result as well as the cause of atmospheric circulation.