Atmospheric Wind - 101

The weight of the air above the earth’s surface exerts a force or pressure on the surface which leads to the development of the winds which influence our daily weather.

The change in pressure measured across a given distance is called pressure gradient. When this force is directed from high to low pressure it is called the pressure gradient force and is responsible for triggering the initial movement of air.

As a result of the earth's rotation, air set in motion by the pressure gradient force undergoes a deflection from its path called the Coriolis force. Air moving from high to low pressure in the northern hemisphere is deflected to the right by the Coriolis force and to the left In the southern hemisphere. Slowly blowing winds will be deflected only a small amount, stronger winds will be deflected more and winds blowing closer to the poles will be deflected more than winds at the equator where the Coriolis force is zero.

Winds balanced by the Coriolis and Pressure Gradient forces are called Geostrophic winds. An air parcel at rest will begin to move from high to low pressure due to the pressure gradient force. The moving air parcel is then deflected by the Coriolis force, As the wind gains speed the deflection continues to increase until the wind blows parallel to the isobars and is then referred to as geostrophic. These winds occur at higher altitudes where they are not affected by surface friction and terrain features.

Since isobars are almost always curved and rarely evenly spaced, winds are now described as being in gradient wind balance. They still blow parallel to the isobars, but are no longer balanced by the pressure gradient and Coriolis forces, and do not have the same velocity as geostrophic winds.

Surface friction in the Boundary Layer 1 to 2 kilometers in height prevents wind from being geostrophic. Winds flow counter clockwise around the centre of a low in the northern hemisphere and with friction, the wind speed slows reducing the Coriolis force and the pressure gradient force inward becomes dominant. The rising air spiraling of into the center of a low pressure system (convergance) creates the probability of clouds, rain and storms to form.

Winds flow clockwise around a high pressure system in the northern hemisphere, but with surface friction the wind again slows down, reducing the Coriolis force and the outward pressure gradient force becomes dominant. In this case, though, the dominent pressure gradient is outward from the center of the high, the surface wind spirals away from the center (divergence) causing a sinking motion which suppresses cloud development and gives us clear skies.