General Wind Forecasting Dynamics
Temperature differences cause pressure differences, which in turn cause air movements. The winds resulting from these differences vary considerably from one geographical location to another. The following sections provide a brief refresher of the forces behind air movement (wind), wind types, and local wind patterns.
Forces That Affect Winds
There are four basic forces that affect the directional movement of air: pressure gradient force (PGF), Coriolis force, centrifugal/centripetal force, and frictional force. These forces, working together, affect the direction air moves.
Pressure Gradient Force
The variation of heating (and consequently the variations of pressure) from one locality to another is the initial factor that produces movement of air or wind. The most direct path from high to low pressure is the path along which the pressure is changing most rapidly. The rate of change is called the pressure gradient. PGF is the force that movesair from an area of high pressure to an area of low pressure. The velocity of the wind depends upon the pressure gradient. If the gradient is strong (weak), the wind speed is high (low).
Coriolis Force
The Coriolis force is the “apparent” force that makes any mass, moving free of the Earth’s surface, appear to be deflected from its intended path. The Coriolis force deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, due to the earth’s rotation. Coriolis only affect winds direction and has no effect on windspeed. The strength of the Coriolis effect varies with latitude.
· At the equator the effect is zero.
· Maximum bending occurs at the poles.
Centrifugal and Centripetal Forces
Centrifugal force throws an air parcel outward from the center of rotation. Its strength is directionally proportional to the speed and radius of rotation. Centripetal force, equal in magnitude and opposite in direction to the centrifugal force, attempts tokeep the air parcel moving around a curved path (such as around curved height contourson a constant-pressure surface).
Frictional Force
An increase in friction causes a decrease in wind speed and subsequently a reduction in the Coriolis force. Consequently, the weaker Coriolis force no longer balances the PGF, and the wind blows across the isobars toward lower pressure. It may cause the wind to blow up to 50° across isobars over rugged terrain and 10° acrossisobars over water. The effect of frictional force reaches to about 1,500 feet above ground level (AGL) over smooth terrain and as much as 6,000 feet AGL over mountainous terrain.
LocalWinds
In the atmospheric circulation system, small-scale wind systems occur with the general circulation pattern. They are a result of the Earth’s rough surface and temperature differences between land and water. These small-scale circulations are frequently called local winds and have names that link them to the place where they occur. The absence of a strong pressure gradient is typically necessary for the development of most of these often thermally-induced winds. Besides the pressure gradient, surface temperatures determine whether the temperature gradient is sufficient to induce such circulations as the land/sea breeze, or in the mountain or valley breezes case, whether the insolation or radiation is sufficient to develop the breeze. With forced circulations like the fall (glacier) and foehn (chinook) winds, the proper orientation and spacing of the isobars (which is a direct result of pressure gradient) is necessary to develop the winds.
Let's now discuss your views on winds forecasting. Click on the SmartNotebook below to begin this discussion. I have given you all a start on a frontal system. I'd bet someone in this workshop can be a bit more artistic than that.
We will take a look at what the winds would be around a frontal system and how an Airman would forecast for an airfield that has this frontal system moving through. Ensure you all remember the wind concept above, you will need to teach these concepts and how they affect windflow.
After we are finished with this exercise, please click on the Voki below for a first day wrap.
