Nov
14
2013

Wind speed tends to diminish after sunset...

Folks have noticed, and asked about how the winds pick up shortly after sunrise and calm down after sunset. It is the daytime heating of the ground by the sun that leads to this difference between calm nights and windy days.

The wind usually increases with height above the surface. The wind several thousand feet above the ground is almost always stronger than that experienced near the ground. Friction is a force that causes the wind close to the ground to move more slowly. Friction decelerates the wind in the same way a rough road surface slows you down on a bicycle.

On many calm nights, there is still wind blowing far overhead. When the sun is up, it warms the surface of the Earth, which in turn warms the atmosphere above it. The warm air rises and the displaced air is replaced by the air above. These thermals mix up the air, bringing the faster moving air from above down near the surface. As the daytime heating goes on, more air from above is mixed down and the wind speed picks up.

When the sun sets, the ground cools down. This cooling ground conducts heat away from the air that it touches and so the air near the ground cools down, cooling faster than the layers higher in the atmosphere. This creates a stable area with cool air near the ground and warmer air above. As the word suggests, “stable” means it is difficult to move the air layer, keeping the fast moving air above from mixing down to the surface.

Of course, if there is a low pressure area in the region, the winds will blow day or night.

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