The cycle of daytime heating and night time cooling explains why, under most circumstances, calm winds are near the surface at night.
The wind usually increases with height above Earth’s surface. The wind several thousand feet above the ground is almost always stronger than that experienced near the ground. Friction causes the wind close to the ground to move more slowly. Friction decelerates the wind in the same way a rough road surface slows down a bicycle.
When the sun sets, the ground loses the solar energy from the sun and continues to lose energy via emission of longwave radiation. This cooling ground conducts heat away from the air near the ground, causing that air layer to cool down 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. This is called a decoupling: This layer is no longer influenced by what is occurring above it.
On many calm nights, there is still wind blowing far overhead. When the sun is up, it warms the surface of Earth, which in turn warms the atmosphere above it. The warm air rises and the displaced air is replaced by the air above. These warm 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. This results in turbulence and mixing of the air near the ground.
Of course, if there is a low-pressure area or fronts in the region, the winds will likely blow day or night.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
