ASK THE METEOROLOGIST
“Why does the wind typically die down in the evening hours?”
Allison Waters, Mae Richardson
The main reason the winds tend to die down in the overnight hours is because there is no sun. At nighttime, the surface and objects at the surface emit longwave radiation. This cools not only those objects but the air near the surface. Cold air is not able to rise (we all know warm air rises) and for this reason, there is little mixing of the air — leading to calm winds.
During the day though, the sun heats the surface and the sun helps to make the ground warm. As the air warms through the day, tiny parcels of air begin to rise when they become warmer than the air around them. These rising parcels of air are known as thermals. When the warm air rises, it leads to the mixing of air in our atmosphere. This rising air will also start to interact with the very fast current of air way up in our atmosphere known as the jetstream. These are all contributing factors to the wind that develops from diurnal heating (from the sun).
These applications do not apply when a storm system is moving through our area. Large pressure differences and temperature differences also contribute to the wind. The stronger the difference in temperature or pressure (differences in density also), the stronger the wind will be. It is a proportional relationship. When the ground is heated by the sun during the day, this creates a large vertical temperature difference from the surface to the cold air aloft. There are horizontal temperature gradients that make the wind too — warm & cold air masses adjacent to each other.