A reader wrote us last week with two questions prompted by careful observation: 1. Why does the wind speed tend to decrease after sunset? 2. Why does this effect seem less pronounced in late autumn and winter?
As it turns out, the answers are somewhat related to one another.
The wind speed tends to decrease after sunset because at night the surface of the Earth cools much more rapidly than does the air above the surface.
As a result of this difference in cooling ability, it doesn’t take long for the ground to become colder than the air above it.
The air in close contact with the ground — say in the lowest 300 feet of the atmosphere — then becomes colder than the air above it.
This circumstance leads to the development of what is known as a temperature inversion. Inversions dramatically reduce the amount of mixing that occurs between different vertical layers of the atmosphere. As a consequence, once the inversion sets up (after sunset), it is much harder for fast-moving air above the ground to mix down to the surface, where it could appear as a gust of wind.
During the day it is very easy for the air to mix and cause surface gusts.
If there is a low pressure area or storm in the region the winds will blow day or night. Late autumn and winter bring the strongest storms of the year to our area. These storms have cloud and temperature structures that can often overrule the tendency for inversions to set up at night.
The much stronger winds near the strong storms, coupled with a tendency to avoid inversions near storms, mean that windy nights are more common in the cold season.