HAVE you noticed that even though it is windy during the day, the wind usually subsides at night? When winds die down after dark, it is usually because those winds were driven by pressure differences caused by the sun's heat.
Wind is the result of air flowing from an area of high pressure to an area where pressure is lower. One of the common causes of pressure differences is differential heating of the Earth's surface by the sun, said San Jose State University meteorologist Jerry Steffens.
For example, in the Bay Area of San Francisco, the winds typically come off the ocean. That is because, during the day, the sun warms the land and the air over it.
The warm air expands and rises, creating an area of low atmospheric pressure over the land. Cool, dense air from over the ocean rushes inland into that low pressure zone, creating our onshore breezes.
After nightfall, the land and the air over it, cool off, and as the air cools, its atmospheric pressure increases. Without the inland low-pressure zone, the sea breezes stop blowing.
In some coastal areas, Steffens said, the land gets colder than the water at night, and the breezes reverse, with colder air from over the land blowing out to fill the lower pressure zone over the relatively warm ocean.