Dear Tom,
Can you explain the cause of the very high humidity levels we experience in this area, year-round, day and night? Unusual, isn’t it, for the center of a continent?
Lee
Dear Lee,
Chicagos annual average relative humidity is 71 percent, and you might be surprised to learn our relative humidity runs a little higher during the winter (75 percent) than during the summer (68 percent).
The explanation for high winter humidity levels is that cold air holds much less moisture than hot air, so it takes less moisture to elevate winter humidities. Because the interiors of continents experience cold winter temperatures, relative humidities then tend to be high even though not much moisture is involved. In the summer, airborne moisture is plentiful (resulting in high humidity) because of high evaporation rates, intense plant growth and occasional infusions of humid air from the Gulf of Mexico.
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Tom Skilling is chief meteorologist at WGN-TV. His weather forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
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