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Dear Tom,
It seems as if Chicago’s air is humid much of the time. True?

Terry Yungsten

Dear Terry,
Chicago’s annual average relative humidity is 71 percent, with winter humidity averaging 75 percent and summer averaging 68 percent. Winter relative humidity is higher because of a peculiarity of air: the higher the temperature, the greater the amount of moisture the air can contain; at lower temperatures, air can contain less moisture. It takes much more moisture to saturate hot air than it does to saturate cold air. Consequently, less moisture is required to elevate humidity in the winter, and so winter humidity tends, on average, to be higher than summer humidity. However, airborne moisture is more plentiful in the summer because of high evaporation rates, plant growth and occasional infusions of humid air from the Gulf of Mexico.