Dear Tom,Is it humid in winter? If...


Dear Tom,

Is it humid in winter? If so why don't we feel it? Why isn't it sticky outside?

Laura Andel, LaGrange, Il

Dear Laura,

There are two primary measures of atmospheric moisture: relative humidity and dewpoint. Relative humidity, a percent of saturation, is temperature dependent. Since cold air holds substantially less moisture than warm air, it is easier to saturate a parcel of cold air. Consequently, relative humidity is actually higher in winter than in summer (76% vs. 66%, according to climatology).

Dewpoint is the temperature to which air must cool to become saturated, without regard to changing temperatures. For that reason, dewpoints better represent the actual moisture in the air, and are preferred by meteorologists. At dewpoints above 65 (degrees), people begin to feel sticky. Since dewpoints cannot exceed the temperature (or the moisture would condense out), the uncomfortable 65 (degrees) threshold requires temperatures at least that warm, and these are seldom reached in winter.



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