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Dear Tom,

I have always thought that at very low temperatures, say zero degrees or lower, the air would be dry. However, I have observed that the relative humidity is usually quite high when temperatures are low. Why should that be?

Jimmy Milson

Dear Jimmy,

First, let’s define relative humidity. Relative humidity is the amount of water vapor in the air divided by the maximum amount of water vapor the air can hold at that temperature, expressed as a percent of saturation. For example, if air at a given temperature contains only one-quarter of the moisture is could actually hold at that temperature, its relative humidity would be 25 percent.

Relative humidity depends on the amount of water vapor in the air and also on the temperature of the air. But there is a catch: the maximum amount of water vapor the air can hold decreases sharply as the air temperature drops. Expressed differently, the relative humidity increases considerably as the temperature falls if the water vapor content of the air remains constant.

For example, a temperature of 30 degrees and a dew point (measure of water vapor present) of 15 degrees yields a relative humidity of 53 percent, whereas a temperature of 0 degrees with a dew point of 0 gives a relative humidity of 100 percent, but the actual water vapor content of the air is much lower.