Relative humidity and moisture in the air

Dear Tom,
The relative humidity was 62 percent when the temperature in Chicago was 23 degrees below zero a few weeks ago. I always thought during bitter cold the humidity would be much lower. Why is it so high with such low temperatures?
Joe Williams, Chicago

Dear Joe,
You have expressed a common misconception. Relative humidity is the amount of water vapor in air divided by the maximum amount the air can contain at a given temperature, expressed as a percent. Relative humidity depends on the moisture content of air and also the temperature of the air. What you don’t realize is that as the air temperature falls, the ability of air to hold water vapor decreases sharply. Therefore, the relative humidity increases considerably as the temperature declines if the moisture content of the air remains constant (and it usually remains constant unless fog develops).

additional for blog:

The dew point temperature is the temperature at which relative humidity will be 100 percent, given the actual amount of water vapor in the air. Here are some relative humidity values at various temperatures and dew points:

 

Note that at a temperature of 23 degrees below zero, the relative humidity will be 63 percent when the dew point temperature is 32 below. Those are the temperature and relative humidity values, approximately, that were specified in the question.

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