How do dew points and humidity levels work in subzero temperatures?

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Dear Tom,

How do dew points and humidity levels work in subzero temperatures?

Sarah Tammen-Gruber,
Essex, Ill.

Dear Sarah,

The dew point is the temperature to which air must be cooled for it to become saturated with water vapor. However, there is nothing special about subzero temperatures. They are a peculiarity of the Fahrenheit temperature scale. (For example, if 200 degrees were added to the Fahrenheit scale, then zero degrees would become 200 degrees on the new scale and your question about zero degrees would refer to temperatures so low that they never occur on the Earth’s surface.)

If the temperature is 20 degrees with a dew point of 0 degrees, its relative humidity (RH) would be 41 percent. If the dew point were 20 below, the RH would fall to 16 percent (with the temperature still at 20 degrees); at a dew point of -40, the RH would be 6 percent.

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