Dear Tom,
Someone from El Paso, Texas said they had a negative humidity reading there. Is that possible?
Eric Swansen, Elmhurst
Dear Eric,
It is impossible to have a negative humidity. Relative humidity is a ratio (expressed as a percentage) of the amount of water vapor actually in the air compared to the maximum amount of moisture the air could hold at that particular temperature. Relative humidity theoretically ranges from 0% to 100%, but since air always contains some moisture it never actually reaches 0%. The person was most likely talking about a negative dew point, one lower than 0 (degrees) F. The dew point is the temperature that the air must be cooled to, for saturation to occur. Chicago frequently has negative dew points in winter, but in a dry climate like El Paso they can occur with readings in the 50s or 60s, resulting in a relative humidity less than 10%.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
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