Dear Tom,
What is the difference between dew point and relative humidity?
Thanks, Tracy
Dear Tracy,
Dew point is a direct measure of the amount of moisture in the air; the higher the dew point, the greater the moisture content of the air and vice versa. The dew point is also the air’s saturation temperature, so when the air temperature equals the dewpoint, saturation occurs and the relative humidity is 100%.
Relative humidity is the ratio of the amount of moisture actually in the air to the maximum amount of moisture the air can hold at that temperature.
On a day when the dew point remains constant, the relative humidity will change as the temperature changes, increasing as the temperature falls and decreasing as the temperature rises. On a typical day, the relative humidity will be highest around sunrise, and lowest during mid or late afternoon.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
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