Could you explain what the dew point temperature is?

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Dear Tom,
Could you explain what the dew point temperature is?
Jan Mildras, Chicago

Dear Jan,
Dew point temperatures aren’t well understood by people outside meteorology (and other related fields). Dew points provide a measure of moisture in the air. They tell is the temperature at which the air holds all the moisture it can — the temperature at which the relative humidity reaches 100 percent. As air cools, its temperature falls but, other things being unchanged, its dew point remains stable, until, when the temperature and dew point are equal, the relative humidity hits 100 percent. In official weather records dating from Nov. 1, 1870, to the present, Chicago’s highest dew point temperature reading stands at 83 degrees, recorded at 7:58 and 8:58 a.m. on July 30, 1999. 

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