Dear Tom,
Is the humidity higher or lower before or after it rains?
Danny Marchan and Marchelle Thurman
Dear Danny and Marchelle,
Most of the time the relative humidity is higher after it rains than before. Relative humidity is computed from the temperature and the dew point. The larger the difference between the air temperature and the dew point, the lower the humidity. However, when the temperature and dew point converge the humidity increases, reaching a maximum of 100 percent when the two are equal. Air temperature usually drops after precipitation begins–a result of cooler air from the cloud level being carried down to the surface with the rain as well as from evaporation of moisture in the lower levels of the atmosphere. It is not uncommon for the humidity to increase from 40-50 percent before a rain to 90 percent or higher after the rain has ended.
———-
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.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
