Dear Tom,
How low did the relative humidity get during the recent excessive heat episode in the Southwest?
—Paul Kubecka, Berwyn
Dear Paul,
During the heat wave, Needles, Calif., logged a temperature of 119 degrees with a dew point of minus 11, yielding a relative humidity of 0.8 percent, reports meteorologist Jeff Masters of wunderground.com. As low as that is, even lower values have been observed. Masters cites two incredibly low relative humidities that vie for the world record. On June 20, 2017, Safi-Abad Dezful, Iran, saw a temperature of about 116 and a dew point of minus 28, and the humidity dropped to 0.36 percent, virtually the same as a 0.33 percent value recorded at Needles on May 4, 2014, with a temperature of 102 and a minus-36 dew point. Chicago’s lowest humidity is 13 percent, logged on several occasions, most recently April 14, 2015.
