Dear Tom,
I’m reading a book that references a killer heatwave in India where the “wet bulb” temperature exceeded 93 degrees. Why is this the killer temperature threshold?
—J. Klonowski, Warrenville
Dear J.,
The wet-bulb is the reading when the thermometer bulb is covered by a water-soaked cloth and ventilated, which usually yields a reading lower than the air temperature, a result of evaporative cooling. That temperature drop is used to compute the dew point and relative humidity. When the humidity is 100 percent, the air temperature, wet-bulb temperature, and dewpoint are all the same. The human body is cooled by sweat evaporation, and when the wet-bulb reading exceeds 90 degrees, the human body will overheat, and eventually shut down around 95 degrees. About 750 people perished in Chicago’s deadly July 1995 heatwave when the wet-bulb readings peaked at 83 degrees.