What is ‘wet bulb’ temperature? How heat and humidity combine to dangerous levels

Extreme levels of heat and humidity are forecast to spread across the country over the next two weeks, blanketing the Southwest, Midwest, and East Coast with high temperatures beginning this June 19 and 20, and extending through the next several days.

High heat alone is a risk to humans, but heat and humidity together make an especially dangerous combination. Humidity has to do with the concentration of water vapor in the air, that cloying stickiness that separates it from “dry heat.” When humidity is high...