Explainer

How Does the Heat Index Work?

Pretty well, if you’re 5-foot-7.

A national heat wave is expected to reach its peak today, as temperatures along the East Coast rise into the 90s and 100s. To make things worse, the high humidity has produced heat indexes of between 110 and 120. What’s the “heat index,” and how does it work?

The heat index tells you how hot it feels at a given humidity. Moist air feels hotter than dry air because it makes sweating less efficient. On a hot, dry day, your sweat will evaporate quickly and cool your skin; under humid conditions, sweat evaporates more slowly and doesn’t do as much. Just as the wind chill attempts to measure how cold it feels under certain wind conditions, the heat index tries to measure how hot it feels given the humidity.

The formula for heat index is based on work completed in the late 1970s. R. G. Steadman wrote a paper called “The assessment of sultriness,” in which he used a list of 20 factors to compute how hot you might feel on a given day. These factors included the rate at which you sweat, the type of clothes you’re wearing, the surface area of your body, and what you happen to be doing.

To isolate the effects of temperature and humidity on the perception of heat, Steadman invented a typical situation: A person who’s 5 feet 7 inches and weighs 147 pounds walks at about 3.1 miles per hour in a light breeze, wearing long pants and a short-sleeved shirt. Then Steadman filled out his 20 variables with information from this scenario and figured out how hot his fictional person would feel at different outside temperatures and levels of humidity. He put the results in a table: Higher humidity would make his exemplar feel hotter, while drier conditions would make him feel cooler than it really is. For any given temperature, there is a percent humidity at which the weather “feels” exactly as hot as the thermometer indicates.