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.
Meteorologists with the National Weather Service used Steadman's table to derive a simpler formula for heat index by creating a function that approximates its values (to within 1.3 degrees Fahrenheit) using only two variables—temperature and percent humidity. Since the formula incorporates all of Steadman's assumptions, how hot you feel may differ from the heat index reported on the evening news. For example, weather reports say that today's heat index in New York City is 106, and that the wind is blowing at 11 miles per hour. But the formula for heat index assumes that the wind is blowing at only 5.8 miles per hour—so the added breeze might make it feel cooler than what's been reported. (Unless it were really hot out—when it gets up into the high 90s, the wind actually makes you hotter.) Likewise, the further your dimensions are from 5 feet 7 inches and 147 pounds, the less likely you are to feel like it's 106 degrees.
Explainer thanks Lans Rothfusz of the National Weather Service.