With record-high temperatures spiking in the U.S. and Europe, and the planet getting hotter in general, it's a good time to answer some questions about the heat index—and why high humidity makes us feel like a limp rag.
Why does humidity make us feel hotter?
Let's talk about sweat. As you may remember from your high school biology class, one of the ways your body cools itself is by sweating. The sweat then evaporates from your skin, and it carries heat away from the body as it leaves.
Humidity throws a wrench in that system of evaporative cooling. As relative humidity increases, the evaporation of sweat from your skin slows down. Instead, the sweat just drips, which leaves you with a damp T-shirt and none of the cooling effect. When the humidity spikes, we effectively lose a key tool that would normally cool us down.
What's relative about relative humidity?
Humidity refers to the amount of water contained in the air. However, as the air’s temperature changes, so does the amount of water the air can hold. (Air can hold more water vapor as the temperature heats up.) Relative humidity compares the actual humidity to the maximum amount of water vapor the air can hold at any given temperature.
What's the deal with the heat index?
While the notion of humidity making days feel warmer is painfully apparent to anyone who has ever been outside on a soupy day, our current system owes a big debt to Robert G. Steadman, a textile researcher. In a 1979 paper called “An Assessment of Sultriness, Parts I and II,” Steadman laid out the basic factors that would affect how hot a person felt under a given set of conditions, and meteorologists soon used his work to derive a simplified formula for calculating heat index.
The formula is long and cumbersome, but luckily it can be transformed into easy-to-read charts available online. You just need to know the air temperature and the relative humidity, and the chart will tell you the rest.
Is the heat index calculation the same for everyone?
Not quite, but it’s close. Steadman’s research was founded on the idea of a “typical” person who was outdoors under a very precise set of conditions. Specifically, Steadman’s everyman was 5 feet, 7 inches tall, weighed 147 pounds, wore long pants and a short-sleeved shirt, and was walking at just over three miles per hour into a slight breeze in the shade. Any deviations from these conditions will affect how the heat/humidity combo feels to a given person.
What difference does being in the shade make?
Quite a big one. All of the National Weather Service’s charts for calculating the heat index make the reasonable assumption that folks will look for shade when it’s oppressively hot and muggy out. Direct sunlight can add up to 15 degrees to the calculated heat index.
How does wind affect how dangerous the heat is?
Normally, when we think of wind on a hot day, we think of a nice, cooling breeze. That’s the normal state of affairs, but when the weather is really, really hot—think high-90s hot—a dry wind actually heats us up. When it’s that hot out, wind actually draws sweat away from our bodies before it can evaporate to help cool us down. Thanks to this effect, what might have been a cool breeze acts more like a convection oven.
When should I start worrying about high heat index readings?
The National Weather Service has a handy four-tiered system to tell you how dire the heat situation is. At the most severe level, when the heat index is over 130°F, that's classified as "extreme danger" and the risk of heat stroke is highly likely with continued exposure. Things get less scary as you move down the ladder, but even on "Danger" days, when the heat index ranges from 105°F to 130°F, you probably don’t want to be outside. According to the service, that’s when prolonged exposure and/or physical activity make sunstroke, heat cramps, and heat exhaustion likely, while heat stroke is possible.
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This article has been updated for 2022.