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Burning questions on the heat index

By Ethan Trex,
Children cool off in the National Gallery of Art Sculpture Garden in Washington, DC.
Children cool off in the National Gallery of Art Sculpture Garden in Washington, DC.
  • With spiking temperatures and humidity levels, the heat index is given a closer look
  • Humidity keeps sweating from being an effective way to cool off, making us feel hotter
  • Direct sunlight can add up to 15 degrees to the calculated heat index
  • On days when the heat index ranges from 105 to 130, you don't want to be outside

( -- Is it really not the heat, but the humidity? With temperatures spiking around the country and weathermen warning of days where the heat index surges into the triple digits, we thought it might be a good time to answer some questions about the heat index.

Why does humidity make us feel hotter?

To answer that question, we need to talk about getting sweaty. As you probably remember from your high school biology class, one of the ways our bodies cool themselves is by sweating. The sweat then evaporates from our skin, and it carries heat away from the body as it leaves.

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Humidity throws a wrench in that system of evaporative cooling, though. As relative humidity increases, the evaporation of sweat from our skin slows down. Instead, the sweat just drips off of us, which leaves us with all of the stinkiness and none of the cooling effect. Thus, when the humidity spikes, our bodies effectively lose a key tool that could normally be used to cool us down.

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What's relative about relative humidity?

We all know humidity is the amount of water the air contains. 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.

Whose idea was the heat index?

While the notion of humidity making days feel warmer is painfully apparent to anyone who's ever been outside on a soupy day, our current system owes a big debt to Robert G. Steadman, an academic textile researcher.

In a 1979 research 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. Today your weatherman just needs to know the air temperature and the relative humidity, and the chart will tell him the rest.

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Is the calculation the same for everyone?

Not quite, but it's close. Steadman's original 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 5'7" 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 certain person.

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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 the 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.

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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, the day is classified as "extremely hot," 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 "very hot" days when the heat index ranges from 105 to 130, 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.

What's the highest heat index that's been recorded?

In their book Extreme Weather Christopher C. Burt and Mark Stroud tackle this topic. The authors explain that the worst spots in the world for a combination of heat and humidity are along Ethiopia's Red Sea coast, along Somalia's Gulf of Aden coast, and around the Persian Gulf.

July 8, 2003, was a particularly punishing day in Dharan, Saudi Arabia. That day the temperature hit 108 Fahrenheit with a dew point of 95 degrees. This combination led to a heat index in the neighborhood of 155 to 160.

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