Community Corner

So, What's a Heat Index?

We asked a professor of atmospheric and oceanic science.

Over the past month or so, we've been bombarded by heat and humidity, and you can't read a weather report without seeing a reference to the "heat index," a number calculated to determine what it "feels like" outside.

But how do forecasters determine how heat feels? We asked Eugenia Kalnay, distinguished professor of atmospheric and oceanic science at the University of Maryland.

"It's a combination of the actually temperature and the effect of humidity," Kalnay said. "Many other factors go into the calculation as well."

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The heat index is designed to reflect how the presence of moisture in the air changes the effectiveness of your sweat, Kalnay said. The more humid it is outside, the slower your sweat evaporates, making it harder to cool your skin.

So, how do they get the number? According to a National Weather Service document, provided by Kalnay, the heat index is calculated using a combination of equations that form a model.

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In order to determine the heat index, forecasters input dozens of parameters into the above-mentioned model, using average values for things like vapor pressure, sweating rate, metabolic activity and size of a person (which, if you were wondering, averages out to 5'7" and 147 pounds, according to NWS).

In addition, forecasters must account for heat lost due to breathing, skin's resistance to heat transfer and clothing fabric's respiration, among other variables. Put all those values into the model and, voila, a head index is born.

Of course, that kind of research and computation isn't always convenient, so the NWS used a multiple-regression analysis to compute average values for each variable and create a more user-friendly way to calculate the approximate heat index.

If you're the do-it-yourself type and want to know how it feels outside, you can plug in the day's temperature for T and percent humidity for R in the following equation:

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x(10^-3)(T^2)
- 5.481717x(10^-2)(R^2) + 1.22874x(10^-3)(T^2)(R) + 8.5282x(10^-4)(T)(R^2)
- 1.99x(10^-6)(T^2)(R)

Or, if complex algebra isn't your thing, you can just check the forecast.


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