Posted on Jun 22, 2012 in Science & nature
I see the “heat index” on my weather reports sometimes, and would like to know what this means and how it’s calculated.
So you’re watching the latest heatwave news and you see that the temperature is 95°F. Bad enough, right? Then you see this “heat index” or “feels like” thing that says it’s 102°F. So which is it?
The “heat index” is a measure determined by a combination of the actual air temperature along with the relative humidity, which attempts to determine the equivalent temperature that we humans perceive.
In short: How hot does it actually feel?
Let’s break down the concept with some easy science. The human body cools itself by sweating: When you sweat, the water released from your skin evaporates, and helps carry heat away from your body. The more humid it is — that is, the more water vapor there is in the air — the less sweat is able evaporate. (Read more of the details here.) The end result is that the more humid it is, the hotter it feels. So when people (like me) in Arizona tells you, “at least it’s a dry heat,” they aren’t just repeating an old, trite saying — 100°F and 7% humidity actually feels cooler than the actual ambient air temperature.
As for how it’s calculated, well, here’s the formula:
T= ambient dry-bulb temperature (in degrees Fahrenheit)
rh= relative humidity (in percent)
Myself, I prefer using the guide at the top of the page, or the heat index calculator at noaa.gov. It’s slightly easier.
Or you can just check one of the many handy weather sites, and it will give you the “feels like” right under the actual temperature.
As either the temperature or the heat index (or both) climb north of 100°F, your body is less able to dissipate heat. It’s important to stay hydrated (with water, caffeine and alcohol are diuretics and will just hasten your dehydration), take frequent breaks, and seek shade and cool areas as much as possible when working outside.
If you feel faint or dizzy, start gettng muscle cramps, and/or have a headache, you are suffering from heat exhaustion. Stop what you are doing, move to a cooler area, and drink some cool water. If you stop sweating, seek medical attention immediately. This is a sign of heat stroke and can be fatal if not treated.
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