What You Didn’t Know About the Heat Index

In these relentless summer temperatures, safety is of course a primary concern. (For information about maintaining a safe workplace in the heat, and on recognizing and treating heat illnesses, see our May blog post.)

It’s also important to plan properly, and if you’re like many people you check the forecast to see what kinds of temperatures you should expect. Bear in mind, though, that the high temperature is only part of the story. Simply put, the temperature tells you how hot it is outside, but for how hot it feels, you need to consult the heat index.

The heat index is also referred as the “apparent temperature,” and like the wind chill in winter, it attempts to account for factors other than the actual temperature. In this case, the main contributing factor is humidity. You’ve heard the saying, “It’s not the heat, it’s the humidity,” and there’s a reason it’s true: Higher humidity levels reduce evaporation of sweat and the cooling effects that go with it, so it really does feel hotter.

Here’s what you might not have known about the heat index, according to the National Weather Service:

While relative humidity is the main factor in the difference between temperature and heat index, the dew point can be a better predictor. The dew point is the temperature the air needs to be cooled to in order to achieve relative humidity of 100%. If the dew point is high, it will likely feel significantly hotter than the actual temperature, even if humidity is fairly low. Again according to NWS, a dew point between 55 and 65 will feel “sticky,” while 65 and above will feel “oppressive.”

The other, even more vital, thing to know about the heat index is that it’s based on being in the shade with light winds. So if you see a heat index of, say, 105 degrees, know that it will feel even hotter in the sunshine.

The advice for extreme temperatures remains the same: Slow down and avoid strenuous activity, dress appropriately, drink plenty of water (and avoid alcohol), and stay in air-conditioned spaces as much as possible. Note also that proteins and some other foods increase your metabolic heat production and also your water loss. Take extra care to stay hydrated.

Questions about workplace health and safety? Contact Heart to Beat, LLC.

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