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Heat Index

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what does the heat index number mean and also what is all the haze we are seeing today? thanks! Kirk T.


Since Kirk sent this a few days ago, the haze was actually smoke from fires to the west of Utah and it was nasty! Some of it has cleared out and actually, our air quality has since improved.

The heat index is number, just like a temperature. It is different than regular temperature and factors in the moisture in the air. As we know, living here in Utah, with our low dewpoints, a temperature of 90 with little moisture in the air feels great, but go to a place like Louisiana, when the high hits 90 and the dew point is at 68 degrees, it feels terrible!

The reason why it feels awful out there, is because that moisture in the air isn't evaporating from your skin as readily. Evaporation is a cooling process. When you step out of the swimming pool or the shower, you feel cool, that water on your skin is evaporating, and cooling you off. If there's a lot of moisture in the air, the moisture doesn't evaporate as quickly, and you stay warm. If you're in the bathroom and it's all steamy, you step out of the shower, you might not feel as cool, that moisture on your skin, isn't evaporating so you don't feel cooler.

The same goes for heat index. When it's more humid outside, we don't cool off as fast and it feels warmer than it actually is. The heat index or HI numbers are useful so we have a guide on when the temperatures get dangerous. When the heat index is above 105 you put your body in danger doing physical activity.

There's an equation to calculate the heat index and it's big and scary and long, but if you're down with some basic Algebra then you could have some fun with it. Since the birth of modern technology, we now have a table that factors that equation in, so we don't have to calculate the heat index every day when the temperature changes. There's a link to a heat index table on the right.

Answered by KSL Meteorologist Dina Freedman.

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