It’s not just the heat; it’s the humidity!

Here’s why the heat index is important

It’s not just the heat; it’s the humidity!
Heat Alerts on Monday, August 12, 2009

SOUTH MISSISSIPPI (WLOX)- We sure know what is means to be “hot and humid” in the South! The heat has been brutal this past weekend and early this week. Not only have temperatures been in the 90s, but the heat index has been well over 100. What exactly is the heat index? We’ll take a closer look at this important number.

We calculate a value called the “heat index” to know how much the humidity will impact on our bodies. This value is the temperature it feels like to the human body when you factor in the air temperature and humidity. We also refer to it as the “feels like” temperature or apparent temperature. From this chart, you can see that higher humidity values at the same temperature gives us a higher heat index.

Courtesy of NOAA
Courtesy of NOAA

Our bodies try to keep us cool by sweating. When sweat evaporates from our skin, it removes heat from our bodies. However, higher humidity makes sweat much more difficult to evaporate. Most of the sweat remains on our skin. Our bodies can’t get rid of excess heat, and our body temperature increases. We include the heat index in our forecast to let you know how much more difficult it is for our bodies to stay cool. The higher the heat index, the greater the risk we have for heat illnesses like heat exhaustion or heat stroke. The heat index is also used by the National Weather Service to issue Heat Advisories, Watches, and Warnings.

Courtesy of NOAA
Courtesy of NOAA

The heat index isn’t an arbitrary number. There is a specific equation that takes into account the relative humidity and air temperature. It also is measuring the heat index for a shady location. In the sun, the heat index may be 10-15 higher than the calculated amount. On the other hand, very low humidity can cause the apparent temperature to be lower than the air temperature. Why? When the humidity is extremely low, your sweat can evaporate quickly. You can calculate the how the humidity impacts the heat index right here.

For more information about the heat index and heat safety, check out these resources from the National Weather Service

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