Weather Blog: What exactly is the heat index?

Posted at 11:18 AM, Jul 11, 2017
and last updated 2017-07-12 04:48:06-04

You often hear me talk about how warm the "heat index" will be in the afternoon. But perhaps you aren't quite sure what that means or how scientists get to that value?

In a nutshell, the heat index is the "feels-like" temperature when factoring in both relative humidity and the air temperature. Extremely high heat index values can be seen in areas like Oklahoma, Texas, or coastal states where humidity is typically very high during the hot summer months.

So what happens when you are out in the heat? Your body's natural response is to sweat. However, when the humidity and the temperature are both very high, evaporation is low. And evaporation is the cooling process you need to help regulate your body's temperature. In turn, if you are outside in the heat when humidity is high, it may be hard for you to cool yourself off. This could lead to heat exhaustion or heat stroke in extreme cases. 

The chart pictured above shows the correlation between humidity and temperature and gives a value for the "heat index" or "Feels like" temperature. If you know both values, you can easily follow this chart from the National Weather Service to see how hot is may feel in the afternoon. 

Something important to take note, the temperature is always recorded in the shade with light winds. So these heat index values (like the 103 degrees that is forecasted for today and Wednesday) is actually what it will feel like in the SHADE! If you are out in the direct sunlight, you can add a good 15 degrees on top of that!

When heat index values reach 105-110 degrees, the National Weather Service will typically issue heat watches, heat advisories, or excessive heat warnings.