Explaining the Heat Index

June 18, 2018 0 Comments

Whenever it gets hot in the summer like it has been the last few days, we’ll often times not just talk about the temperature, but add in the heat index to our weather forecasts and social media posts. We usually keep the explanation short on TV and typically it goes something like “it’s currently 93° in Peoria right now, but the heat index is 101°.” But have you ever wondered exactly what the heat index is and how it is calculated?

The short and sweet definition of the heat index is how hot it feels outside. To get this number, temperature and humidity (measured in either dew point or relative humidity) are combined. In a way, this is similar to how the wind chill describes how cold it feels based on temperature and wind speed.

There is some science behind the concept of the heat index and why heat and humidity are used together.

When our bodies get warm, we begin to sweat. While it can be annoying sometimes, this actually serves as a vital function to help regulate our body temperature. The excess water on our skin evaporates, which is actually a cooling process. This then helps cool the body.

To put this into practice and explain the heat index, let’s do a quick thought experiment. Imagine two scenarios, both of them are 90°, but in one the dew point is 40° (dry air) and the other has a dew point of 70° (humid air). Even though the temperature is the same in both, the heat index for the first scenario is 86°, and the second has a heat index of 96°. This is because in the second scenario there is more water vapor in the air compared to the first one, which means sweat won’t be able to evaporate as easily, which in turn means we won’t be able to cool down as much.

So while yes, 90s are going to be hot regardless, humidity plays in important role in just how hot it really feels outside. Here is a full chart of heat index values based on temperature and dew point.

Once heat index values get into the 90s and above, prolonged time outside and/or insufficient ways of cooling down can become dangerous. Usually either Heat Advisories or Excessive Heat Warnings will be issued when heat index values become high. Be smart and be safe in any extreme heat.

-Chief Meteorologist Brian Walder

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