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What is heat index, how is it calculated?

The "feels like temperature" can be calculated with this chart if you know the current relative humidity and air temperature (Courtesy of NWS).

With summer in full swing, meteorologists often use the phrase “heat index” or tell you the temperature outside feels like a higher degree, but what exactly does that mean?

The heat index is a combination of temperature and relative humidity. It is sometimes called the “apparent temperature” because the heat index reflects how the current temperature feels on the human body.

On a typical hot summer day, sweat helps our bodies cool down by evaporating off our skin and into the air. Evaporation happens when liquid turns into gas, which requires more energy. That energy is taken from our bodies in the form of heat, which results in cooling. However, if there is a lot of moisture already in the air, we cannot sweat as effectively, so our bodies are unable to perform it’s natural cooling process and we feel warmer in a more moist environment.

Meteorologists use “relative humidity” to show the ratio of moisture in the air to the maximum amount of moisture possible. Relative humidity is measured as a percentage – the higher the percentage, the more humidity. If the dew point and the air temperature are the same, relative humidity is 100% and it's incredibly muggy outside.

There is a direct relationship between air temperature and relative humidity when it comes to the heat index – if either the air temperature or the relative humidity increase, so does the heat index.

You only need to know the relative humidity and air temperature to determine the heat index. The chart above from the National Weather Service shows how air temperatures can take a quick and dangerous turn with a little humidity.

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