How Do Meteorologists Calculate 'Feels Like' Temperature?

On the heels of another visit from the polar vortex in parts of the northern hemisphere and the extreme heat in Australia and other parts of the southern hemisphere, meteorologists have been talking not only about what the temperature will be outside, but also what that temperature “feels like.” Why are these numbers sometimes so different? How do meteorologists know what the temperature will feel like to you?

Although numeric, temperatures are not universal. Most of us have a sense of what a temperature of 75 degrees Fahrenheit means, but add a brisk wind and you will feel colder. Raise the humidity and the temperature your body experiences feels a lot higher too. Precipitation and cloud cover also play a role in making that 75 degrees a more relative experience. So how are “feels like” temperatures calculated?

What Is the Heat Index?

A simple measure of how a temperature of 85 degrees Fahrenheit, for example, will likely feel different in Houston, Texas versus Los Angeles, California is the heat index. The heat index combines the temperature and a measure of the relative humidity to give a better indication of how hot that temperature will actually feel.

On a hot day, our bodies sweat to cool down. In a low humidity, dry environment, that sweat is quickly evaporated into the air leaving us feeling effectively cooled. In humid locales, the air is already full of moisture and so our sweat evaporates more slowly (or not at all), leaving us feeling hotter.

Cold air can hold less water vapor so as temperatures decrease, eventually a temperature called the dewpoint is reached when the air is too cold to hold anymore moisture. Since water vapor can no longer remain in the air as humidity, it instead begins to condense on surfaces.  The temperature and the dewpoint together give the relative humidity, a measure of how much moisture is in the air relative to how much moisture the air could potentially hold. That relative humidity, combined with the temperature, then determines the heat index or an estimate of how hot the air actually feels.

So 85 degrees at 10% humidity, the temperature feels closer to 79 degrees, but at 90% humidity, it will feel closer to 100 degrees outside.  You can find the heat index in your city without actually doing any math thanks to...

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