Meteorologists talk 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.

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.

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 online heat index calculators like those from the National Weather Service and the The Washington Post.

What is the windchill factor?

On the other end of the temperature spectrum is the windchill factor. Wind can cause sweat from our bodies to evaporate faster and can carry heat away from us quickly to make us feel colder.  Thus, for example, a temperature of 75 degrees can feel more like 65 degrees with windspeeds of 60 miles per hour.

How do you estimate a “feels like” temperature?

When a weather service makes a prediction about what a temperature will feel like, they typically include these basic measurements of the heat index and the windchill factor. A more complex estimate comes from the metric established in the 1990s and patented by Accuweather called the RealFeel Temperature. Their model includes heat index and windchill, but also other weather factors like cloud cover or UV index, sun intensity, visibility, and ground cover. They further incorporate expected heat transfer rates across an average sized human body with an average metabolism. And they assume the person is dressed appropriately for the weather.

Ultimately, “feels like” temperatures are an effort at understanding and quantifying the human response to temperature, a relationship with multiple complicating factors.

Ultimately, “feels like” temperatures are an effort at understanding and quantifying the human response to temperature, a relationship with multiple complicating factors. For starters, I can sit in a conference room in a scarf and sweater while my colleague next to me is comfortable in his t-shirt. Some of this difference comes down to our varying metabolisms—how fast we burn calories to produce heat and how fast that heat is dissipated over the surface area of our bodies. Our bodies may prefer different temperatures as well, like when a person is said to “run hot” or “run cold.”

So while there is still much estimation involved in determining “feels like” temperatures, they do a pretty good job of preparing us for what the weather has in store. And as we heard from meteorologist Morgan Miller in an earlier episode, local weather forecasters usually know what estimates work best for their region and can combine that knowledge with their personal experience to give you the best “feels like” estimate.