On a recent cold morning in Washington DC, I looked up the weather. The temperature was 38°F, but with winds occasionally gusting to 8 miles per hour, the wind chill was officially 32°F. Freezing.

Except it wasn't actually freezing. There weren't any puddles on the streets turning into ice. The precipitation that was falling was clearly coming down as rain. And Weather Underground reported that it "felt like" 36°F. The wind chill indicator gave a misleading picture of what things were really like outside.

There's a good reason for that: Wind chill simply doesn't mean what most people think it means.

The wind chill index is designed for a very precise, very narrow purpose. "It was developed solely to assess the risk of frostbite on unclothed parts of the body," says Krzysztof Blazejczyk, a Polish researcher who studies the thermodynamics of the human body. In other words: If the temperature is 38°F and the wind chill is 32°F, that means you'd develop frostbite on exposed skin just as quickly as you would if the temperature was 32°F and there was no wind. That's it. This formula also assumes you'll be walking directly into a steady wind continuously, with your face totally bare.