A cold morning is bad enough, but when it’s both cold and windy, it's tempting to just stay in bed. There’s something about the wind that makes a winter day feel downright oppressive, and—science to the rescue!—we have a way to help you understand just how miserable it’ll make you feel. It’s called the wind chill, and even though some cynical folks like to brush it off as a made-up scare number, it’s a real value that’s grounded in sound science.

The wind chill is a value that tells you how cold it feels to your body when you combine the actual air temperature with the wind speed. If the air temperature is 25°F and the wind is blowing at 10 miles per hour, for instance, the wind chill is 15°F; in other words, the combined chill of the cold air plus the breeze makes your body react the same way as if the air temperature were really 15°F.

National Weather Service

Scientists used detailed human studies to figure out how a cold wind affects your body’s ability to retain heat, and they came up with an equation to figure out what it feels like to your skin when it’s a certain temperature with a certain wind speed. Colder temperatures and stronger winds bring about colder wind chills. Once the wind chill reaches a certain level, though, it gets to be downright dangerous. If it’s exactly 0°F and the wind is blowing at 15 MPH, the wind chill at that moment is -19°F—if you’re outside without gloves in that kind of weather, it would only take 30 minutes for your hands to develop frostbite. As the wind chill gets colder, frostbite (and hypothermia) set in even faster.

Why does the wind affect how your body reacts to cold temperatures? Your body generates a lot of heat, and you lose a significant portion of that heat through exposed body parts like your hands, arms, and face. Some of the heat radiating away from your body lingers near your exposed skin sort of like a bubble. This bubble of warm air acts like a natural blanket, protecting you from experiencing the full effects of the cold air.

When the wind blows, however, it removes this layer of warm air around your skin, putting the full force of the frigid air in direct contact with your body. As your skin grows colder, your body has to work harder to generate enough heat to prevent injury. The longer your body is exposed to the cold air, the harder it has to work.

If you stay outside too long, your body will eventually start working to warm your internal organs instead of your extremities, chilling your arms and legs to the point of frostbite. Stay in the great outdoors unprotected a little longer and your internal temperature could drop low enough that you slip into hypothermia that might lead to organ failure or even death. 

Don’t ignore your friendly neighborhood weather person when he or she tells you that the wind chill is dangerously low. Even though it seems silly that meteorologists make a big deal about cold weather in the middle of the winter, sometimes the combination of the cold and wind makes it downright dangerous to be outside for any length of time without extensively covering every square inch of exposed skin. Covering up from head to toe isn’t always possible, though, so if you find yourself stuck outside when it’s so cold it could kill you, shielding yourself from the wind is a good way to stave off injury long enough to get inside unharmed. So if you're in one of the regions in the U.S. going through a brutal cold snap this weekend, be smart and stay warm.