When a record-breaking winter storm hit the northeast earlier this year, temperatures at the Mount Washington Observatory in New Hampshire plummeted to -47°F, but when the observatory factored in the 127 mph wind gusts battering the weather station, they produced a wind chill temperature of -110°F.

Wind chill, mentioned in morning weather reports and noted in weather apps, is a measure of how cold the air feels on your skin. And, it’s an important forecast for public safety.

People exposed to -10°F and light winds can experience frostbite in 30 minutes, but increase those winds to more than 60 mph and frostbite can occur in under five minutes.

How do meteorologists calculate wind chill?

When a person stands outside in the cold, their body begins to lose heat. But just like a person blowing heat away from a hot bowl of soup, cold winds whisk away body heat more quickly, making it feel colder outside.

Calculating wind chill can help people better prepare for harsh, outdoor conditions. Cold weather can cause hypothermia when the body’s temperature falls below 95°F, or frostbite when body tissue freezes and could be permanently damaged.

Meteorologists calculate wind chill using this formula calculated by the National Weather Service (NWS) that considers wind speed and air temperature, though other factors like a sunny day can make the temperature feel warmer.

Why did we use it?

The term “wind chill” was first coined in 1939 by geographer and Antarctic explorer Paul Siple. With his fellow explorer, Charles Passel, the two experimented with how long it took to freeze water under different air and wind conditions. Using this data, they created formulas to determine how wind might influence how the temperature feels on the skin.

Over time, their formulas have been updated with more precise computer models and experiments with humans.

A study published in 2002 exposed six male and six female subjects to different temperatures and wind speeds and measured heat loss through sensors on their face.

While these experiments helped scientists establish baseline data on how heat leaves the body in healthy adults, some populations including children, the elderly, and adults with health issues face a greater risk from wind chill.

Other ways to measure how it feels outside

The formula used by the NWS to determine wind chill is not the only method used to calculate how environmental conditions influence how hot or cold the temperature feels.

Weather forecasting service AccuWeather has its own proprietary index called the ReelFeel temperature. According to AccuWeather’s chief meteorologist Jonathan Porter, their index factors in weather conditions that NWS’s wind chill does not, such as dew point, cloud cover, and precipitation.

Another model, more commonly used in Europe, is called the Universal Thermal Climate Index, and it also considers conditions like humidity and sunshine.

The NWS uses only air temperature and wind speed in their wind chill estimate to provide a snapshot of the most influential weather conditions that influence how the temperature feels, using the simplest calculation, according to Michael Muccilli, the NWS Winter Program Coordinator.

How to stay safe when the wind chill drops

Wind chill and other estimates of how cold it feels outside are ultimately forecasted to help inform people about potentially deadly weather.

Stay inside when there’s risk of frostbite or hypothermia, but if you must go outside, plan accordingly. Dress in layers, as many as three or more for extreme cold. Cover extremities like fingers and toes, wear a hat to prevent heat loss from your head and make sure outer layers and shoes are waterproof. And take shelter from the wind, advises Muccilli.

Remember that dangerous health issues, especially when wind chill is at its most extreme, can take hold in minutes.