"Wind chill" is how we talk about heat loss caused by wind. Your body produces heat and warms itself when you stand outside in the cold. Because of convection, you're also heating the air around you. If the wind is still, that warmed air close to your skin helps you feel warmer. Strong winds, on the other hand, keep moving the warmed air away from you, putting you in contact with colder air which makes you feel colder.
While this basic definition has stayed the same, the way we measure wind chill changed back in 2001. So, when you hear a wind chill factor today, it means something different than it did in the 20th century. In general, the new system calculates wind chill factors into adjusted temperatures that are warmer than what the old system would have come up with. So, if its 20 degrees F outside, and the windspeed is 15 miles per hour, the old system would have put the wind-chill index at -4 degrees F. Today, we'd say it's more like 6 degrees F. Balmy.
The first wind chill formula and tables were developed by Paul Allman Siple and Charles Passel working in the Antarctic before the Second World War, and were made available by the National Weather Service by the 1970s. It was based on the cooling rate of a small plastic bottle as its contents turned to ice while suspended in the wind on the expedition hut roof, at the same level as the anemometer. The so-called Windchill Index provided a pretty good indication of the severity of the weather.
In the 1960s, wind chill began to be reported as a wind chill equivalent temperature (WCET), which is theoretically less useful. The author of this change is unknown, but it was not Siple and Passel as is generally believed. At first, it was defined as the temperature at which the windchill index would be the same in the complete absence of wind. This led to equivalent temperatures that were obviously exaggerations of the severity of the weather. Charles Eagan realized that people are rarely still and that even when it was calm, there was some air movement. He redefined the absence of wind to be an air speed of 1.8 meters per second (4.0 mph), which was about as low a wind speed as a cup anemometer could measure. This led to more realistic (warmer-sounding) values of equivalent temperature.
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.