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In practical weather terms, there is an actual air temperature and a "feels like" temperature. Television meteorologists often provide both numbers during especially hot summer days or cold winter days. In the summertime, the "feels like" temperature is called the heat index, but during the winter, the "feels like" temperature is often called the wind chill factor. This is a combination of air temperature and wind speed that affects the freezing rate of exposed skin.
A quick demonstration of the effects of wind chill can be performed at your desk right now. Simply blow a fast stream of air across your exposed forearm or hand. The area receiving the fast-moving air should feel noticeably cooler than the rest of your arm. This is the wind chill effect. When the accelerated air from your mouth moved across your exposed skin, the normal evaporation rate was temporarily raised. The heat radiating from your arm was also affected by the change in wind speed.
The actual wind chill effect works like this on a much larger scale. On a relatively warm winter's day, the air temperature might be 32°F (O°C). This would not be especially uncomfortable for the average person wearing suitable winter clothing, but what if the wind speed were raised to 25 mph (40 kph)? The "feels like" temperature would drop to to 19°F (-7°C). At this temperature, exposed skin could suffer frostbite within a few hours.
If the air temperature continued to drop and the wind speed remained constant, the wind chill factor could become cold enough to cause immediate frostbite to exposed flesh. This is why cold weather experts always recommend covering the face, head, neck, hands and ears if you have to venture outside during a winter storm event. This factor primarily affects human flesh, not most metal or machinery.
Different agencies responsible for official weather information use different formulas to calculate the wind chill factor. The easiest way for average people to calculate wind chill factor on their own is to access an online conversion table provided by official weather services. Air temperature and wind speed can be entered into dialog boxes, and the approximate chill factor can be calculated within seconds. There are also charts available that show the wind chill factor at different temperatures, combined with the estimated exposure times before the risk of frostbite appears.
If anyone really wants to calculate this factor like the professionals do, here's the formula for Fahrenheit calculations: WC=.0817(3.71 V^.5 + 5.81 - .25 v)(T-91.4)+91.4, where V=wind speed in mph and T=°F. We'll leave a light on for you.