The wind chill is a measure of how cold it really feels outside. The wind chill formula relies on environmental data including the wind speed and the ambient air temperature. The wind chill is only calculated when temperatures drop below 40 degrees Fahrenheit (4.4 degrees Celsius).
Wind chill makes it feel colder than it really is due to the loss of body heat when exposed skin is in direct contact with the wind and cold temperatures. Heat is drawn away from the body at a faster rate when wind speed increases. For example, your weather station may report that the outdoor temperature is 20 degrees Fahrenheit (-7 degrees Celsius) with a wind speed of 10 mph (16 kph). The temperature at which these two values intersect is the wind chill. In this example, it will feel like it is 9 degrees Fahrenheit (-13 degrees Celsius).