The windchill factor is the temperature of windless air that would have the same effect on exposed skin as the combined effects of wind speed and air temperature.

Increased wind speeds result in accelerated heat loss from exposed skin, since it reduces the boundary layer of stagnant warm air on the skin. This stagnant air film acts as an insulator. When this insulation layer is reduced, the body will lose heat more rapidly.

On November 1, 2001 a new windchill temperature index was implemented by the National Weather Service. This new formula to calculate windchill is:

Wind Chill (F) = 35.74 + 0.6215 T - 35.75(V0.16) + 0.4275T(V0.16)

where V is the wind speed in miles per hour, and T is the outside temperature in degrees Fahrenheit.

Windchill temperatures (F) as a function of air temperature (F) and wind speed (mph):

```                      Temperature (F)
45  40  35  30   25   20   15   10    5    0
---------------------------------------------------
5  |  42  36  31  25   19   13    7    1   -5  -11
10  |  40  34  27  21   15    9    3   -4  -10  -16
15  |  38  32  25  19   13    6    0   -7  -13  -19
Wind   20  |  37  30  24  17   11    4   -2   -9  -15  -22
Speed  25  |  36  29  23  16    9    3   -4  -11  -17  -24
(mph)  30  |  35  28  22  15    8    1   -5  -12  -19  -26
35  |  35  28  21  14    7    0   -7  -14  -21  -27
40  |  34  27  20  13    6   -1   -8  -15  -22  -29
45  |  33  26  19  12    5   -2   -9  -16  -23  -30
50  |  33  26  19  12    4   -3  -10  -17  -24  -31
55  |  32  25  18  11    4   -3  -11  -18  -25  -32
60  |  32  25  17  10    3   -4  -11  -19  -26  -33

Temperature (F)
-5  -10  -15  -20  -25   -30   -35   -40   -45
------------------------------------------------------
5  |  -16  -22  -28  -34  -40   -46   -52   -57   -63
10  |  -22  -28  -35  -41  -47   -53   -59   -66   -72
15  |  -26  -32  -39  -45  -51   -58   -64   -71   -77
Wind   20  |  -29  -35  -42  -48  -55   -61   -68   -74   -81
Speed  25  |  -31  -37  -44  -51  -58   -64   -71   -78   -84
(mph)  30  |  -33  -39  -46  -53  -60   -67   -73   -80   -87
35  |  -34  -41  -48  -55  -62   -69   -76   -82   -89
40  |  -36  -43  -50  -57  -64   -71   -78   -84   -91
45  |  -37  -44  -51  -58  -65   -72   -79   -86   -93
50  |  -38  -45  -52  -60  -67   -74   -81   -88   -95
55  |  -39  -46  -54  -61  -68   -75   -82   -89   -97
60  |  -40  -48  -55  -62  -69   -76   -84   -91   -98
```

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