Note that bike speed into still air is the same as wind speed.
Cyclists are often moving at 15mph even on very cold days. A quick glance at the chart
indicates that you loose heat at a rate that seems much colder than the outside
While cycling in 40 degree still air, at 15 mph, it will feel like 23 degrees, or 9
degrees below freezing. Does this mean you could get frostbite at 40? No. But you will
cool down at the same rate that you would if it really were 23 degrees, you will never get
colder than it really is.
The wind-chill temperature is a measure of relative discomfort due to combined cold and
wind. It was developed by Siple and Passel (1941)and is based on physiological studies of
the rate of heat loss for various combinations of ambient temperature and wind speed. The
wind-chill temperature equals the actual air temperature when the wind speed is 4 mph or
less. At higher wind speeds, the wind-chill temperature is lower than the air temperature
and measures the increased cold stress and discomfort associated with wind.
The formula the U.S. National Weather Service uses to compute wind chill is:
T(wc) = 0.0817(3.71V**0.5 + 5.81 -0.25V)(T - 91.4) + 91.4
T(wc) is the wind chill, V is in the wind speed in statute miles per hour and T is the
temperature in degrees Fahrenheit.
The formula to calculate a Celsius wind chill using V as the wind speed in kilometers
per hour and T in degrees Celsius is:
T(wc) = 0.045(5.27V**0.5 + 10.45 - 0.28V) (T - 33) + 33
The effects of wind-chill depend strongly on the amount of clothing and other
protection worn as well as on age, health, and body characteristics. Wind-chill
temperatures near or below 0 F indicate that there is a risk of frostbite or other injury
to exposed flesh. The risk of hypothermia from being inadequately clothed also depends on
the wind-chill temperature.