APPENDIX F: WIND CHILL
Wind chill is a term that is used to describe the rate of heat loss from exposed skin
caused by the combined effects of wind and temperature. If there is no wind, heat
emanating from an object will remain near the object and warm the air around it, thus
providing a measure of insulation that inhibits further cooling. Air movement, though,
will conduct heat away from the warm object, a process known as advection. As the
wind speed increases, heat is advected away more quickly, resulting in more rapid
cooling of the object. The wind chill temperature is a calculated temperature that pro-
vides a better indication of the cooling capacity of the wind in conjunction with low
temperatures. It was originally based on the length of time required for a container of
near-freezing water to become frozen under various combinations of wind and tem-
perature. The wind-chill temperature is equal to the dry bulb temperature that is re-
quired to cool the object at the same rate as if there were no ambient air movement.
The concept was first quantified in 1941 by Paul Siple, an Army major and geogra-
pher, and Charles Passel, a geologist, while stationed at Little America, Antarctica.
Their experiments were based on the time required to freeze a known volume of water
under various combinations of temperature and wind speed during the winter darkness
of Antarctic. Since the publication of their results (Siple and Passel 1945), the concept
of an equivalent wind chill temperature has enjoyed widespread use as a means of
describing the combined severity of the wind and low temperatures on human beings.
In the years since, several individuals have suggested improvements to the Siple and
Passel model. Their criticism stems from the fact that cylinders of water are not life-
like because they have no metabolic heat source as does the human body and were not
clothed as a human would be. As such, cylinders of water will freeze faster than flesh,
so the original heat-loss relationship underestimates the time of freezing and accord-
ingly overestimates the chilling effect of the wind.
The model in current use by the National Weather Service (Quayle and Steadman
1998) is
WC = 0.0817 (3.71 V0.5 + 5.81 0.25 V) (T 91.4) + 91.4
(F1)
where WC = equivalent wind-chill temperature
V = wind speed (statute mph)
T = temperature (F)
or
WC = 0.045(5.27 V0.5 + 10.45 0.28 V) (T 33) + 33
(F2)
where WC = wind-chill temperature
V = wind speed (km/h)
T = temperature (C).
These formulas are only valid for wind speeds ranging from 4 to 40 mph (6.4 to 64 km/
hr). Increasing wind speed will not cause an exposed object to be colder than ambient
temperature. The object will achieve a temperature equal to ambient, and higher wind
speeds will only cause the ambient temperature to be achieved more quickly. Table F1
shows equivalent wind chill temperatures in both English and metric units.
123
Previous Page
