Question:
I would like to know what the actual formula is to
calculate the wind chill factor. Or is there such a thing?
Replies:
I do not know if there is a formula, but I seem to recall
(this was learned way back in high school so no guarantees)
that it was evaluated by take the temperature of a wet thermometer
at a particular wind speed and comparing that to a dry
thermometer - this gives you a temperature decrease caused
by the wind evaporating water that should closely approximate
the effect of wind on the human body. I assume the wind
chill factors quoted by weather people are derived from tables
of this sort...
Check out:
http://observe.ivv.nasa.gov/nasa/earth/wind_chill_applet.html
Actually, different countries use different measures of
wind chill and different units even...
The relevant quantity is not the final temperature as I said
earlier, but the rate of heat loss per unit area of exposed
skin. There are formulas for heat loss (I think there is one
ascribed to Newton) but basically in still air it goes linearly
in the temperature difference between the inside (body temperature)
and the air outside. When it is windy the heat loss is increased
due to evaporation of water from the surface of your skin. The
"wind-chill factor" then gives an equivalent temperature that
it would have to be outside in still air for the same rate
of heat loss that you get in still air with the current wind.
I think it is possible that in a really strong wind the effective
temperature could go below absolute zero, which would not make
too much sense from a temperature perspective - just that the
heat loss rate is greater than you could ever get by standing
in still air no matter how cold. Anyway, that is the basic idea.
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