Wind chill

I am sure glad that there weren't any traffic cops
around when I stopped in the middle of an
intersection to take this photo last week!

In Illinois, one unofficial meteorological term is "bitterly cold".  It is in the forecasts whenever the temperatures drop to single digits, and wind chill factors are in the minus teens to twenties.  This made me actually look up how wind chill is calculated.

The term "wind chill" was coined by a polar explorer, Paul Siple in his Ph.D. thesis research in which he measured the freezing rate of water under different conditions in trhe Antarctic.  He was a member of Admiral Richard Byrd's first Antarctic expedition in 1928.  When he was a young child, his family moved to Erie, Pennsylvania (a very cold place), and he went to my alma mater, Allegheny College, graduating with a B.S. in biology.  He was the author of four books.  He went to Clark University in Massachusetts for grad studies in geography and climatology, doing a thesis "Adaptations of the Explorer to the Climate of Antarctica."

With Charles Passel, he hung water-filled, plastic cylinders from a long pole in different conditions in the Antarctic, believing that these simulated heat loss from human flesh.  They finished the research in 1940, but it wasn't published in the open literature until 1945 because of potential military applications during the war.  They came up with an equation, the Siple-Passel equation" to determine an equivalent temperature under light wind conditions.  The coefficients in this equation were refined over 50 years, and then in 2001 it was replaced by the formulation now in use.

The model is based on engineering heat transfer from a bare face of a person walking directly into the wind at 1.4 m/s (3.1 mph).

The equations used now in North America are

T = 13.12 + 0.6215*T-11.37* V^0.16 + 0.3965*T*V^0.16

where T is the wind chill temperature and T is the air temperature, both in Centigrade.  V is in km/hour, measured at a height of 33' off the ground. In this equation the * indicates multiplication, and the ^ indicates that the next number is an exponent (e.g., ^0.16 above).  I can't do exponents in this blog program.

In Fahrenheit and mph, the equation is

T = 35.74 + 0.6215*T-35.75*V^0.16 + 0.4275*T*V^0.16.

A cool piece of trivia that I found on the Wiki site about wind chill is that it can't be used on other planets! If you did this calculation for Titan, a satellite of Saturn, where the average temperature is -179 C, and wind speeds can be as high as 430 km/h, you get a wind chill of -315 C.  This is 42 degrees lower than absolute zero, i.e., impossible!