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Friday, February 26, 2010

Wind Chill

"I've got to go home - Oh, baby, you'll freeze out there
Say, lend me your coat - It's up to your knees out there"
- song lyrics from Baby It's Cold Outside Slate.com recently published an article, "It's time to get rid of a meaningless number", that is opposed to the idea of "Wind Chill factor". The columnist believes that "no amount of tweaking will make wind chill more comprehensible."
Let's start at the beginning. In 1945 two Antarctic explorers, Paul Siple and Charles Passel, realized that a bottle of water would freeze faster on a windy day than on a calm day. This is because the wind is more efficient at carrying away the heat in the water. They carefully recorded wind speed and freezing times to create a chart and formula uniting wind and temperature. So given 5 F and a wind speed of X, you could calculate that a bottle would freeze in the same amount of time as a calm day of -40 F.
The concept of wind chill was first used by Canadian Weather men in the 1970s. The Siple-Passel formula was used for 30 years before two researchers, Randall Osczevski in Canada and Maurice Bluestein in the United States, decided to retest the numbers. They found that the Siple-Passel formula was too extreme in its predictions. So instead of -40F above the same 5F and wind would feel more like -19F.
So this is confusion #1 - the system of wind chill was changed at the turn of the century.
Confusion #2 - If it's a windy day at 35F, the wind chill might "feel like" 20F but no bottle of water (or anything else) is going to freeze. The coldest anything can get in this example is 35F - safely above freezing. All wind chill does is tell you how quickly you'll cool down to the outdoor temperature - not how cold you will become. Wind does not make things colder than the air temperature - it just makes them as cold, faster.
Confusion #3 - Your results may vary. The actual rate at which your body cools will depend upon the clouds & sun, how much skin you have exposed, how insulating your coat is, how exposed you are to the wind, and so on. The modern Wind Chill model is based upon people who are 5 feet tall, somewhat portly, and walk at an even clip directly into the wind.
Bottom Line
Wind Chill is useful as a reminder about the dangers of a cold wind but keep in mind the limitations mentioned above. Also remember that wet clothes draw out heat and will cool you down faster than dry clothes. Stay dry and out of the wind.
Check out one of my early blogs for more details on Hypothermia.