Wind Chill
Dateline: 12/11/98
For people to adapt to the world around them, particularly in cold climates, an analysis of the air temperature is required, and then a judgment of how that temperature will
affect their bodies. In still air, the equation is reasonably simple - in 1714, Gabriel Daniel Fahrenheit developed the mercury thermometer, which registers temperature in the scale named after him. Once a wind
comes up, however, calculating the effects becomes much more complicated.
In 1939, Antarctic explorer
Paul A. Siple, in his dissertation Adaptation of the Explorer to the Climate of Antarctica, coined the term "wind chill" to describe
the combined effect of cold air and wind. Two years later, he and Charles Passel developed a chart to relate that effect to still-air temperatures. There continues to be a great deal of study on the physical effects of wind at
low temperatures, and
disagreements among research teams are common. You will not only find 2 completely different
interpretations of wind chill used by Canada and the United States, but even some variation in the results of the charts available on the Internet using the US model. For a detailed look at the formulae used in the
various calculations, have a look at
Gene Nygaard's page.
I find the wind chill charts as used in the United States to be much more fun than the Canadian ones - it's much more dramatic to tell people in California that the wind chill is currently
minus 65° than it is to say that it's extreme. To see an example of a US-style chart,
this chart, by David Lister, is in Fahrenheit, down to minus 30.
Here's another one that goes much further into the
thermal basement. With his chart, Lister uses the following notes to describe the effects:
Wind Chill | Possible Effects |
30°F or greater | Chilly: Generally unpleasant |
15°F to 30°F | Cold: Unpleasant |
0°F to 15°F | Very Cold: Very unpleasant |
-20°F to 0°F | Bitter Cold: Frostbite possible |
-20°F to -60°F | Extremely Cold: Frostbite likely. Outdoor activity becomes dangerous |
-60°F or less | Frigidly Cold: Exposed flesh will freeze within half a minute |
For automatic calculation of the wind chill, check out
Scott Blanksteen's Calculator. If you're looking at a weather map, it may also be useful to understand the symbols for
Beaufort's Wind Scale in your calculations of the wind chill.
Environment Canada's calculations are expressed in terms of the loss of energy, in
watts per square meter - see a chart of that type
here. In Canadian weather reporting, a Wind Chill Warning is issued when the cooling rate drops to
2000 watts/square metre for at least 12 hours, or the equivalent temperature is expected to be -40C or colder for at least 12 hours.
Complicated? - I think so. More useful may be the conversions from the "watts per square meter" figure on the chart to a description
of the effects:
Wind Chill Factor Description
- 700 -
Conditions considered comfortable when dressed for skiing
- 1200 -
Conditions no longer pleasant for outdoor activities on overcast days
- 1400 -
Conditions no longer pleasant for outdoor activities on sunny days
- 1600 -
Freezing of exposed skin begins for most people, depending on the degree of activity and the amount of sunshine
- 2300 -
Conditions for outdoor travel such as walking become dangerous. Exposed areas of the face freeze in less than 1 minute for the average person
- 2700 -
Exposed flesh will freeze within half a minute for the average person
Often, the strength of the wind isn't the only factor that needs to be taken into consideration when assessing wind chill danger. If movement at any appreciable speed is to be expected during an
activity, the relative wind may be the most important figure. If you choose to go out snowshoeing, it will at most be a minor consideration - however, if
cycling, downhill skiing, dogsledding or snowmobiling are planned,
the speeds involved could very quickly turn a comfortable day into one during which the danger of
hypothermia is high. For example, if you take a snowmobile
out when it's 0° Fahrenheit, and cruise along at only 35 miles per hour, the wind chill is minus 50°!
With the numbers and descriptions that you've seen above, many of you will be wondering how anything survives in the Arctic (or in the case of humans, why they would choose to live there).
The secret is all in what a mammal surrounds its body with, whether that be blubber, hair, fur or clothing. The cold air must be kept from cooling the skin too much, and even more critically, from cooling the internal organs.
Arctic animals have got great exposure on the Internet, and their various adaptations to the cold are well described.
For humans, Chris Kallio, our Backpacking guide at The Mining Company, gives a good introduction to dressing in layers for maximum warmth and flexibility.
For further study of meteorology, the
Weather Glossary gives an excellent summary of terminologies used. On my Weather & Climate links page, you can explore Northern weather conditions,
and Joseph Bartlo's Meteorology site here at The Mining Company provides a broad range of resources.
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