About Wind Chill...

About Wind Chill....

by Steve Horstmeyer, Meteorologist, Cincinnati, Ohio, USA

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Look for more on wind chill in Weatherwise, Oct/Nov 1995


Robert Frost on Wind Chill

		The sun was warm but the wind was chill.
		You know how it is with an April Day
		When the sun is out and the wind is still,
		You're one month on in the middle of May.
		But if you so much as dare to speak,
		A cloud comes over the sunlit arch,
		A wind comes off the frozen peak,
		And you're two months back in the middle of March.

				Two Tramps in Mud Time (1936)

The Concept

When the wind blows your body loses heat more quickly than if the wind is calm (as long as air temperature is lower than skin temperature). The reason is that each air molecule that touches exposed skin, carries away some body heat. If the wind blows faster, more molecules touch your skin and more heat is removed. This can be enhanced by increased evaporation with higher wind speeds, but wind chill strictly refers to the removal of "sensible" heat, heat that a thermometer can measure. Heat carried away through the evaporation of perspiration, that kind of heat is called "latent heat" is not part of the original concept.

A Brief History

Wind Chill is controversial, not the concept, but how it is calculated and used. In the 1940's Antarctic explorers Siple and Passel exposed a plastic cylinder to varying conditions and timed how long it took the water to freeze. They developed the Wind Chill Index which expressed how quickly a human body cooled when the wind increased. So far so good......not really!

Problems

Not only is the accuracy of Siple's and Passel's measurements questioned, but there are many other problems.

  1. A plastic cylinder is a poor representation of the human body.
  2. The Wind Chill Index assumes skin is bare, absurd in cold conditions, and there is no generation of body heat.
  3. There is no allowance for extra heat generated during exercise, as a distance runner even in the coldest weather during grad school in Nebraska (-33 deg. F.) it was hard to stay cool.
  4. Sunshine can warm you considerably.
  5. Wind Chill has never been tested and experimentally verified.

The most FAQ about wind chill

If the air temperature is 35 deg. F. and the wind chill 20 deg. F., will water freeze?

No! Wind Chill expresses how quickly you will lose body heat. At 20 deg. F. and 20 mph, the wind chill is -10 deg. F. The index (-10 deg. F.) is the equivalent temperature telling you that your body loses heat as fast as if the temperature is -10 deg. F. and the wind light.

How cold a glass of water can get is dictated by the second law of thermodynamics which says the water cannot freeze until the temperature hits freezing, that is it can only cool to whatever the air temperature is. The wind chill will just get it there faster.

© 2008 Steven L. Horstmeyer, all rights reserved

For more see Weatherwise Oct/Nov 1995, pg 24.


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