When temperatures fall and the wind howls, we hear about the danger of "wind chill."
Robert Frost said it through his poetry:"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."
- Robert Frost
in the Mud Time, 1936
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" and is not part of the original concept.
Wind chill is controversial, not the concept, but how it is calculated and used. In the 1940s, 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.
However, the accuracy of these measurements has been questioned, but there are other problems.
- A plastic cylinder is a poor representation of the human body.
- The Wind Chill Index assumes skin is bare and there is no generation of body heat.
- There is no allowance for extra heat generated during exercise.
- Sunshine can warm you considerably.
- Wind chill has never been tested and experimentally verified.
A frequently asked question about wind chill is: If the air temperature is 35 degrees F. and the wind chill is 20 degrees F., will water freeze?
The answer is "no." Wind chill expresses how quickly you will lose body heat. At 20 degrees F. and 20 mph, the wind chill is 10 below zero. This measurement tells you that your body loses heat as fast as if the temperature were 10 degrees below zero 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 to say, it can only cool to whatever the air temperature is. The wind chill will just get it there faster.