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Wind chill

"Wind Chill" is the temperature that it feels like outside to people and animals. It is not actually a temperature scale but a measurement of heat loss from the combined effect of wind and low temperatures. Another way to look at the wind chill factor is that it is the temperature a person feels because of the wind. If you've ever used a fan to cool yourself on a hot day, then you've felt the effects of a wind chill. A breeze doesn't make the temperature drop, but it can make it seem as if the temperature is cooler than it actually is.

Here's how it works

The basic law of thermodynamics says that any object warmer than its surroundings will lose heat. Normally we have an invisible layer of still air on the surface of our skin that acts like a blanket of insulation and slows our loss of body heat. Blowing wind reduces this insulating layer of warm air next to our skin and increases our rate of heat loss. The faster the wind blows, the more quickly we lose heat. Secondly, wind draws away body heat by quickly evaporating any moisture that forms on the skin; the stronger the wind, the greater the evaporation and the colder you feel. For instance, if the temperature is 0 degrees Fahrenheit and the wind is blowing at 15 mph, the wind chill is -19 degrees Fahrenheit. At this wind chill temperature, exposed skin can freeze in 30 minutes.

The wind chill factor becomes critical when the air temperature drops below the freezing point.

If wind is taking away heat faster than our bodies can replace it, we can end up with frostbite. The danger of frostbite increases sharply as the air temperature falls and the wind speed climbs.

Frostbite occurs when skin and the underlying tissues freeze. The areas most likely to be affected by cold temperatures or low wind chill factor are the hands, feet, nose, and ears. The symptoms of frostbite are a definite lack of sensitivity to touch, although there is probably a sharp, aching pain. The skin becomes hard, pale, cold and white patches may be seen. In severe cases the blood vessels are damaged. To treat frostbite, never rub or immerse the affected area in hot water. Use warm water, 100º F to 105º F. Or warm the area with dry, gloved hands. Do not walk on feet. If the skin tingles and there is a burning sensation when warming, the circulation is returning. If numbness remains, seek professional medical care immediately.

Wind Chill Temperature Index

The Wind Chill Temperature Index was issued by the National Weather Service beginning with the 2001-2002 winter season. The chart is a wind chill guide to winter danger and frostbite. The wind chill factors shown in darker blue indicate the danger zones for when frostbite can occur within only 15 minutes of exposure. To use the chart, you'll need to know two things. First: the actual temperature and secondly, a good guess at the wind speed. Find your wind speed in the left-hand column and then read across the row until you find the column that comes closest to matching the actual air temperature (listed in the first row of numbers across the top of the chart). The number you find in the box at the intersection of the wind speed row and the temperature column is the wind chill factor. For example: An air temperature of 40º F when combined with winds of 20 MPH results in a wind chill factor, or equivalent human heat loss rate, as if the temperature were actually only 18º F.

See NOAS's National Weather Service Web site for the Wind Chill Chart