Ask Jack: Wind Chill
Q: If the temperature is above 32 degrees and the wind chill is below 32 degrees, will water freeze?
—Kirsten H., Somerville, MA
A: No. Most people are aware that they feel colder when the wind is blowing, but this is because the wind carries away the layer of air around you that your body’s heat has warmed.
The big danger of air temperatures below 32 degrees Fahrenheit is that the water in the cells of your skin can freeze to cause frostbite. When the air is just a little below freezing and no wind is blowing, heat from your body warms the air around you enough to prevent frostbite.
As the air grows colder, however, your body doesn’t supply enough heat to keep the air around you above 32 degrees. One of the basic principles of the science of heat—thermodynamics—explains this. Heat moves from warm objects to cold objects, cooling the warm object and warming the cold object. The greater the temperature difference, the faster heat moves from warm to cold.
Since heat can’t flow from toward a warmer object, no matter how fast the wind blows, it can’t cool anything below the air’s temperature.
However, since wind carries warm air away from your body, it has the same affect on cooling you that a lower temperature would. For example, the U.S. National Weather Service wind chill chart tells you that if the temperature is 20 degrees F and the wind is blowing at 35 mph, you will cool off as quickly as you would in zero-degree air with no wind.
While the air temperature has to be colder than 32 degrees for frostbite, hypothermia can occur at much warmer temperatures. This refers to a great enough loss of core body temperature (not just a cooling of the skin) that internal organs and the brain no longer function normally. While frostbite can cause serious injuries, it’s rarely deadly. Hypothermia is deadly.
Since the body’s temperature is normally around 98 F, the first signs of hypothermia, such as shivering, begin when the body’s core temperature drops below approximately 95 degrees. If the victim isn’t immediately warmed, he or she will die.
The Weather Service’s wind chill chart starts with 40 degrees F, and gives a “wind chill temperature” of 25 degrees in a 60 mph wind. If you were out in such conditions you wouldn’t have to worry about frostbite since even with a 60 mph wind your skin would never cool below the air’s 40 degrees. But, you’d be in real danger of hyporthermia unless you were wearing very warm clothing.
Chapter 11 The AMS Weather Book: The Ultimate Guide to America’s Weather has more on wind chill, frostbite and hypothermia, including the story of the Antarctic explorer who came up with the idea of wind chill.
The National Weather Service has its wind chill chart and other information online.
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