Last week, I wrote about the heat index. If you remember, this index is an “apparent or perceived temperature.”
In other words, it’s a measure of how hot air feels against your skin due to lack of evaporation; it can be much higher than the actual air temperature.
On the other side of the comfort scale is the wind chill index, and it’s based primarily on wind speed versus relative humidity for the heat index. As warm-blooded mammals, we are constantly transfering thermal energy to the atmosphere through conduction, convection and thermal radiation.
Believe it or not, air is a wonderful insulator from the cold. In fact, it’s one of the better ones we know of because it’s a gas. In a gas, molecules and atoms are spread out considerably farther than a substance in a solid or liquid phase like running water. Gases reduce heat transfer due to conduction. Conduction is the transfer of heat through a material in direct contact with another body, like a cast iron skillet on an electric stove.
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On a molecular level, heat is transferred with the collisions of particles, like atoms and electrons, within the material. In other words, rate of heat transfer decreases when atoms are farther spaced apart, like in air, which is comprised of 78 percent nitrogen, 21 percent oxygen and 1 percent of other gases like argon, carbon dioxide and water vapor.
On a frigid and blustery day, the thin boundary layer of heated air in contact with your skin and the thermal energy it contains is whisked away by the wind replaced with colder air. We call this process convection, which simply means movement.
When this occurs, your body has mechanisms to maintain its core temperature. For example, shivering activates muscles and generates heat. Goosebumps cause the hair on your skin to extend upward and partially block the wind. In extreme cases of hypothermia, your body restricts blood flow to extremities to maintain its core temperature of 98.6 degrees Fahrenheit.
Theoretically, you could be on top of a snow-covered mountain on a sunny and dead-calm day absorbing the sun’s heat through thermal radiation. And without any wind to take your thermal energy away, you would probably feel perfectly comfortable in shorts and T-shirt despite well below freezing temperature.
Unfortunately, days like this are rare, and the wind can rapidly take your body’s heat away, which can lead to potentially dangerous consequences like frostbite.
To help protect the public, the National Weather Service developed the wind chill temperature index chart, much like the heat-index chart, to increase public awareness and safety.
This chart utilizes air temperature and wind speed to make an approximation on how cold it really feels on skin. Remember, there are other factors that can affect how frigid it feels, such as relative humidity and the amount of sunshine. Regardless, it’s vital to take wind seriously on cold days to reduce your risk of hypothermia and frostbite.
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