The Truth About Wind Chill

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How many times have you heard it on a TV weathercast?   "The temperature is 35F, but with the stiff winds today, the wind chill temperature is 24F, so bundle up!"

 TV folks sometimes refer to the wind chill temperature as the apparent or equivalent temperature, the temperature it would feel like if the wind was calm.

Now if the temperature is 35F and the wind chill temperature is 24F, will water freeze? (Answer: No)

Will your car battery have a harder time starting your car if the wind chill is lower, but the temperature is the same? (No)

What exactly is this wind chill business?

 The original wind chill index was created during the early 1940s by two polar explorers (Siple and Passel), who measured how long it took for plastic bottles of water to freeze at various wind speeds.   If the temperature was below freezing, the bottles would freeze up more quickly when the wind speed increased.  Their original index was not in terms of temperature, but rather in cooling rate.

   In the 1960s, wind chill was expressed in terms of equivalent temperatures--the still air temperature that would produce roughly the same cooling rate compared to the observed temperature and wind speed.   Finally, in 2001 the National Weather Service produced an upgraded wind chill chart that more accurately described the cooling rates at various temperatures and wind speeds (see below):

The whole idea of wind chill is based on the fact that the loss of heat on our skin is related to the difference in temperature between our skin and the cooler outside air (more cooling when the difference was larger) and the wind speed (stronger winds remove heat from our skin faster).   Under calm or near calm conditions, a veneer of "dead air" stays near our skin, and this air, warmed by our skin, act as an insulator.   If the winds pick up, this protective layer is progressively removed.

 The efficiency of batteries only depend on temperature.  If the wind is strong perhaps the battery might cool off faster if it enters an area of cooler temperatures, but the chemical reactions inside the battery only vary with temperature.  Freezing of water depends on the average speed of the water molecules (which is directly related to temperature, the average kinetic energy--energy of motion--of the molecules).  Temperature alone decides on whether water will freeze.  The winds could be blowing 500 mph, but water will not freeze at 33F!

Tonight, eastern Washington is going to be hit by some of the coldest temperatures yet this winter, and the National Weather Service has a freeze warning for the lower elevations of eastern WA.  Here is the forecast temperature for 5 AM.  Most above freezing.

And here are the wind chill temperatures at the same time (lot of them below freezing)

 Wind chill or equivalent temperatures are useful, but there are many other factors that influence cooling rate, such as the amount of solar radiation (sun) and clothing.

 Seattle School Board Information

Jack Whelan, who had run for the Seattle School Board this year, has written a wonderful piece on the current election.  If you would like to read it, do so here.

My recommendations for the Seattle School Board election is here.

And today, another State Auditor's report found that the pressure to underpay for the MLK School came from the highest levels in the School District.
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