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The cold truth about the wind chill

I recall both Andy Rooney and David Letterman taking shots at the wind chill index as TV weather gimmicks, both with inventive sarcasm and wit. Since I can be quite the smart aleck myself, I admit it can be a good target. It doesn’t help if a weathercaster takes air time to give you the wind chill temperature when the wind speed is 5 mph and the temperature is 38 degrees. That’s not time well spent.

Just about anyone who lives in a northern latitude (and sometimes in the Deep South) is aware wind chill is something you need to know about if you are going to be enduring prolonged exposure to cold accompanied by stronger winds. The wind can make all the difference for comfort and safety. Wind chill doesn’t change the actual air temperature. It is a measure of how much heat exposed skin loses to moving air and wind chill.

While not changing the temperature, it is a good gauge for determining how quickly a surface will be cooled to the actual temperature. The wind chill temperatures are tied to human exposure, but wind chill impacts virtually any exposed surface. That includes roofs, exterior walls, windows, and things like car engines with weak batteries facing into a cold wind.

Under those conditions, if the temperature is minus 5 degrees and your battery is weak, it will drop down to minus 5 and stay there for a longer time than if the wind was light. If you live in a cold windy spot, your heating bill will tend to be higher than a sheltered location. There is more heat exchange occurring where the wind is blowing harder.

One of the flaws, in my view, in the use of heating degree days to measure heating energy requirement is the absence of wind chill in those calculations (heating degree days are determined by calculating the number of degrees a day’s mean temperature is below 65. At 65 or higher for a mean, that day has had zero heating degree days).

Mike Nowak is bundled up as he shovels snow on Mary Lou Lane in Depew, Tuesday, Nov. 18, 2014. (Derek Gee/Buffalo News)

If you have a poorly insulated or leaky house, you will hear your furnace firing up much more often on windy days than on tranquil days. A home in a sheltered valley in Allegany County will typically have more frigid low temperatures than a home on a bluff overlooking Lake Erie, and pile up more heating degree days. But that exposed home above the lake may have a higher heating bill due to wind chill on the home, with far more heat exchange on the home’s surfaces.

As for us humans, research into wind chill began long before there were TV weathercasts, increasing in the 1930s and accelerating during World War II. In 1945, the National Weather Service began using a wind chill index based on the cooling rate of a container of water hanging in the wind. It turned out this method underestimated the time to freezing for human skin, and overestimated the actual amount of heat lost. Here is the Old Wind Chill Index:

This old index gave us wind chill temperatures during the teeth of the Blizzard of ’85 in the minus 50s, and occasionally in the minus 70 range during the Blizzard of ’77.

In 2000, cooperation between the U.S. and Canadian governments in a multidisciplinary study led to a new Wind Chill Index based on more complete and reliable data. The study included six male and six female volunteers who were exposed in wind tunnels to varying levels of wind chill, with special instrumentation and more modern theory of heat transfer. Here is the current Wind Chill Index:

This more reliable wind chill index took those minus 50s from 1985 and minus 70 from 1977 to the minus 30s and minus 40s, respectively.

The NWS issues wind chill advisories and more dangerous wind chill warnings at different thresholds for different regions in the country. In South Florida, an infrequent wind chill in the 30s is stressful to the locals there, not acclimated to our kind of chill.

In WNY, the NWS Buffalo Forecast Office issues a wind chill advisory when wind chill temperatures of minus 15 degrees are anticipated and a wind chill warning for minus 25 degrees, and minus 30 near Watertown. Human acclimatization is part and parcel of these determinations.

There is also an underlying body of research for measured human hazards to different levels of harsh to severe wind chill. The time it takes for skin to suffer frostbite and actual skin freezing, as well as the likely time for a high risk of hypothermia are factored in. There are elements that the wind chill index cannot readily measure. Hypothermia in wet clothing and for immersed people can set in amazingly quickly at higher temperatures than you might expect. Exposed wet hikers can be at a major risk, even more so in windy locations.

At the other end of the spectrum is the level of human activity. Someone who’s shoveling out his car from big drifts is working up an internal head of steam, and probably can stave off dangers from wind chill for a lengthier time span than some kids standing in a whipping arctic wind waiting for a delayed bus.

I seldom editorialize and advise people to “stay home” except in exceptionally harsh cold. On those few days, I’ll either state the old but reliable bromide to “dress in layers.” Or, I might hone that down, if it’s REALLY brutal out there, to “dress in a layer of house.”

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