On yesterday's episode of Weather Break, we talked about the meaning of the wind chill temperature. I think pretty much everyone knows that the air FEELS colder when the wind is blowing than when it is calm outside. As we discussed yesterday, this is because our sense of "how cold it is" is really a measure of how fast your body is losing heat. When the wind is blowing, your body loses heat more rapidly that it does under calm conditions, and so you feel colder. Meteorologists have found complex mathematical formulas to determine these rates of heat loss, which they summarize as the wind chill temperature.
The modern way in which wind chill temperatures are computed involves a complex understanding of the thermodynamics and the fluid mechanics involved, but the original formulation was purely empirical, meaning that it was based on experimentation and observation. The original inventors of the wind chill factor were Paul Siple and Charles Passell, who were American research scientists working in near the South Pole as part of the third Antarctic Expedition from 1939 to 1940. Siple and Passell came up with a clever way to simulate the way a body loses heat when exposed to very cold temperatures and strong winds. They filled containers made of pyrene, which is a type of plastic, with water and placed them outside in the brutal Antarctic conditions. To Siple and Passell, these plastic, water-filled containers were a good approximation to the human body, which is almost made mostly of water held together by a flexible container. Using a system of sensors, Siple and Passell were able to observe how look it took before the container of water froze. Not surprisingly, when the temperatures were colder, the water frozen more quickly, but the plastic container of water also froze more quickly the stronger the wind was. In one case, the entire container froze in about 35 seconds, whereas under milder conditions the water sometimes took more than an hour to freeze. Using a large number of these observations, Siple and Passell were able to use complex mathematics and statistics to come up with a system of equations that could estimate how long it would take for the containers of water to freeze under any combination of temperature and wind conditions. This eventually became a way to calculate a "wind chill temperature", which is a type of "apparent temperature" and has become familiar to most people.
If it seems to you that the wind chill temperatures aren’t as low as they used to be in years passed, you’re right. Since the development of the original wind chill temperature scale, there has been rumblings in the meteorological community that wind chill temperatures seemed to exaggerate the apparent coldness of the air. An official weather report, for example, might say that the wind chill temperature is thirty degrees below zero, and while there is not doubt that it FEELS very cold out there, does it really FEEL like it is thirty below? Probably not. In the 1990s, more rigorous calculations of the rate of heat loss were made, and it was found that Siple and Passell’s original method of computing wind chill temperature did have a significant cold bias. Starting in the year 2001, the National Weather Service adopted a new way of computing the wind chill temperature, and the result is that the wind chill is never as low as it used to be on the old scale. The Weather Service is more confident now that these wind chill temperatures are a meaningful way of expressing the rate at which bodies lose heat when exposed to extreme temperature conditions. This information is used in part to help the Weather Service issue various winter weather advisories that help the public make good decisions about travel and school closures.
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