Factoring in the wind chill

The humorist Carl Hurley once said that he wished they had never invented the wind-chill factor given in weather reports. “Use to when they said it was 29 degrees it felt like 29 degrees,” he commented. “But now they say it’s 29 degrees with a wind chill of 20 and you freeze to death!”

The wind chill temperature was adopted by the National Weather Service to reflect the cooling effect of wind on cold days. In theory, the wind chill temperature is the temperature in still air that would cause the same rate of heat loss from exposed skin as that brought about by prevailing winds. For example, a 10 mile-per-hour wind on a 20 degree day has the same chilling effect as a 3 degree day with no wind. Hence the wind chill temperature is 3 degrees.

The original formula used to determine wind chill temperature was developed from research done in the 1940s. To determine rates of heat loss, scientists sealed a thermometer in a plastic bottle of water and timed how long it took the water to freeze under a variety of wind speeds and air temperatures. The freezing times were later converted into a chart of temperature equivalents. But the whole point of wind chill is how it impacts your comfort, and the thermal properties of a plastic bottle do not resemble those of human flesh.

To resolve this, a scientist named Osczevski literally stuck his head in a refrigerated box with sensors on his cheeks until his skin temperature came close to the freezing point. His reasoning was that any attempt to revamp wind chill should start with the face, which is the most exposed part of the body, and therefore most vulnerable to frostbite. In 2000 Osczevski created a mathematical model of heat transfer in the human face and tested it with volunteers who braved fierce strong winds in a wind tunnel.

The result was a gentler wind chill prediction. A 20 degree day with a 10 mph wind now has a wind-chill rating of 9 degrees instead of 3. It turns out that there is not much difference between the old and new formulas at low wind speeds, but at higher speeds the new formula is quite a bit warmer. The National Weather Service adopted the new calculation method, and so less teeth chattering temperatures are given.

For the record, the normal temperature of the skin is 94 degrees F. Exposed skin becomes uncomfortable when it cools to around 59 degrees, and painful at 50 degrees. Below that skin temperature starts to feel numb, and under certain conditions, frostbite can occur within minutes. Skin freezes at 23 degrees.

Steve Roark is the area forester in Tazewell, Tennessee, for the Tennessee Department of Agriculture, Forestry Division.