Weather 101 - Wind Chill

In summer, "it's the humidity" that makes the heat unbearable. In winter, "it's the wind" that makes the cold so dangerous. And wind is something many of us have had to address during this late fall to winter season (with some locales reporting wind chill readings in the minus 40 to minus 60 degree range). When combined with low temperature, the wind chill index helps us gauge how the cold may affect us. We may dress more warmly or cover exposed skin better as wind chill values tumble.

According to the National Weather Service (NWS), "wind chill is the temperature it 'feels like' outside and is based on the rate of heat loss from exposed skin caused by the effects of wind and cold. As the wind speed increases, the body is cooled at a faster rate causing the skin temperature to drop. Wind Chill does not impact inanimate objects like car radiators and exposed water pipes, because these objects cannot cool below the actual air temperature." Thus, don't expect water to freeze with an air temperature of 35^oF and a wind speed of 35 miles per hour (wind chill of 21^oF).

AccuWeather, a private weather forecasting service, has taken the wind chill concept further with its "RealFeel Temperature." Their index addresses the effects of "temperature, wind, humidity, sunshine intensity, cloudiness, precipitation and elevation on the human body."

While the initial wind chill index, designed by two Antarctic research scientists (Paul Siple and Charles Passel) in 1941 keyed on how wind and air temperature affected the freezing of water. Their studies (in the Antarctic cold) keyed on heat loss rates and water freezing times as a function of air temperature and wind speed using water-filled, plastic cylinders suspended from a long pole.

Following questions raised by several meteorologists and engineers, a recent study by a joint U.S.-Canadian group keyed on how wind, temperature, solar radiation and other factors affected the human face, "the part of the body most often exposed to severe winter weather." It recommended changes to the wind chill index that were implemented in 2001. NOTE: You can read the entire report by accessing the hyperlinks shown here. You can also view the updated NWS wind chill chart online.

Basically, here is how wind chill works.

Under calm wind conditions with a person standing still, heat given off by the human body warms the air adjacent to exposed skin by conduction (touching or contact). This air layer insulates the body somewhat, lessening heat loss to already warmed air. It's as though the atmosphere was covering the skin with clothing.

But, when the wind blows (or the person walks faster than 3 miles per hour, that insulating effect vanishes and the body loses heat to the air. Up to a certain point, the faster the air is removed from the exposed skin or the faster one moves, the faster heat is lost.

The new index emphasizes the effects of wind chill on humans. It keys on frostbite (the injury to skin tissue due to freezing) and hypothermia (the dropping of core body temperature to unsafe levels).

Frostbite can be identified by a loss of feeling or color in the affected area. The extremities - ear lobes, fingers and toes are most at risk. Hypothermia's warning signs can include any of the following: uncontrollable shivering, memory loss, disorientation, incoherence, slurred speech, drowsiness, and apparent exhaustion.

If you or someone with you exhibits any of these potentially life-threatening signs, get inside and slowly warm the affected areas or the entire body. While rapid warming may appear to be what is needed, you don't want to drive chilled blood too quickly into the body's core region.

Note that our pets are equally susceptible to its effects, although the wind chill formula isn't directly applicable to them (their size, hair or fur covering, skin temperature are so different).

It is obviously better to avoid frostbite or hypothermia than to treat it. The first line of defense is to know what is going on outside. Weather forecasts are available almost everywhere nowadays thanks to technology. Pay special attention when wind chill advisories or warnings are in effect. This holds true even in southern climes where wind chills don't have to drop to minus readings to be dangerous.

Refer to the NWS' wind chill chart to determine wind chill readings for yourself. You can even "eyeball" what the wind chill is (at least for temperatures above 10^oF) by using this simple formula:

Wind Chill = Air Temperature - ½ Wind Speed.

Then "dress for the weather." The NWS offers the following tips:

• Wear layers of loose-fitting, lightweight, warm clothing. Trapped air

between the layers will insulate you. Outer garments should be tightly woven, water repellent, and hooded.

• Don a hat, because 40% of your body heat can be lost from your head.

• Cover your mouth to protect your lungs from extreme cold.

• Wear mittens, snug at the wrist, rather than gloves. This concentrates heat in one place and lessens heat loss surface area.

• Try to stay dry and out of the wind.

Finally, just remember that summertime (it's coming soon to a community near you) will bring high heat indices (content on this subject forthcoming), and you'll be wishing for cold weather!