Lesson 7 - Understanding Wind and Humidity Factors

Wind, Humidity, and Temperatures

Cold and Windy...the wind chill factor

A cold snowy day doesn't seem too bad...until suddenly a blast of icy wind tears around the corner. Just as temperatures begin to show serious drops, weather reporters start leading into their forecasts with words of warning:

"Sharp winds coming at us this morning at about 20 miles an hour, giving us a wind chill of almost ZERO DEGREES...if you have to go outside, make sure you throw on that ski parka, cause it's gonna be cold, cold, cold!"

Although radio announcers and news meteorologists often talk about wind chill as if it were a completely different temperature, this can be misleading. Only people and animals are affected by wind chill factors--a thermometer left outside on a calm day when the temperature has plummeted to 6° will still read 6° even after the wind kicks up. However, a pedestrian struck by a sudden gust of wind will immediately notice that "cold" just seemed to get a whole lot colder. Wind chill, the combination of wind and temperature, can make 40° feel closer to zero.

To calculate just how much colder the wind will make people feel, the the National Weather Service uses the following formula to compute wind chill effects:

Where T(wc) is the wind chill, V is in the wind speed in miles per hour and T is the temperature in degrees Fahrenheit.

Rather than do the math, many people prefer to use The Wind Chill Index, shown below. Although the Index offers the wind chill as a "temperature," the degrees listed are intended as guidelines, to indicate how much clothing people should put on before venturing outside, or whether their family pets should stay indoors.

Wind chill only impacts humans and animals because only living things need to be surrounded with heated pockets of air to keep them warm outside. Inanimate objects are notaffected: if the antifreeze in a car is good to 10° on a day when the temperature reads 10°, but a 15 mph wind creates a wind chill effect of -18°, the temperature inside the car's engine block will not fall below 10° unless the actual temperature drops below 10°.

That same 15 mile an hour wind, however, can literally rip the heat away from an animal's fur or a person's clothing.

In general, a human's body heat will warm the layer of air nearest to it. In extremely calm weather, this inner layer stays in place, providing a slight insulation from the colder air further away. A person running, walking very quickly, or simply standing in a wind feels cold because the warm pockets of air in that inner layer are literally being swept away and replaced by colder air.

Antarctic explorer Paul A. Siple coined the phrase "wind chill" in his 1939 dissertation, "Adaption of the Explorer to the Climate of Antarctica."

During the 1940s, Siple and one of his associates, Charles F. Passel, conducted experiments to see how long it would take water in a plastic container to freeze after the container was exposed to the elements.  Siple and Passel soon realized that the correct answer was "That depends!"  The time varied according to three factors:  the temperature of the water when it was first exposed, the temperature of the outside air, and the speed of any wind blowing on the day of the experiement.