Wind Chill

Wind chill is the apparent temperature felt by humans as a result of air blowing over exposed skin. The temperature that humans actually feel, called the sensible temperature, can be quite different from the

temperature measured in the same location with a thermometer. The reason for such differences is that the human body constantly gives off and absorbs heat in a variety of ways. For example, when a person perspires, evaporation of moisture from the skin removes heat from the body, and one feels cooler than the true temperature would indicate.

Wind chill formulation was begun by the U.S. military during World War II (1939–1945). These tables, called the Windchill Temperature (WCT) index, were provided to the U.S. National Weather Service, a part of the National Oceanic and Atmospheric Administration (NOAA), in the 1970s. In 2001, these tables were revised in order to include additional information that had been collected over the years. The new formula for wind chill figures was based on technological advances in science, computer modeling, and general technology. The formula provides a more accurate and better understandable means for calculating wind chills, especially when dangerous high winds and low temperatures are present.

In still air, skin is normally covered with a thin layer of warm molecules that insulates the body and produces a sensible temperature somewhat higher than the air around it. When the wind begins to blow, that layer of molecules is swept away, and body heat is lost to the surrounding atmosphere. An individual begins to feel colder than would be expected from a thermometer reading at the same location. The faster the wind blows, the more rapidly heat is lost and the colder the temperature appears to be.

A example of a wind chill chart that shows the relationship among actual temperature, wind speed, and wind chill factor, or the temperature felt by a person at the given wind speed is shown in Table 1. according to this chart, no observable change in temperature is sensed for wind speeds of 4 mph (6 km/h) or less. At a wind speed of 15 mph (24 km/h) and a temperature of 30° F (-1.1° C), however, the perceived temperature is 19° F (-7.2° C).

The colder the temperature, the more strongly the wind chill factor is felt. At a wind speed of 30 mph (48 km/h), for example, the perceived temperature at 30° F (-1.1° C) is 15° F (-9.4° C), but at -40° F (-40° C), the perceived temperature is -80° F (-62.2° C). It is noted, however, that bright sunshine may increase the wind chill temperature by 10 to 18 degrees Fahrenheit.