When you go outside on a windy winter day, the cold air seems to take your breath away and it feels even colder than the thermometer would lead you to expect. This is the physical manifestation of the phenomenon known as windchill. During the last century, scientists have developed ways to quantify the effect of this factor on human activities.
The movement of air causes heat loss through a process called convection. When the air is moving faster, the heat loss occurs more rapidly. To illustrate this, think about a cup of coffee that you pour and set on the table. If you leave the coffee sitting there for a while, the heat will transfer to the surrounding air through the mechanism of convection, and your coffee will get cold. If you blow on the surface of the coffee, you accelerate the transfer of heat to the surrounding air, and the coffee gets cold even faster.
The measurement of windchill attempts to quantify the reduction in temperature caused by moving air. It is expressed as a relative temperature that compares the cooling effect of a certain air temperature combined with a specific amount of air movement to a lower temperature under calm conditions, i.e. with little or no air movement. It does not express an actual lower temperature but indexes an equivalent amount of heat loss.
The first windchill calculations were published by Major Charles Siple and geographer Charles Passel in 1945, the result of experiments they conducted while working in Antarctica. Their experiments involved the amount of cooling measured in containers of water under different wind and temperature levels. Authorities began using these conclusions as the basis for warning people about potentially dangerous weather conditions which would require extra concern and the use of heavier or additional clothing.
In recent years, the original windchill index has been revised to reflect scientists’ thinking that human bodies don’t react to windchill in the same way as water, because the body retains heat through metabolic processes and the insulation of body fat and clothing.
The current windchill chart published by the National Weather Service (NWS) shows windchill-adjusted relative temperatures for various combinations of ambient temperature and wind strength, as well as exposure times that are likely to result in frostbite on unprotected skin areas at given temperature levels. According to the NWS, the current index is based on a human face model, assumes a calm wind threshold of no more than 3 mph, uses a consistent standard for skin tissue resistance, and assumes no mitigation of conditions from sunshine. It calculates wind speed at five feet above the ground, the average height of a human head, based on a standard height of 33 feet above the ground, the typical mounting height of an anemometer, an instrument that measures wind strength.
The windchill factor is not intended to scare anyone, despite the overemphasis sometimes given to it in the media. It is intended to help people know when to take extra precautions against exposure and to dress appropriately for potentially severe conditions.