Wind chill

This page is for the term "wind chill." For the film, see Wind Chill.

Wind chill is the apparent temperature felt on exposed skin due to the combination of air temperature and wind speed. Except at higher temperatures, where wind chill is considered less important, the wind chill temperature (often incorrectly called the "wind chill factor") is always lower than the air temperature.

Explanation

Air movement increases the rate at which the temperature of an object reaches the temperature of the ambient air. Humans feel this increased rate of heat transfer as wind chill.Any wind increases the rate at which moisture evaporates from the skin and carries heat away from the body. The phase change of water (in sweat) from liquid to vapor requires that the molecules reach a higher energy state. That energy is acquired by absorbing heat from surrounding tissue by conduction (see heat transfer). Wind also increases forced convection, which directly transfers the heat away from the body.

Definitions

Some meteorologists disagree that wind chill temperature should always be less than air temperature, on the grounds that the average wind speed is not calm anywhere on Earth. They propose the establishment of a benchmark wind-speed figure, typically in the range of 8–13 km/h (5–8 mph). Any wind speed slower than this benchmark would actually result in a higher wind chill temperature than the air temperature.

The official definition of wind chill in meteorology was originally based on measurements taken at a distance above the ground. The exact definition of wind chill has been controversial because it is a composite index, because animate and inanimate bodies behave differently, and because wind chill reports have a major impact on winter tourism.

Significance

The concept of wind chill is of particular significance in very cold climates such as the Arctic and Antarctic, at high altitude, at high speeds, or in very high winds. It is of great importance to the survival of humans and animals, and can even affect machinery and heating systems.

Formulae and tables

The first wind chill formulae and tables were developed by the United States military during World War II, initially by Siple and Passel working in the Antarctic, and were made available by the National Weather Service by the 1970s. In 2001 the formulae were revised to reflect more accurate theories and testing than those done by the military. These formulae are designed specifically for the human body, or even more specifically for the human face. Wind chill also affects animals, and wet, inanimate objects, but different formulae apply to them.

Original model

It has generally been conceded that the original model for wind chill was not necessarily the best possible for all purposes. The physical basis for the calculation of wind chill is now the relationship between the temperature, volume and pressure of a fluid. Moving air reduces air pressure and increases the cooling effect. Still air can actually insulate, which is why wind chill was measured a number of metres above the ground rather than at ground level.

The US and Canadian formulae are best suited to extremely cold climates. Other formulae such as the Steadman wind chill index (developed by Australian environmental scientist Robert Steadman) have been developed for temperate climates, but are less well known. Some wind chill indices also take humidity into account—and the wind chill and heat index are sometimes collectively referred to by a single term, either "apparent temperature" or "relative outdoor temperature".

New wind chill index

The new wind chill index used by the US and Canadian weather services is determined by iterating a model of skin temperature under various wind speeds and temperatures. The results of this model may be approximated, to within one degree, from the following formula:

<math>T_{wc}=13.12 + 0.6215 T_a-11.37 V^{0.16} + 0.3965 T_a V^{0.16}\,\!</math>

where <math>T_{wc}\,\!</math> is the wind chill index based on the Celsius scale, <math>T_a\,\!</math> is the air temperature in °C, and <math>V\,\!</math> is the air speed in km/h measured at 10 m (33 ft, standard anemometer height).^[1] The equivalent formula in US customary units is:

<math>T_{wc}=35.74+0.6215 T_a-35.75 V^{0.16}+0.4275 T_a V^{0.16}\,\!</math>

where <math>T_{wc}\,\!</math> and <math>T_a\,\!</math> are measured in °F, and <math>V\,\!</math> in mph.

As the air temperature falls, the chilling effect of any wind that is present increases; that is to say, a 16 km/h (10 mph) wind will lower the apparent temperature by a wider margin at an air temperature of −20°C (−4°F), than a wind of the same speed would if the air temperature were −10°C (14°F). Winds stronger than 65 km/h (40 mph) are assumed to have no significant additional chilling effect beyond the effect felt at that velocity, and the wind chill phenomenon is thought to be absent altogether at air temperatures above approximately 20°C (68°F).

Celsius Wind Chill Chart w/Frost bite info

Comparison of old and new values of Wind Chill at minus 15 C

References

External links

CAUTION: Some heat index charts are misleading because they use an arbitrary level of humidity, not 0%, as their reference level. For example, they may describe 32°C (90°F) as feeling like 32°C (90°F) at 30% humidity, and like a lower temperature at a lower humidity; but 32°C (90°F) can correctly be said to feel like 32°C (90°F) only at 0% humidity. For calculating wind chill, ensure that you use a modern chart for accurate results.

• National Weather Service Wind Chill Temperature Index Table of Wind Chill temperatures
• National Science Digital Library - Wind Chill Temperature
• [1] Environment Canada's Wind Chill Calculator
• An Introduction to Wind Chill A lesson plan on Wind Chill
• A Cold Wind Blowing An article on wind chill and tables in SI units
• Wind Chill and Humidex Criticism about the use of Wind chill and humidex
• Windows program for Heat Index, Wind Chill, etc

See also

Frostbite

Categories

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