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Wind chill

Wind chill forecast

The term wind chill goes back to the Antarctic explorer Paul A. Siple, who coined it in a study: "Adaptation of the Explorer to the Climate of Antarctica".
In 1940 Siple and Charles F. Passel conducted experiments on the time needed to freeze 250 grams of water in a plastic cylinder that was exposed to the elements. The three parameters involved are:

  • the initial temperature of the water;
  • the outside temperature;
  • and the wind speed.

The wind chill factor or equivalent temperature uses a neutral skin temperature of 33 C as a baseline value, so only two parameters remain.

Definition: Wind chill accounts for loss of heat when warm air around a body is replaced with colder air. The factor is an indication of the effect of the combination of air temperature and wind speed on human comfort and safety.

  • The ability to forecast wind chill is especially important to yachtsmen since:
  • offshore weather forecasts rarely include a wind chill factor;
  • both the wind speed and the apparent wind speed are likely to differ from the forecast true wind speed;
  • the nature of sailing implies that the duration of exposure to wind chill can not be predicted;
  • the threat of hypothermia can be anticipated by using the factor.

Indeed, the wind chill factor is a highly underrated tool to prevent hypothermia and frostbite on board!

This table provides equivalent temperatures and is based on the Steadman equation below, though many other variations exist:

ET = 0.045(7.1766 x √KNOTS + 10.45 - 0.5145 x KNOTS)(Celsius - 33.0) + 33.0
  • Forecasts made with these equations are reasonably sufficient, though work is being done at this very moment to obtain a better index or factor.

Current inadequacies include:

  • the Siple and Passel experiment did not take into account that the water was stored in a container with properties of its own;
  • there's no heat being generated in a water filled object, whereas the human body is constantly generating heat.
  • the original experiment uses wind speed at 10 metres above ground.
  • the current wind chill index also ignores other environmental factors such as sun shine, air humidity or precipitation;
  • the equations are not valid above 40 knots and below 5 knots!

safety
Conclusion: For your safety: anticipate and use the equivalent temperature to adequately forecast wind chill.