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".
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
- 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
equation below, though many other variations exist:
ET = 0.045(7.1766 x √KNOTS
+ 10.45 - 0.5145 x
- 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
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!
Conclusion: For your safety: anticipate and use the equivalent
temperature to adequately forecast wind chill.