Thursday, March 16, 2006

Spray Chill Factor

OK - all you amateur sailor/scientists who like to argue about something none of us really understands ... we'll return another day to the confusing question of how sails don't work the way all the books say they work.

Today I have another question for you, thanks to Steve Cockerill of Rooster Sailing. We all know about "wind chill" which takes into account the combined cooling effect of air temperature and wind speed. But this doesn't adequately reflect the total chill experienced by us crazy frostbite sailors - in particular that invigorating feeling you get when a wave breaks over your head or icy spray slaps you in the face.

Steve says ...

I am trying to develop a new Chill Factor that takes into account the situation that dinghy sailors find themselves. Wind Chill takes into account the extra cooling effect of the air on bare skin and is only half of the equation. It does not take into account the additional cooling of water (a substance that can cool 27 times faster as it has a higher conductivity) or the dew point - a measure of how dry the surrounding air is (dryer air encourages water to evaporate, leaving you colder) and the water temperature.

So what do you think? How would we calculate a Spray Chill Factor?

In line with the high standards of scientific debate already established on this blog by the author (and others) do not allow your lack of qualifications to deter you from answering this question. If in doubt answer another but apparently similar question, provide links to dubious authorities who can't answer the question either, or indulge in gratuitous name-dropping. Go for it.

1 comment:

Litoralis said...

The best estimate is probably the Wet Bulb Globe Temperature equation. The WBGT is measured by a simple three-temperature element device.

The first temperature, (Tg), is measured by a black globe thermometer, which usually consists of a 150 mm (6 inch) black globe with a thermometer located at the centre. The black globe temperature represents the integrated effects of radiation and wind.

The second thermometer measures the natural wet-bulb temperature (Tnwb). It consists of a thermometer with its bulb covered with a wettened cotton wick supplied with distilled water from a reservoir. Evaporation from the wettened bulb cools the thermometer. The natural wet-bulb thermometer, like the black globe thermometer is not shielded from wind or radiation. This thermometer represents the integrated effect of humidity, wind and radiation.

The final temperature element is the (shade) air temperature (Ta). It is measured by a thermometer shielded from radiation - generally by being placed in a weather screen. It is the standard temperature normally quoted in weather observations and forecasts.

The three elements Tg, Tnwb, and Ta are combined by into a weighted average to produce the WBGT.

WBGT = 0.7 × Tnwb + 0.2 × Tg + 0.1 × Ta

Obviously an experimental measurement of the WBGT (although the most accurate because it automatically takes into account the wind speed) is impractical most of the time. So here's a mathematical best fit for the experimental data using air temperature and relative humidity (this equation does not take into account the relative wind speed):

WBGT = 0.567 × Ta + 0.393 × e + 3.94
Ta = Dry bulb temperature (°C)
e = Water vapor pressure (hPa) [humidity]

The vapor pressure can be calculated from the temperature and relative humidity using the equation:
e = rh / 100 × 6.105 × exp ( 17.27 × Ta / ( 237.7 + Ta ) )
rh = Relative Humidity [%]

The only other factor that is missing from these equations is the the conductive cooling that takes place when the cold water hits the skin.

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