Published: 2008-02-24 11:03:34
Updated: 2008-02-24 11:03:34
Posted February 24, 2008
By Ryan Oberlin
MIKE MOSS SAYS: Ryan, Under typical circumstances, water freezes at a temperature of 32 degrees Fahrenheit or lower. Regardless of the wind chill value, if the temperature of air and water is 36 degrees, the water will not freeze due to any wind chill effect. This is because wind chill is not an actual temperature. It is intended to represent the actual temperature that would produce a similar rate of heat loss from human skin in the absence of wind.
So, if we had a temperature of 36 degrees and a wind of 15 mph, for example, the wind chill value is 27 degrees. What that really says is "the combination of 36 degrees and 15 mph of wind makes a person lose body heat (generated by our metabolism of food) at about the same rate as a temperature of 27 would if the winds were negligible." Since our perception of cold is tied to the rate of heat loss, we "feel" about the same in 36 degrees and 15 mph wind as we do in nearly calm winds at 27.
Water, on the other hand, does not produce any heat on its own, so there is no wind chill effect. If the temperature of the air and water is 36 degrees for both, air blowing across the water will not transfer heat out of or into the water because they are the same temperature to begin with. Remember that for a person, the 36 degree air is blowing across skin that will try to maintain itself at a temperature of around 90 degrees or a little higher, so that there is a strong transfer of heat from the skin into the air. All this means that the wind chill value of 27 degrees is a nearly meaningless number from the point of view of the water or any other inanimate material that doesn't have an internal heat source.
As an aside, one could probably postulate a scenario in which evaporation of the water, the rate of which would depend in part on the humidity of the air and on the wind speed, could cool the water enough to reach the freezing point. This would occur more rapidly the higher the wind speed was, but involves a lot of complexities - for example, if the air remains at 36 degrees and the water cools due to evaporation, heat will be transferred into the water from the air, which would warm the water and cool (plus humidify) the air. What happened from that point would depend on the source of the air, how it came to be 36 degrees in the first place, how much water is involved and what sort of container it is in, and so on. As another example, if the air was saturated so that its relative humidity was already 100% or very close, then regardless of how strong the wind was there would be no net evaporation from the water surface and the water would not cool down. That's all a bit of a diversion from the original question, though, and isn't directly related to wind chill.