wolit (07/08/82)
Well, here it is July in New Jersey (and elsewhere), so I guess it's a good time to start a discussion of that old nemesis of weather-watchers, the Wind-Chill Factor (hereafter, WCF). This was brought on by a piece in net.auto, where some Honda-owner in Wisconsn was bragging about how well his buggy started last winter when the temperature was -20 and the WCF -80 or so. As I understand these things, the WCF is only of concern to us evaporation-cooled, warm-blooded beings. A car, sitting outside all night, is gonna be pretty much at the ambient temperature by morning, no matter HOW hard the wind is blowing. Unless you start dowsing it with some volatile liquid, there's no way it'll get any colder. Even then, it won't get below the wet-bulb temp, which is NOT the same as the WCF. The WCF basically tells you how bad you're gonna feel, by measuring how FAST you'll cool off. Now for the bad news: All the WCF charts I've ever seen plot the WCF as a function of the ambient, dry-bulb temperature, and the wind speed. Nowhere is the relative humidity taken into account. It seems to me that the RH should have some effect on how cold you feel, since you'll cool off faster in dry air, because of evaporation, than you will in moist air. Any ideas on why there is no RH factor in the WCF? Next winter, we'll talk about the Temperature-Humidity Index, and what the Energy Factor number on your air conditioner means... Cheers, Jan Wolitzky (...rabbit!wolit)
soreff@sri-unix (07/09/82)
I would think that cooling rate of a human (in reasonably cold weather) should depend only very weakly on the relative humidity. I'm assuming that a thin layer of air neer the human reaches about body temperature or so, and that the cooling rate depends on the rate of replacement of and conduction through and evaporation into this mass of air. If the source of this air is ambient air at some low temperature (eg below freezing) then the initial partial pressure of water vapor will be very low (compared to the saturation pressure of water at body temperature) even if the air is fully saturated at ambient temperature. Cold air should always feel like dry air to a warm blooded animal, basically because the vapor pressure of water is strongly temperature dependent. Warning: This is only a guess, I don't know the details of the physics of evaporative cooling. Does anyone out there have a solid reference? -Jeffrey Soreff
kpmartin (07/12/82)
I suspect that people perspire so little in cool weather that the relative humidity makes little difference. Also, when the cold air is warmed by your skin the relative humidity drops, since the total capacity for water vapour has increased. I suspect that most of the wind chill effect is merely due to the stirring of the air. In still air, a thin layer of warm air forms over the skin (and rises to form a plume above you). As the wind increases, this layer becomes thinner and cooler. So the temperature you feel when it is -10 is not really -10 unless it is VERY windy; it is probably closer to +10 (celsius). For high wind speeds, the cooling effect will approach that of a steel block, or a body of fluid, both of which have higher heat capacity and better cunductivity than air.