Date: Sun, 31 Mar 2002 23:38:35 -0500 From: Steve Smith <sgs at aginc.net> To: WSFA members <WSFAlist at keithlynch.net> Subject: [WSFA] Re: Interesting Inventions Reply-To: WSFA members <WSFAlist at keithlynch.net> "Keith F. Lynch" wrote: > > Steve Smith <sgs at aginc.net> wrote: > > Take a large shallow pond. Water will evaporate, cooling it. Cover > > it with straw during the day to keep it from warming up. Remove the > > straw at night to expose the pond to the sky (radiation temperature > > about -268 degrees C (5K)). The water will freeze. > > 3K, not 5K. But the freezing has little to do with the radiation > temperature of the sky, and plenty to do with the relative humidity. > Water will always seek the dew point. That's how a wet-bulb > thermometer, for measuring relative humidity, works. If the dew point > is above the freezing point of water, water will not freeze. If the > dew point is below the freezing point of water, water will eventually > freeze unless there's a source of heat, such as direct sunlight. 5 degrees. The extra two degrees comes from water vapor in the atmosphere. (Not much water vapor in the desert). The data came from a Scientific American article a few years back that talked about various "traditional" means of keeping cool in the desert. The article stated that radiation cooling was the main reason that it actually froze. Evaporation (dew point) helps, of course. I can personally vouch for the fact that the Arizona desert cools off *very* quickly after dark. Very little water vapor in the air. To bring a SF note to the discussion, at Iguanacon II in 1978 in Phoenix, Harlan Ellison was the GoH. When he found out that Arizona hadn't ratified the Equal Rights Amendment, he talked about doing all sorts of nasty things to let people in AZ know what assholes they were (he used stronger language, of course). One of them was to have as many people as possible avoid staying in the hotel and camp in the desert. Now, desert camping is no more dangerous than any other form of camping -- if you know what you're doing. Fortunately, the locals talked him out of it -- not only does it get cold at night, Labor Day is right about at the beginning of thunderstorm season ... -- Steve Smith sgs at aginc.net Agincourt Computing http://www.aginc.net "Truth is stranger than fiction because fiction has to make sense."