 The Clarinet BBoard
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Author: Robert
Date: 2002-09-19 18:57
I've seen that The Doctor and others refer to "relative humidity". What does that mean exactly?
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Author: Dee
Date: 2002-09-19 20:44
The amount of water that the air can hold depends on its temperature. When the air is fully saturated, you have 100% relative humidity. However the actual quantity of water that the air can hold is much higher at 100F than 0F.
The air is seldom fully saturated. So meteorologists calculate how much moister the air is holding compared to how much it could hold at the current temperature. That is relative humidity. If the air is holding half of its maximum capacity for that temperature then the relative humidity is 50%.
The humidity given in the standard weather report is relative humidity.
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Author: Dee
Date: 2002-09-19 20:45
Can't type today. "moisture" not "moister".
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Author: Don Berger
Date: 2002-09-19 20:56
Well, Omar, how should we [and hopefully others, please] attempt to explain the Physical Chemistry of the air-water system [exactly!!], but likely desired only in layman's terms?? My first refuge was my Chem Engr's Handbook, several pages of it! Most libraries will prob have it and the Kirk-Othmer books and others, besides meteorology texts, for explanations. Maybe it will be sufficient just to say that Rel Hum [in %'s {percentages}] is the concentration of water in air OELATIVE to the "saturation" concentration at "that" temperature, sinple-mindedly, how wet is the air!, not getting into engr's steam tables! What think you?? Don
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Author: Don Berger
Date: 2002-09-19 21:06
Great minds, same channels [and typos] Dee, my oelative [vs Relative!!]. As in My Fair Lady, "I believe we've got it" !!! Don
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Author: L. Omar Henderson
Date: 2002-09-19 21:12
Don - It depends on how you measure RH - in practice RH is usually measured by absorption of water on a material with some change in a property of that material - e.g. length, weight, electircal resistance, dielectric constant, etc. So, in the usual case water is measured by water activity and roughly translated in it's ability to do watery things such as participating in chemical reactions, absorption onto hydrophilic molecular groups, etc.. Other kinds of sensors include psychrometers, Dew point detectors, etc. The actual formula (I won't bore anyone with the calculus) depends on the saturation vapor pressure, dependence on temperature and atmospheric pressure. The partial pressure is dependent on the actual composition of air which has several partial pressure components. And one could go on, and on but for our purposes the amount of water vapor in the air is probably good enough (below 50% is low, above 70% is high, New Orleans Clarinetfest was absurd!).
The Doctor
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