[CAUT] RH? Please enlighten me.

[Fred Sturm]

Relative Humidity means relative to saturation at that temperature. At  
saturation, when the air has "absorbed" as much water vapor as it can  
hold, condensation takes place, which may happen as dew. At 100% RH,  
anything slightly lower in temperature will "sweat," meaning water  
from the air will condense on it. When you lower the temperature, the  
air can't hold as much water vapor, so the RH goes up, even without  
adding any moisture. And vice versa when you raise temperature.
	Another way of looking at it is "dew point." This has to do with what  
temperature will cause condensation for the water vapor content of the  
air. It can be measured by taking polished stainless steel and  
lowering its temperature. At the point when it fogs up, that is the  
dew point. Or you can get an idea by putting a glass of ice water out  
and seeing how much and how fast it "sweats." If it doesn't sweat at  
all, your dew point is below freezing, and your RH is probably pretty  
low (how low depends how warm or cold the temperature is).
	Does that help?
Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu



On Dec 9, 2008, at 10:30 AM, Jim Busby wrote:

> List,
>
> The recent post on relative humidity has me wondering what RH really  
> means. Don Ms funny post about “death” with humidity being too low  
> shows that many (myself included) don’t understand RH.
>
> For instance, when I heard that the RH was 100% in Louisiana I  
> figured the whole state was under water.
>
> And, obviously life can be sustained when RH registers at near zero,  
> as is does occasionally in our music building. (Interestingly the  
> static electricity in such low humidity makes it essential that you  
> “discharge” the charge in your body before touching an ETD, or  
> before pumping gas, as multiple fires at gas stations have shown.)
>
> Any scientifically minded tech out there care to enlighten me?
>
> Thanks.
> Jim Busby

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