> Does anybody know an equation for calculating relative humidity
> when wet bulb and dry bulb temperatures are known?
The first hit when Googling for "bulb relative humidity equation" was:
If that doesn't do the job, do your own web search and check out other sites.
Wet-bulb temperature is the dew point temp. of an air-water mixture, that is, the temp. at which water droplets begin to condense when an air-water mixture is cooled. Dry bulb temp. its temp. before cooling. The former is always less than the latter except when relative humidity is 100%.
Every M.E. learns that there's a corresponding unique saturation pressure for each saturation temperature, another word for the dew point temp., or wet bulb temp, for water vapor. At 100C or 212F, this saturation pressure is
14.7 psi, or atmospheric pressure. At 35F, just above the freezing point, it is 0.09992 psi.If you're from the metric world, atmospheric pressure (14.7 lbs/square inch, a.k.a 1 bar) is about 101 kilopascals (kPa).
Relative humidity is the ratio of the saturation pressure corresponding to the wet-bulb temp., call it Pwb, to the saturation pressure corresponding to Pdb, the saturation pressure corresponding to dry bulb temp, or,
RH (%) = Pwb / Pdb * 100%
In winter, when the air is very dry, the dew point temp. is often well below freezing.
No kidding! As in -60. C or F makes no difference. They are both cold enough to freeze the balls off a brass monkey.
Walter.
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