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Hi Randy & All, Aug 15, 2008
It depends upon how clean the air is. Condensation is usually
initiated around hydroscopic nuclei. My Meteorology book (Taylor) says
that "With perfectly pure air from which all nuclei have been removed by
repeated washing, condensation will not start until a relative humidity
of about 420% is reached...".
But in all but ultra pure air, supersaturation so far as I know is
transient as caused e.g. by sudden cooling as in a cloud chamber where
as I recall the detection of charged particle movement relies on
charge-induced condensation of supersaturated air.
Frankly I would suspect probe calibration [I tried recording RH
using Omega (?) probes about 1992 and none of three (?) new probes
worked well enough to be of use due to output drift] to be faulty if
checks of the headspace of constant humidity solutions have not been
run. Any Chemistry and Physics Handbook edition should have this
information indexed as 'constant humidity'. For example, the RH of air
over a saturated solution of ZnSO4.7H2O is 90% at 20oC. One could get a
rough idea of calibration just using a covered jar with excess crystals
in a dark closed closet but a covered wide-mouth Dewar flask in a
constant temperature chamber would give more reproducible readings.
Yt, DW
Randy Lauff wrote:
> Every so often, a student will preamble a question with, "You're going
> to think I'm stupid but..."
>
>
>
> So this sounds like one of those questions. I bought some data loggers
> which record relative humidity. They are routinely recording over
> 100%. Can relative humidity be over 100% (i.e. is the air
> supersaturated with water)? And if so, what is the maximum Relative
> Humidity possible?
>
> Thank you,
> Randy
> _________________________________
> RF Lauff
> Way in the boonies of
> Antigonish County, NS.
>
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