In article et,
"Dave Liquorice" writes:
On Sat, 14 Feb 2009 12:02:34 -0000, Pete Zahut wrote:

Dunno about that one but if you have a rotary clothes line outside,

Depends on the outside temperature really, the warmer it is, the quicker
stuff dries.

No it's the relative humidity not the absolute temperature that enables
the clothes to dry. It could be 40C but 100% RH and your clothes would
never dry, conversly it could be 4C - 10% RH and your clothes will dry
very quickly.

I take your point, but at the particular combination you've given,
4C - 10% RH, the wet bulb temperature (i.e. the temperature of your
clothes) would extrapolate back to about -3C. Now they won't actually
drop below zero, but this means the clothes are only 4C colder than
the surrounding air, which is less energy transfer to supply the
latent heat for the evaporation that it would be if the full 7C temp
difference was obtainable, so it will significantly slow down
the drying of your clothes. At 10% RH, this effect kicks in below
about 8C (the point at which the wet bulb temperature is 0C).

So it's not actually the relative humidity so much as it is the
difference between wet bulb and dry bulb temperature which affects
drying times. This varies with relative humidity (and is zero at
100% humidity), but it's also this temperature difference which
passes the energy into the clothes to provide the latent heat for
evaporation of the water, and the larger the difference between
wet bulb and dry bulb temperature, the faster the energy will pass
into the clothes to evaporate the moisture and the more moisture
will be evaporated. Any air movement helps lots too, by both
transferring energy from the air to the clothes faster, and by
removing the air which has absorbed moisture from the clothes
faster.

You can read off the wet bulb, dry bulb, and humidity values from
the following chart:
http://upload.wikimedia.org/wikipedi...eaLevel-SI.jpg

--
Andrew Gabriel
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