On Mon, 20 Mar 2006, Lance A. Brown wrote:
We've had an abnormally dry winter....
OK we get dry air from the high pressure system over Canada
in winter. Winters are normally dry. but it would be the
same inside if it rained all the time in winter here (except
for wet clothes drying). The real problem is taking cold
air from the outside (which doesn't hold a lot of water) and
bringing it inside and warming it. Whether the outside air
was saturated (raining) or dry (clear blue sky) doesn't
matter a whole lot since there is so little water in the
cold air.
I couldn't find any graphs of saturated water vapor pressure
in the range 0-30C on the web. Instead look at the wet-bulb
depression (which is related to vapor pressure)
http://www.novalynx.com/reference-rh-table.html
Take saturated air at 50degF (10deg C left side) warm the
air 10degC (to 70F), by going across the table, and you have
humidity of <10%. If the air had been dry initially, say at
30% humidity, then you would only have to warm the air 8degC
(to say 65degF) to get the same low humidity.
(anything below 40% is regarded as dry)
If instead you brought in air saturated at 15deg C (say
60deg F), (rather that at 10degC) and warmed it 5degC (say
to 70deg) then the humidity would be 50% rather than <10%.
Air outside at temperatures less than 50degF are going to be
bone dry once it's brought inside and warmed to anything
we're likely to be sitting around in.
The main determining factor for humidity inside a building
in winter is the temperature of the air you're bringing in
not whether it was dry or wet to start.
Joe
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