why humid is better!

[Roger Heflin]

Bruce wrote:
> 
> Hmm, I thought to change states, ie from liquid to gas, takes energy.
> So for water to change states, it has to boil to turn into a gas, taking
> many calories to do this.  Water can evaporate, but in that case it's still
> liquid.  Water can exist as a gas only when the temp is above *about* 212dF,
> under normal circumstances.  Second it cools is when it returns to liquid.
> Bruce
>
The first part is correct, but when water evaporites it is *not* liquid.

At 212 F all water is vapor, and lower temps you have what is referred to
as a partial pressure, as say for instance 70F you can have up to .36psi
of water vapor and this would be 100% humidity, 50% is half of that.
At 80F you can have up to .50psi of water vapor, and a 90F you can have
.69psi of water vapor, and at 100 it is .95psi, now at 212F this number 
becomes 14.7psi (full atm pressure), but you always have some real
vapor even though the temp is not high enough, unless it is really
cold.   Temperature is only an average, even though the average of the 
water is 70F, some molecules have higher energy and some have lower
energy, the partial pressure is a measure of how much will have enough
higher energy to exist as vapor and not as liquid water, even though the
temp is not above 212.  Notice that as the temperature of water rises
the amount of "steam" increases even though the water on average is 
not yet hot enough to be steam, some of it is, and notice that as the
temperature rises the steam increases, this is the visual of the partial
pressure increasing.

The dew point at 70F is when you have exactly 0.36 psi of water vapor,
the air cannot hold anymore and the water will start to fall out as
dew (this is liquid, because the air cannot hold anymore water).

Ice melts the same way, on ice you always have some liquid water, unless
things are quite a bit less than 0F (this is the point at which salt quits
melting ice).  What happens when you put salt on the ice, and  the liquid
water on top of the ice mixes with the salt and now freezes at a much lower temp,
so is not allowed to refreeze so it stays liquid, and slowly over time more
of the ice turns to liquid and goes into solution with the salt and 
then stays melted.  This is why salt quits working if things are really
cold, there is not liquid to go into solution with.

Something else that you notcie from the above numbers is that 50% humidity
at 100F is .475psi of water vapor and 100% humidity at 70F is .18, so 50%
humidity at 100 is much much worse than 100% at 70.

				Roger
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