On Thu, 21 Nov 1996, Paul Hunt wrote:
On Tue, 19 Nov 1996, Paul Hunt wrote:
> > >
> > > Blowing cold air on the screen will clear it much faster than hot
> > > air, in fact hot air will often make it worse.
And Ulix responded:
> > >
> > I doubt that very seriously. Would you care to elaborate?
> >
> > Ulix __/__,__
>
>
And on Thu Nov 21, Paul Hunt retorted:
>
> It is similar in principle to A/C, although not nearly as effective. I have
> seen comments in the list about how A/C users don't have the same problem,
>and
> the handbook in a previous A/C equipped car recommended switching it to chill
> when first getting in the car on a cold and damp day, and only slowly
>changing
> it to heat. It works.
> PaulH
Oh, my. This threatens to become one of those, "slow the water down so
it will have more time to cool in the radiator" threads.
Paul, you are comparing apples to oranges. Switching on the A/C in a
modern car improves demisting because the air passes over the cooling
coil, which condenses water out of the air, before it is blown on the
windshield. The water you see dripping from the bottom of air conditioned
cars is water that condensed on the cooling coils. Because the cooling
coil dries the air, switching the A/C on clears the window quickly because
the air has a low relative humidity.
If you have no air conditioner, then the rules are different. You want
the relative humidity of the air you blow on the windshield to be as low
as possible, because air with a low relative humidity will clear the fog
faster than air with a high relative humidity.
How do you lower the relative humidity if you have no air conditioner to
remove water from the air? You heat the air. Passing the air over a warm
coil warms the air, but does not change the amount of water vapor in the
air. Warm air can hold a larger amount of water vapor than cold air.
Because relative humidity is a measure of how much water vapor is present
expressed as a percentage of how much the air can hold, heating the air
lowers the relative humidity of the air. The warm air can now hold more
water vapor than the same air could hold when colder, so it will demist
more effectively.
Bottom line: in a car with no A/C, warm air will demist the windshield
faster than cold air.
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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