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Frost and Surface Orientation
Name: Thomas E.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 8/20/2003
Question:
Why does frost form faster and heavier on horizontal surfaces than vertical surfaces?
You will notice this when scraping frost from your car's windshield; the side windows, which are
more vertical, will have much less frost on them. Gravity should not play a role because frost
and dew do not fall from the sky.
Replies:
Thomas.
The sloped windshield is looking toward the empty sky. The side windows are facing objects on the
ground. The ground-based objects are emitting infrared heat waves. The empty sky is radiating
nothing back to the windshield. When the air's temperature and moisture content is just right,
the frost will form better on the windshield than the side windows because the side windows are
being ever-so slightly warmed by radiation from nearby objects.
Regards,
ProfHoff 717
For frost to form on a surface the surface temperature must by less than 0 C. On a clear night (and particularly when there is no wind) this is possible even though the air temperature may be several degrees above freezing. The reason is the surface radiates infrared radiation back to the sky, and in calm air convection is not adequate to equilibrate the surface and the air temperatures. A related observation is that frost forms less on a cloudy night than on a clear night because radiation loss is less. All of the above applies to the formation of dew if the surface temperature is greater than 0 C.
Frost (or dew) will not form unless there is sufficient water vapor present in the air so that the frost point (or dew point) temperature is greater than the surface temperature. This is "why" frost frequently does not form on very cold nights -- all the water vapor is "squeezed" out of the air and it is too dry to form frost.
The actual detailed mechanism of frost formation is very complicated my research discovered. Surface charge, thin films on the surface, and other parameters play a role. However, ignore these factors.
The air close to the cold surface is denser than the "ambient" air because of its proximity to the colder surface. This causes a "brushing" motion of air across the surface and water vapor is deposited directly on the surface without any intermediate formation of liquid; however, the detailed hydrodynamics is complicated. On a horizontal surface the hydrodynamics are even more complicated. The air next to the surface loses water vapor to frost, increases in density due both to this loss and to the lower temperature. In both cases there is a pressure gradient between the higher density air near the surface and the "ambient" air which quickly equilibrates. This expansion "drives" the trailing "fresh" air toward the surface so that the process continues. In the absence of gravity this convection would not occur.
In the case of a vertical surface, all of the above pertains, but the flow near the vertical surface tends to be more laminar and less convective. Hence the less frost.
This "explanation" is still incomplete but frost formation, and ice itself, is very complicated.
Vince Calder
Moist air is less dense than dry air. The simple
explanation is that O2 and N2 molecules have more mass than H2O;
therefore, the less water in the air per unit volume allows more O2 and N2
molecules to fill that volume and increase the density of the air. In
aerodynamics, the more humid the air the less performance you get from an
airfoil due to the decreased density of the air.
The vertical movement and moisture content of air in the atmosphere is
more a function of temperature rather than densiy.
However, for clarity,
density is inversely proportionate to temperature
(i.e. cold air is more dense, higher temperature air
is less dense). But, the temperature of the surface
upon which the frost forms is the determining factor
in where it will form rather than the total density of
the surrounding air from whence the moisture is
derived.
Todd Nelson, ATP
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