Ask A Scientist General Science Archive

name         Thomas E.
status       educator
age          40s

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.
------------------------
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
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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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