MadSci Network: Earth Sciences

Re: Why does frost formation stop?

Date: Tue Sep 6 14:58:23 2005
Posted By: Ken Harding, Science and Operations Officer
Area of science: Earth Sciences
ID: 1124295256.Es


You've almost answered your own questions. Good job doing some research on 
your part.

Frost formation ceases for several reasons. Most likely, environmental 
changes (temperature, dewpoint, or wind) become unfavorable for continued 
formation. As frost forms, water goes directly from the vapor stage to the 
solid form - deposition. This process skips the liquid phase and allows 
the formation of frost on surfaces below 0 C. Interestingly, this 
depositional growth of ice crystals in car windshield is the same process 
that grows snow flakes in clouds, and for the same partial pressure 
gradients you mentioned. 

A smaller affect of slowing deposition would be as the frost builds up 
from the removal of vapor from the air, without a significant 
of vapor, the frost formation will slow or stop. With continual vapor 
present, frost will continued to be deposited.

When the wind is blowing even a small amount ( 2 to 3 m/s) the atmosphere 
stays well mixed, making it harder to deposit frost. A well mixed 
atmosphere, even on a clear night, will redistribute heat more evenly, 
oftentimes keeping the temperature warmer. 

Mathematically, the vapor pressure differences are described by the 
clausius clapeyron equation. Google has many resources that describe this 
equation and give examples. 


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