|MadSci Network: Earth Sciences|
GL, 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 replenishment 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. Ken
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