Frost Point
Frost Point
Definition
The temperature to which a given parcel of humid air must be cooled, at constant atmospheric pressure, for water vapor to be deposited on a surface as ice crystals without undergoing the liquid phase. (The gas to solid process is called deposition as opposed to the gas to liquid process know as condensation.) The temperature must be below freezing and the air must have cooled enough to become saturated, given that the relative humidity of the air will increase as the temperature falls until it reaches 100%.
Discussion
The dew point or dew point temperature of an air parcel is the temperature at which liquid water (dew) forms. Specifically, it is the temperature as which the partial pressure of water vapor in the parcel is equal to the equilibrium vapor pressure of liquid water. Partial pressure is the fraction of total pressure due to one element of the air, in this case water vapor. It can also be thought of as the fraction of total molecules in a parcel of air. The equilibrium vapor pressure is the pressure (or again number of molecules) at which a surface is in steady state with the air above it, in this case liquid water. Overall, there is no evaporation or condensation. If the partial pressure of water vapor in the parcel exceeds this, i.e. more water vapor in the air, then we have condensation.
The frost point temperature is the temperature at which the partial pressure of water vapor in the parcel is equal to the equilibrium vapor pressure of ice. Above 0 C, the equilibrium vapor pressure of liquid water is higher than the equilibrium vapor pressure of ice. Below 0 C, the equilibrium vapor pressure of liquid water is less that the equilibrium vapor pressure of ice.
Above 0 C, when the air becomes saturated condensation occurs and dew will form. Below 0C, when the air becomes saturated, deposition occurs and frost will form.
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