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Frost on the Pumpkin - Page 2

© Keith C. Heidorn

Frost Formation

When the temperature of a surface falls below the dewpoint temperature but remains above freezing, dew forms through the condensation of water vapour onto it. But if the dewpoint is below freezing (now known as the frost point), we get frost rather than dew. (The frost point is the temperature at or below 0°C to which air must be cooled to achieve saturation with respect to ice.) Therefore, frost, the cold cousin of dew, usually appears over an area during clear, cold nights with light winds when the surface temperature falls below the frost point.

Frost forms first on rock, glass, or metal surfaces that lose heat more rapidly through radiative cooling than the surrounding air. This is why car windshields frost over before frost forms on surrounding vegetation. If the surface on which it forms has a temperature below the frost point, frost may even appear when the officially reported air temperature is above freezing.

Frost formation may be sporadic across an area, particularly in hilly or mountainous terrain. Since cold air is denser than warm air, it flows like water downhill to pool in low areas or hollows. Areas that are low-lying are thus more susceptible to frost formation. They called frost pockets or frost hollows. They are undesirable places for planting gardens.

Frost can form by either of two processes. At temperatures from about 0° to -18° C (32° to 0° F), frost formation will most likely begin by water vapour condensing as a liquid on a surface and then quickly freezing. However, once the first ice crystals have formed, further frost accumulation proceeds more rapidly through water vapour depositing directly as ice crystals than through condensation and freezing.

On certain surfaces in the above temperature range, frost may deposit directly without formation of that initial ice crystal through condensation/freezing. In such circumstances, some surface characteristic or impurities on it can seed the process by mimicking the structure of an ice crystal and thus providing a preferred growth site. This could be a surface flaw such as a scratch on a piece of glass or a chemical impurity, or a particle of salt or dirt lying on its surface.

At temperatures colder than -18° C (0° F), frost form on the surface through the direct deposition of water vapour as ice. This process is called deposition. (Some call this sublimation but technically this is the opposite process, the direct conversion of ice into water vapour without an intervening liquid phase.)

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