Frost on the Pumpkin

Short days and low sun angles enhance the dreariness of November skies frequently flushed with stratus clouds from dusk to dawn. But at times, November mornings dawn with a quiet, spectacularly brilliant beauty. These are the mornings touched by the brush of Jack Frost.

Frost has several related meanings. To farmers and gardeners, frost means subfreezing temperatures that halt or destroy plant functions. A frosty day is thus any day when temperatures are subfreezing, and degrees of frost is a term used to refer to the number of temperature units below 0^oC or 32^oF over the period. The aspect of frost I will discuss here, however, refers to ice crystals which appear on solid surfaces in the absence of precipitation. There are two forms of such frost: rime and hoar.

Rime frost occurs when the rate of formation, usually under conditions of high atmospheric water content (vapour and/or liquid) and at least moderate wind speeds, is rapid and adds a dull, matte finish to the surface on which it adheres. Rime formation is most common during cold fogs when water droplets come in contact with subfreezing surfaces. It may also occur when moisture-laden clouds are forced over cold mountain slopes, growing toward the wind direction prevailing at the time of formation. In some cases, rime accumulations can reach thicknesses of a metre or more.

Hoar frost, on the other hand, forms through the slow deposition of water vapour directly on a surface as ice. It forms best when winds are light, which is often the situation during clear, cold nights. By accumulating slowly, hoar frost forms delicate, interlocking crystals that grow outward from the surface with a feather, fern, or flower pattern. Hoar frost appears as definite, delicate crystalline structures of elements growing on seed elements in steps or layers.

Hoar frost's white colour is caused by small air bubbles trapped in the ice crystal that reduces its transparency. The smooth faces of the hoar crystals cause them to glitter in the sunlight, particularly at the low, early-morning sun angles. Frost crystals are usually small, but under the proper conditions if left undisturbed, frost ferns and frost flowers may grow quite large, particularly in locations well sheltered from wind and sun.

Hoar frost is the most common frost for those dwelling in non-mountainous areas. When we speak of frost on the pumpkin, we are usually referring to hoar frost rather than rime frost, and hereafter I will also do so.

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^oC 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^o to -18^o C (32^o to 0^o 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^o C (0^o 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.)

A Final Thought

The beauty of frost is one good reason to turn weather eyes downward. The beauty of crystals can be further enjoyed with a magnifying glass. ...I wonder how spiders view frost clutching their webs?

(Rime on tower photo courtesy NOAA US Dept of Commerce.)

Learn More From These Relevant Books
Chosen by The Weather Doctor

Schaefer, Vincent J. and Day, John A.: A Field Guide to the Atmosphere, 1983, Houghton Mifflin Company, ISBN 0-395-33033-5.
Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.

Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
November 1, 2005
A version of this material was previously published by Keith Heidorn on Suite 101: Science of the Sky, 2001