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Frost on the Pumpkin

© Keith C. Heidorn

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°C or 32°F 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.