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Weather Almanac for November 1999A Frosty Morning  He comes,---he comes,---the Frost Spirit comes! (John Greenleaf Whittier, The Frost Spirit) When I think of the month of November, the first visions that pop into my head are those of cloudy and dark days. In each region of North America where I have lived, the low sun and short days of November enhanced the gloom of skies frequently flushed with stratus clouds from dusk to dawn. But often November mornings dawn quite the opposite with a quiet, spectacularly brilliant beauty. These are the mornings touched by the brush of Jack Frost. Our friend Jack Frost, it appears, is a benevolent artist compared to some of the other frost beings of mythology. Jack is likely the son of the Norse god of wind Kari, born Jokul ("icicle") Frosti ("frost"). When Jokul Frosti immigrated to England with the Norse, he became Jack Frost, an elf-like being who colours tree leaves and paints patterns on windows. Other frost beings including the Frost Woman and Frost Man, important weather deities in Finland and northern Russia, who control blizzards and other cold, wintry elements in these northern regions. Elsewhere in Russia, folks believed Father Frost to be a mighty blacksmith who forged great chains of ice to bind water to the earth each winter. And when Old Mother Frost shook the white feathers from her bed, they fell on German soil as snow. In Japanese folklore, the Frost Man was the roguish brother of the Mist Man. Australian aborigines attribute frost to icicles thrown down to earth from seven sisters whose bodies sparkle with ice. These frosty sisters could not live with men on earth, so they sought a home in the heavens, each one becoming a star of the Pleiades constellation.
Frost, the cold cousin of dew, usually appears over a large area during clear, cold nights with light winds when the air temperature near the ground falls below the frost point. (The frost point is the temperature to which air must be cooled -- at constant pressure and humidity -- to achieve saturation with respect to ice at or below 0°C.) Frost can even form when the officially reported air temperature is above freezing; for an explanation of this situation, see Weather Whys: Frost Formation.
Frost formation may also be sporadic around 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 and thus more susceptible to frequent frost formation are called frost pockets or frost hollows. They are bad places for planting gardens or building homes. When the temperature of a surface falls below the dew point but remains above 0°C, dew forms on the surface through the condensation of water vapour on it. (This happens when water vapour molecules condense on the surface at a rate faster than they can evaporate from it and liquid water accumulates.) But if the dew point temperature (known as the frost point) is below 0°C, we get frost rather than dew. Frost can form in one of two ways, both similar in nature to the formation of dew but ultimately leaving solid water -- ice -- on the surface rather than the liquid water state. At temperatures from about 0° to -18° C (32° to 0° F), frost will most likely form initially when water vapour condenses as a supercooled liquid on a surface and then quickly freezes. Once the first ice crystals have been formed, further frost accumulation may proceed by the deposition of water vapour directly to the solid (ice) state.
At colder temperatures, ice will be deposited directly on the surface through the deposition of water vapour directly to the solid state. This process is called deposition. (Some meteorologists call it sublimation but technically this is also the opposite process, the conversion of ice directly into water vapour without an intervening liquid phase and the use of the same term for the two processes, I find confusing.)
There are two forms of frost: rime and hoar. Rime frost occurs when the rate of frost formation is rapid, usually under conditions of high water content in the air (vapour and/or liquid) and at least moderate wind speeds. Rime formation is common during fogs where supercooled or near-freezing water droplets come in contact with subfreezing surfaces. It may also form when clouds are blown over sub-freezing surfaces, such as occurs when moisture-laden clouds are forced over coastal mountains during the cold season. In these cases, accumulations can be several metres thick, growing in the direction of the prevailing wind at the time of formation. Rime frost, on close inspection, has a grainy appearance, like sugar or salt, forming spikes, needles, or feathers and not having a recognizable crystal structure. Rime is opaque, less transparent than glaze ice formed during freezing rain episodes. Rime is denser and harder than hoar frost. Hoar frost forms from the slow deposition of water vapour directly to ice on a surface. By accumulating slowly, hoarfrost can form interlocking crystals that grow out from the surface. Hoar frost generally has a feather, fern, or flower pattern growing from the initial seed. Hoar frost forms only when winds are light, which is often the situation during clear, cold nights; thus it is the most common type of frost to those dwelling in non-mountainous areas. When we speak of frost, we are usually referring to hoar frost rather than rime frost. Hoar frost shows a definite, delicate crystalline structure of elements growing on elements in steps or layers. The crystal's white colour is caused by small air bubbles trapped in the ice, thus reducing its transparency. The smooth faces of the hoar crystals allow it to glitter in the sunlight, particularly at the low sun angles of early morning. This appearance is in sharp contrast to rime frost which adds a dull, matte finish to the surface on which it adheres. Hoar frost crystals are usually small, but under the proper conditions if left undisturbed, frost ferns and frost flowers may grow quite large. In the quiet conditions found in an ice cave or ice-filled sink hole, for example, they may rival true ferns and flowers in size.
When dews and frosts spread across the countryside, the landscape wears a veneer providing dazzling beauty as the sunlight is scattered, reflected and refracted by the many crystal surfaces. Such frosty November mornings reveal the nightshift labours of Jack Frost, bringing much needed beauty to a month generally characterized in northern latitudes by dull, dark days.
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The word frost can take any one of several related meanings in the English language. To farmers, gardeners and botanists, frost means those subfreezing temperature conditions which halt or destroy plant functions. A frosty day is generally considered to be any day when temperatures are subfreezing regardless of other weather elements. Degrees of frost refer to the number of temperature units below 0°C or 32°F. Finally, and the aspect of frost we will discuss here, frost refers to ice crystals appearing on non-liquid surfaces without the aid of precipitation processes, most often forming during the night.
For frost to form, it must have a surface whose temperature is below the frost point. Thus, frost may form on rocky, glass, or metal surfaces that lose heat more rapidly than the surrounding air through radiative cooling. This is why car windshields may frost over but no frost forms on the pavement or vegetation surrounding the vehicle.
On certain surfaces, frost can begin to be deposited directly at these temperatures without condensation/freezing to form that initial ice crystal. In such cases, some characteristic of the surface or impurities on it can seed the process by mimicking the structure of an ice crystal to provide a growth site. This can be a surface flaw such as a scratch on a piece of glass or a chemical impurity or particle lying on its surface. For example, crystals of silver iodide so closely mimic the structure of ice crystals that they are used for condensation nuclei to seed clouds in rain- and snow-making endeavours.
And, while frosts provide great beauty on the large scale, they are best appreciated close-up. The delicate crystal patterns of hoar frost can make even the most common surfaces a treasured vision. I heartily recommend, the next time you wake up to a frosty morning, to take a few minutes to study a variety of frosted objects through a magnifying glass. The vision will open a new world of intricate natural beauty. This is one time when Weather Eyes should be looking down.