The Frost Spirit

November can sometimes seem an endless parade of grey, stratus cloud filled days. It is also a month of frosty nights and mornings and of course, we can't have frost without thinking about Jack Frost!

The story of Jack Frost has Nordic origins, where he was called Jokul ("icicle") Frosti ("frost",) son of the Norse god of wind, Kari. When Jokul moved to England, he became Jack Frost, and was pictured as an elf like being who coloured tree leaves and the ground with frost and painted patterns on windows. Being popular, Jack then emigrated to the US.

Other cultures had frost beings too. Frost Woman and Frost Man were weather deities in Finland and northern Russia. They controlled cold weather events in their domains, from frost to blizzards. In another part of Russia, Father Frost was a blacksmith who forged chunks of ice and sent them to Earth. The Germans had Mother Frost, who shook white feathers from her bed which fell on Germany as frost and snow.

The Australian Aborigines have an interesting story about frost that connects it to the Pleiades, the little open cluster of stars seen now in our eastern skies after dark. To the Aborigines, these stars are seven sisters whose bodies sparkle with ice and who hurl icicles down to Earth as frost.

Farmers and gardeners are not concerned with the actual physical frost, they usually think of frost as any below freezing temperature conditions which can cause damage to plants. The rest of us think of frost as the appearance of ice crystals on surfaces without precipitation.

Frost most often forms on cold, clear windless nights, or nights with light winds. The air temperature near the ground fall below frost point - the temperature to which air must be cooled (at constant humidity) to achieve saturation with respect to ice at or below 0 C. Interestingly, this can happen when the air temperature is above freezing.

Frost forms first on rocky, glass and metal surfaces, things which lose heat most quickly. That is why you will often see car windshields frosted over while there is no frost on the surrounding ground.

When frost begins to spread to the ground, it starts with low regions. This is because warm air rises, being less dense than cold air. So valleys and hollows will get frost before hilltops. Such areas are often called frost pockets, and are bad places to have gardens.

Frost can form in one of two ways, both similar in nature to the formation of dew but ultimately leaving ice on the surface rather than water. 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 as ice.

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.

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.

Rime frost 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 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.

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 and prepare to be dazzled!

Constellations

The Solar System