Diamond Dust: Snow Without Clouds

During the depths of January cold, we often hear some self-proclaimed weather sage observe, "It's too cold to snow." At first, the statement appears to jive with general experience. Are not the coldest days and nights clear and dry? While this is generally true -- large, frigid arctic airmasses are usually characterized by dry, mostly clear weather -- it is not the cold that prevents snow falling in these airmasses, but the lack of ascending air to start the precipitation process.

Truth is, it is never too cold to snow, though the conditions which bring the coldest temperatures are usually non-conducive to producing precipitation. But, it is also true that the colder the temperatures, the less water vapour the air may contain. In fact, the amount of water vapour at saturation drops by about a factor of two for every 10 Celsius degrees (18 Fahrenheit degrees) drop in air temperature.

At very cold temperatures, 40 below zero (C or F) and colder, snow can actually fall out of the cleanest, clearest blue sky without intervening clouds. Temperatures need not be so cold if there is dust, or other minute particles, in the air on which the water vapour may deposit. When condensation nuclei are present, diamond dust may form at temperatures just below minus 20^oC (0 ^oF). At such temperatures, the water vapour in the air spontaneously forms ice crystals which slowly settle earthward. When these falling crystals are caught in the light, they sparkle like gemstones, a weather condition known appropriately enough as diamond dust.

I recall one frigid arctic outbreak in southern Ontario when the temperature barely hovered around minus 20^oC ( minus 4^oF). As I walked home in the late afternoon, the soon-to-be-setting sun illuminated the horizon in brilliant light through the crystal-clear sky. Looking beyond my parka hood, I noticed a soft flutter of snow encircled me. The snow, so fine that individual crystals were distinct on my dark coat sleeve, floated around me gently and slowly drifting earthward. In the brilliant sunlight, the snow crystals glittered against the darkening sky.

At such low temperature, ice crystals form as irregular hexagonal plates, or as unbranched ice needles or ice columns directly from water vapour in the air. The formation of hexagonal-plate crystals is favoured at air temperatures from minus 10^oC to minus 20^oC (14^oF to minus 4^oF). Ice plates resemble dinner plates with a hexagonal pattern in their long dimension and are thin relative to their width. Ice columns, on the other hand, look like minute stubby pencils. Columns typically form in temperatures below minus 25^oC ( minus 13^oF).