As defined by Merriam Webster’s Collegiate Dictionary, snow is “precipitation in the form of white ice crystals formed directly from the water vapor of the air at a temperature of less than 32˚F (0˚C).”1 This definition will be the primary focus for my examination of snow physics.
Clouds are the primary source of snow. Air contains tiny particles of water. Relative humidity is a measurement of how much water air can hold at a given temperature. Air is saturated with water at 100 percent humidity. Clouds form when the air cools and the relative humidity increases above 100 percent.2 Water droplets or ice crystals in clouds are very small having “a diameter of a hundredth of a millimeter.”3 Typically, ice crystals form “9 to 12 kilometers (30,000 to 40,000 feet) above, temperatures cool to -60˚C (-75˚F), a level rarely reached even at the poles.”4 Ice forms best when there is already a template to order the molecules such as dust particles or ice that is already present.
Clouds consist of water droplets, ice crystals, pollen, bacteria, chemicals produced by humans, dust, and fungi to name a few. The contents of the cloud determine the chemical make-up of the snow that is formed. These particles become the nuclei of snow. This can lower the formation temperature to just below freezing “whereas a pure water droplet will remain unfrozen until the temperature falls to between -18 to -31˚C, depending on its size.”5 The number of particles in the air capable of becoming a nuclei for snow will increase as temperatures reduce.
Snow has the potential to form numerous patterns. Some of the
patterns formed are diamond dust,
stellar dendrites, sectored plates, columns and needles,
capped columns, triangular crystals, split stars and split
plates, twinned crystals, twelve-sided snowflakes and double
stars, chandelier crystals, spatial dendrites, and rimed
Each pattern will
be created within a different set of parameters. They are created
when specific conditions of humidity, temperature, and air
density are met. This
diagram shows the morphology:
Clouds associated with snow formation and snowfall are Cirrus and Altocumulus, which produce light showers. Nimbostratus clouds produce continuous snow showers. Cirrus and Altocumulus clouds are associated with warm fronts. Cumulus and Cumulonimbus clouds are associated with cold fronts. Cumulus clouds produce light snow showers and Cumulonimbus clouds produce heavy snow showers.7