I have been taking some account of snow and how it falls. It does not come down everywhere the same, of course, and in some places hardly at all. In just a few miles of distance there are great variations in winter's blanket. Mere elevation does not always account for the difference.
Why did snow fall here and not in some other valley? How is it that other winter storms have darkened these skies and passed on without leaving a flake?
As in making bread, I have learned, there are ingredients to the formation of snow and conditions to be followed. Cold air is not enough, nor clouds, nor moisture.
At the nucleus of every snowflake there is a particle of dust, or fumes, or even tiny microorganisms. It is upon such smidgens of matter that snow crystals form and grow.
The moisture in a cloud, if it is cool enough, will collect on these crystals and cause them to expand. If they grow large enough, gravity with grab on and pull them down to earth. If not, they may drift along for days, carried in some cloudbank until they dissipate.
Clean clouds usually produce no snow. Where there is no dust or other tiny particles of matter the crystals have nowhere to grow. A cloud may be laden with moisture, like thick fog, but until some catalyst triggers crystallization there will be no snow.
The process of cloud seeding attempts to make up for the lack of particulate matter in the atmosphere by introducing some. Silver iodide is commonly used. Dropped from aircraft above or projected from the ground below, it is offered to clouds as nuclei for snow crystals. Sometimes the offering is accepted; sometimes not.
Lead particles from automobile exhaust can serve the same purpose. The fumes rise toward heaven, mingle with moisture, and bring down snow. But add too much lead, or too much of any substance, and snow crystals will be stunted. Clouds can get clogged.
During the Dust Bowl years of the 1930s black snow fell in New England. In the Alps, a pale blue snow came down in 1972, fed by copper salts that had blown across the Mediterranean from the Sahara Desert. Other winds from the Sahara have produced pink snowfalls.
Here in the Northwest, snow commonly occurs near a low pressure system drifting down from the Arctic. Where the cold air from the north meets warm, moist air from the south is where snowfall is most likely.
If a cloud's temperature is fairly warm, say from 10 to 32 degrees, then large, intricate snow crystals are more likely to form. The warmth encourages the crystals to clump together, producing the big, puffy snowflakes that make the heaaviest snowfall.
If temperatures reach subzero or below, the atmosphere holds less moisture. Snow crystals may still form, but only as fine powder, and oftentimes they'll just blow on by.