So beautiful … Falling snow usually wafts gracefully down from the clouds, shrouding us in cool quiet. Drifts of fallen snow soften the contours of the ground and mini-drifts accent the tops of branches and twigs.
The magic starts with sequentially freezing droplets. "Snow" is a form of solid water that develops in clouds of certain temperatures.
Clouds are made of water vapor that is liquid, frozen or supercooled (below freezing, but not frozen). In the right range of temperatures, droplets of water vapor in a cloud don’t all freeze at once: snow forms when not-yet-frozen droplets condense against a frozen droplet, the molecules positioning themselves as they freeze against each previous tiny bit of ice.
Snow flakes are always six-parted because as the droplets freeze, the water molecules arrange themselves in a hexagonal lattice. (The lattice takes up more room than when the same number of molecules are just jostling around each other, as they do in liquid water and water vapor — which, by the way, is why ice floats.)
The ultimate form of the flake is determined primarily by the temperatures of the air and water vapor, the amount of water vapor, and the size of the water vapor droplets. Similar to raindrop growth and descent, eventually enough vapor freezes to the snowflake to make it too heavy for the cloud’s air currents to keep aloft.
Temperature variations during the flake’s development determine how the flake grows. Ice crystals generally build the most complex shapes as they move through a cloud with temperatures between -10° and -20° C. The arms of a classic snowflake are similar because they experience the same variations: truly symmetrical snowflakes are not at all common. In fact, most Oregon Coast snowflakes are very large, wet, irregular clumps.
The classic, six-branched snowflake is probably the most familiar of the seven to dozen-plus classified snowflake shapes. The hexagonal plate (with rather simple, straight sides) and divided hexagon (with internal divisions) are also widely-recognized snowflake patterns. While more common than classic flakes, snow crystals and flakes shaped like needles, sheaths, capped columns, and irregular shapes are less familiar to us — likely because they're less decorative.
Yes, it can be "too cold to snow.” Generally, by the time an air mass chills down to about -20° C the water vapor has already formed snow and has fallen from the cloud.
There’s vastly more snow above us than makes it to the ground. Temperature changes experienced by the falling flakes/droplets affect the end result of the clumped cloud vapor: nearly all our snow melts to become rain as the falling flakes drop to lower elevations.
Temperature changes along the path of water vapor droplet to the ground result in a variety of precipitation in addition to rain. "Sleet" is rain (or melted snow) that freezes as it falls to the ground; "freezing rain" is rain (that may have started out as snow) that freezes upon contact with the ground or other surfaces. "Hail" is an ice particle that is tossed about in the cloud among frozen and liquid water, repeatedly collecting liquid water then freezing. The concentric layers of ice make the resultant hail stone look like an old-fashioned candy jaw-breaker. “Graupel” is “soft hail,” snow flakes that collected bits of rime ice as they fell, tumbling into tiny snowballs. (Hail bounces, graupel doesn’t.)
“Rime” or “hoarfrost” forms when droplets in a cloud on the ground–fog — freezes on contact with a very cold item or previously-frozen fog, building delicate structures.
The Oregon Coast’s temperate climate makes for a rare, short-lived existence for these beautiful and infinitely diverse solid-water constructions. How fortunate that we usually receive just enough snow to delight those who watch and play in it, without too much tribulation for those who need to travel through it.
For more information — and some beautiful photos — visit Kenneth Libbrecht’s informative website: http://www.its.caltech.edu/~atomic/snowcrystals/.
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