Fast Falling Flakes Fun

Sublime Fun

A faithful CED reader, amused by the connection between decomposed organic matter and the kind of snowflakes that form, wanted to know if that also means that biogenic snowflakes fall at special rates. That was nice, smooth aerodynamic thinking.

Three types of snowflakes fall by the rule the-bigger-they-are-the-faster-they-fall: graupel, rimed crystals, and needles. Falling graupel [aka snow pellets, soft hail or tapioca snow] is white sleet-like stuff that bounces when it lands and ranges in size from a bit less than 1.5 mm to as big as 5.5 mm. The 1.5 mm graupel falls at about 1 m/s while the 5.5 graupel reaches a terminal earthbound velocity of 2.5 m/s. [1m/s = 2.2 mph]. So 5.5 mm graupel falls at a pelting 5.5 mph.

Rimed crystals (ice coated ice crystals -- sort of ugly, irregular and non-smooth stuff) start falling out when the reach about 1.5 mm in maximum dimension and they get as big as 4 mm. Their terminal velocities range from 0.8 to 1.1 m/s.

The third kind of ice crystals that fall faster the bigger they get are needles. Needles start to fall out of clouds when they are only 0.5 mm in the direction of largest diameter. They get as big as 2.5 mm. The 0.5 mm needles fall (creep) downward at 0.4 m/s while the largest needles fall at 0.6 m/s.

A second group of crystals speed to earth in a size-invariant manner. Ice crystals that are plate-like are in this category. They fall at the same speed over their entire size range. They start to fall out of clouds when they reach about 1 mm in size.

These six-sided, often glorious dendritic-crystals fall the slowest of all (0.25 m/s) . While graupel pelts you, star-like dendritic crystals just sort of come to rest on you. You might say that biogenic snow crystals formed around 12 C are the most gentle of all snowflakes. You can find these numbers on snowflake fall speed in Cloud Dynamics by Houze, R. A. from Academic Press, (1993).

So what is so sublime? Charlie Brown is a noted snowflake-on-the tongue-catcher. That is sublime. When gaseous water (vapor) freezes directly on another crystal a without a liquid phase it is called sublimation and also the reverse when vapor evaporates from the ice without passing through the liquid phase. That too is sublimation.


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