Yesterday started with leaden skies, but with the dark hills bright white on the skyline. Snowflakes drifting in the dry air exploded into individual crystals as they hit my windscreen, each one a perfect microstar. Proper snow!
The pellets of last week, called graupel apparently, look like beads of polystyrene. They form aloft when microdroplets of supercooled water freeze onto the surface of a snow crystal, building up into a porous pellet.
Wet snow is the norm here. Large sticky flakes. Great for snowball fights, building snowmen and lots of fun, but there for a day or two and then it is gone. Usually. As a child I saw snow with flakes the size of saucers – bigger than my open hand. Sparse, flat discs falling very slowly like parachutes and collapsing into wet smudges on the ground. Rare.
In the Alps I remember heavy mist settling over the mountain, only to realise it was individual snowflakes, each 1-2mm in diameter but perfect in their beauty. Like these:
Source of images and info: Snowcrystals.com (Caltech.edu) – a fascinating website.
The basic six pointed star shape is of course due to the hexagonal crystal lattice structure of water, but the way the ice crystal grows is down to many factors. The shape depends on the balance between the formation of facets and branches, which in turn depend on humidity and temperature; “branching instability” results in complexity.
Figure from: Libbrecht, K. G. The physics of snow crystals. Rep. Prog. Phys. 68 (2005) 855–895 (available here). Fascinating – I’ll never look at snow in the same way again..
Back in 1991, British Rail (as it was then IIRC) experienced disruption during severe wintry weather and the excuse that it was “the wrong kind of snow” was splashed across headlines. It was seen as a lame excuse at the time as has never been forgotten (the other one being “leaves on the line” which seem to come as a surprise each Autumn). BR was used to plowing wet, heavy snow, but the dry powdery stuff that hit in 1991 got sucked up into machinery, causing havoc.
Guide to Snowflakes “The best powder snow, where you sink to your knees while skiing, is made of stellar dendrites. These crystals can be extremely thin and light, so they make a low density snowpack.”
I’m not sure of the provenance of this quote, that I read the other day, but now can’t remember the source, but it is lovely: “Snowmen fall from Heaven, Some assembly required”. It is the weekend, go have fun!