A fascination with snow

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!

Graupel - pellet snow (Source Wikipedia)

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.

The snow crystal morphology diagram, showing different types of snow crystals that grow in air at atmospheric pressure, as a function of temperature and water vapour supersaturation relative to ice. The water saturation line gives the supersaturation of supercooled water, as might be found within a dense cloud. Note the morphology switches from plates (T ≈ −2°C) to columns (T ≈ −5°C) to plates (T ≈ −15°C) to predominantly columns (T < −30°C) as temperature is decreased. Temperature mainly determines whether snowcrystals will grow into plates or columns, while higher supersaturations produce more complex structures.

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. 

Stellar plate crystal

Stellar dendritic crystal

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!

Advertisements
This entry was posted in Weather and tagged . Bookmark the permalink.

5 Responses to A fascination with snow

  1. E.M.Smith says:

    Well, a Google search on the phrase gave 1070 results, so I’ll not post them all here 😉

    One included “The Snowman’s Prayer”:

    Snowman’s prayer: Please freeze. Amen.

    Snow makes folks whimsical. I think I’m going to like the next couple of decades… 😉

    • Verity Jones says:

      Thanks. Such was my state of tiredness in the last few days that it did not occur to me to Google it.

      I also think we’d better get used to it. I remember childhood winters like this although perhpas not just as bad, although certainly there was talk of a coming ice age (in the 70s). Did you ever read Tonyb’s article on Dickens and winters at that time? http://noconsensus.wordpress.com/2010/01/06/bah-humbug/

      We had a sprinkling of snow for about a week – just low temperatures, then about an inch arrived on Friday evening and has persisted. The roads are covered with black ice and I mean really bad – the side roads that have not been gritted, you can barely even walk on, no matter how steady on your feet normally.

  2. John F. Hultquist says:

    Your comment about “black ice” made me chuckle. The expression is also used in the US. Some of our roads are paved with a red cinder rather than black asphalt and so we have “red ice” too, as far as I am concerned. In either case the ice is clear – that which makes it so dangerous.

    But the chuckle: We had a comedian that went by the name Flip Wilson. One of his best characters was “Geraldine Jones, always referring to boyfriend ‘Killer’ and whose line “The devil made me do it” became a national expression.” http://en.wikipedia.org/wiki/Flip_Wilson

    I have always likened the term “black ice” to Geraldine’s “the devil” in the following way. A person drives to fast, hits a patch of ice, and lands up-side-down in the ditch. (A friend did this thinking she could pass another car.) She, like most others that live to tell, will say the accident was caused by black ice. I lay the blame on inattentive driving; as long as no one gets hurt it still brings a chuckle as I think of Geraldine.
    ———————–

    Now that you have mastered graupel, go for virga.

    [Reply – thanks for that! I know the phenomenon well, I just didn’t know it had a name. Verity]

  3. tonyb says:

    Verity

    I would like to make an official complaint. Here in South Devon we didn’t actually get any snow despite the Met Office-15 miles away-forecasting that we would. Who can we sue?

    tonyb

    [Well, much as I love snow the last week has not been a happy one for us here – more soon, V.]

  4. Ian Beale says:

    Off thread but check http://joannenova.com.au/2010/12/breaking-the-abdication-of-the-west/

    [Thanks for the link – at least the ‘deal’ now is an agreement of what we ‘should do’ but postpones for another year any agreement of a mechanism of how it will be achieved. Another year of continued digging required 😉 Verity]

Comments are closed.