Guide's trip

On the way up to Austerdal glacier. Photo: Mathilde Andersson

The past few days from the 12th to the 14th Eirik and Sara third years, Mathilde and I second years and five first year students drove up to Veitastrond for a short glacier trip. We were walking on Austerdal glacier with the view of the beautiful Odin, Thor and Loke glacier when I figured out I wanted to write a post about snow.


Formation and Growth of Snow Crystals in the Atmosphere
The atmospheric clouds compounded of supersaturated water vapor droplets, are the mother of all snow crystals. Droplets are formed by water building up on small particles of salt, dust or soil in the air called condensation nuclei. The droplet will grow when the air is supersaturated and water vapor condensate on the surface. Snow can be formed by tiny ice crystals when the air temperature is below zero degrees however, small water droplets will persist in a supercooled state. Freezing ice crystals are only formed when foreign particles are being crystallized. The freezing nuclei are less common compared to the condensation nuclei. Their special structure promote freezing individually by the temperature gradient and they increase in numbers when the temperature decreases. Droplets will freeze without assistant from the freezing nuclei at minus 40 degrees (Clung & Schaerer, 2006;43).

Crystallographic axes of a snow crystal
(Wergin & Rango, 2002).

The ice crystals will fall when they gain enough weight. On the way down they will become larger by colliding with other supercooled droplets that would freeze onto them, this process is called riming. When the branches on the crystal are entirely filled in by an extended riming process, it becomes a rounded crystal called graupel. Hail is formed by graupel alternating in a freeze and thaw cycle (Clung & Schaerer, 2006;45).

Basically the growth of a snow crystal occurs in two directions, the a-axis that form plate like crystals and the c-axis that form needle like crystals. On vapor growing crystals there will always be six sides due to the basal plane of the crystal. The temperature is the most important of all the complicated variables that form the crystals both on the way down and on the earth after the snowfall. The growth direction switches from a-axes to c-axes when the temperature changes. The crystals that have fallen through cold air are generally smaller than those falling through warm air due to its ability to hold more moister (Clung & Schaerer, 2006;46).


The Perfect Flake
Snowflakes form in warm and moist air with calm to light wind speed. When the cloud cover is high, the fall time for the snow crystal is long and the temperatures are a few degrees below zero with relatively dry air, we got the perfect powder light snow skiers are looking for (Clung & Schaerer, 2006;47, Landrø, 2007;35).


References:

Mc Clung, D & Schaerer, P.(2006). The Avalanche Handbook 3rd Edition, The Mountaineers Book, USA.

Landrø, M.(2007). Skredfare - En håndbok om skred for fjellskiløpere, klatrere og løssnøkjørere, Fri flyt, Oslo.

Wergin, P. W; Rango A; Foster, J; Erbe, F. E; Pooley, C.(2002).Retrieved from: http://modis-snow-ice.gsfc.nasa.gov/uploads/pap_R_irreg02.pdf, 21.10.2011.