Thursday 4 August 2005
Drifting Snow
I realise that I've been making quite a few weather related posts recent but I couldn't resist one more as I've got some snazzy video clips to go with this one.
As you can see from the bad weather video clip, when the wind starts blowing above a certain strength it is tends to pick up loose snow from the surface. The exact speed required depends a bit on the snow conditions but it usually starts at around 12 knots. Quite often when it's windy it's also overcast, which makes it difficult to see the spindrift close to the surface.
Occasionally though you are lucky enough to get strong winds combined with clear skies and good contrast. On days like this the snow can be seen as it snakes across the surface, hugging the floor like dry-ice. When the sun is rising or setting at the same time it sometimes looks like the whole ice shelf is on fire. It's quite hypnotic to watch as small areas of turbulence propagate across the ground.
Earlier this week we had some wind at lunchtime when it's light and I managed to capture some video footage on Miriam's camera from the Piggott open platform. You can even see how turbulence from the platform is disrupting the smooth flow of snow in the centre of the frame. Click on the image to view the video (5.8MB, MPG file):
When the wind is blowing you tend to get areas of the ground which are scoured free from snow, revealing the hard icy surface below. In other places the snow seems to gather together to form crescent shaped piles of soft snow. What I didn't immediately realise was quite how co-ordinated these crescents are. Instead of forming and disappearing at random, individual crescents actually move slowly across the surface in the direction of the wind. It takes several hours for them to move any appreciable distance so it's not obvious that it is happening unless you're looking for it. Presumably each crescent is made up of different snowflakes as they move, with some being pulled off the tips while new ones are trapped and build up on the windward side.
The video clip below shows the effect pretty clearly. It is actually made up of a load of individual photos, each taken 1 minute apart, during a windy day last year. The whole video clip (2.4MB, MPG file) probably spans a few hours of time:
Posted by simon at 12:44 PM | Feedback (6)