Received from an Antarctic met man: The F?hn effect is dominated by "blocking". Wind, approaching a ridge, will either go up and over the ridge (normal) or come to a stop and then flow round the sides. South Georgia often experiences this latter.
Which of the two (up and over or round the sides) depends on the temperature gradient (stability), and wind speed,. If it's stable, the air at the ground is cold and "heavy" and wont flow up and over the top, it goes round the side. BUT, the air at the top of the ridge and just above the ridge, flows over and then down. Air aloft is already warm (because its stable, cold at the bottom), it then descends and gets even warmer and dryer ...a F?hn. South Georgia has these often. If the air is just between the flow round the sides (Froude > 1) of up and over.
Passage is slow though, and heavy on fuel. Most of all, it takes an experienced and well informed ice-pilot to be confident in entering such ice so as not to be locked into the pack should the wind direction change and consolidate the ice.
If the wind gets up and closes the ice, it could well be goodbye to the boat and the people in it too.
During this frozen-in time, it is possible to travel out across the sea-ice and walk right up to the bergs. This guy in the picture has an almost identical picture to this - with me in front of the berg though.
These bergy bits are trapped in the frozen sea-ice in the winter making it possible to walk out to them. In the distance can be seen trapped icebergs and the long low landmass of a nearby island, the two peaks to the left are about 40 miles (64 kilometres) way.
Icebergs are made up of snow that has fallen over many hundreds or even thousands of years. The stripes and different coloured layers in icebergs represent different layers of snowfall and the weather conditions under which the snow fell. If it is very cold then a light open layer with much air included will be formed, this gives a paler or white layer. The darker, bluer layers come from snow fall in relatively warm, maybe even wet conditions when little or no air is trapped in the layer.
In addition to this, air is squeezed out of the lower layers of a glacier as more and more snow falls and so the weight of snow builds up.
In this case the algae is predominantly a red-coloured species, but further down the slope, green and blue-green colours are discernable. This is relatively short-lived spring phenomena as soon the very snow and ice layer that the algae are living in will melt and the algae will flow down to the sea with the water that provides them with their nourishment. It is not unusual to see distinctly red, green or blue-green topped ice bergs in the spring as a result of the growth of such algae.
Sometimes, walking across such an area will leave behind red footprints as the algae are concentrated by the walker as the snow is crushed, and sometimes there will be a a faint smell of fresh watermelon accompanying the phenomena.
Large irregularly shaped bergs tend to be the most interesting to visit, but also the most dangerous and most unstable. They will break up at some point and they will tilt and move around a lot before settling to a new stable position. If you're in the vicinity when this happens, you may get some big pieces of ice dropped on you or at the very least there will be some major waves and disturbances of the sea. Having said that I've never heard of anyone actually being hurt in such an event - a combination of the rarity of it happening, alertness and speed of the boatman/boat and people just not going near big bergs very often in small boats.