We were up on Grisedale Pike above Keswick today enjoying the snow. There was a bitter wind blowing on my back as I took the photograph. Notice the rime ice on the fence posts and rocks. It is growing into the wind. Apparently the water droplets in the air become supercooled. In other words, they remain liquid even though they have been cooled below 0 degrees Celsius. This is because there is nothing to help ice crystals to nucleate. The wind blows them onto cold objects which then make the supercooled liquid freeze instantly - but on the side that the wind makes them hit. Hence the rime grows into the wind.
