I have probably already mentioned rime ice in previous entries and will certainly be mentioning it quite a bit in the future. Because of this, I figured now would be a good time to talk about this phenomena and explain it.
To put it most simply, rime ice is frozen fog. I am sure that you already know that fog is composed of very tiny water droplets. When the air temperature drops below freezing, these tiny water droplets begin to freeze onto any exposed surface to form rime.
This happens with great frequency on Mount Washington in the winter, roughly every other day I would say. It also happens occasionally during the summer time. Any day that we have fog (I often refer to this as being in the clouds) and the temperature is below freezing, we will have rime ice forming on everything. When I say everything, I mean EVERYTHING, including a persons jacket, gloves, goggles, and even eyelashes!
Rime ice is actually a form of job security for me. This is because it is one of the biggest reasons there could never be an automated station on the summit of Mount Washington. Typically, we get rime ice accumulation rates of 1 or 2 inches per hour. However, we can get rates of 3 or 4 inches per hour on a somewhat regular basis. Some of the most severe storms we have seen will see rates even higher than that. We have to go up and de-ice the instruments once per hour on average, more often during severe icing events. Most winter instrumentation is heated, but we still need to go up and knock ice off the structures supporting the instruments and make sure that the heating is not failing.
Here is a picture that I just took this afternoon showing rime ice formation on an anemometer mount from last night.
Notice how the ice grows horizontally and can grow to impressive lengths, in this case about 1 to 2 feet. The rime actually forms INTO the wind, which is opposite of what people would expect for some reason.
To finish up this post, take a look at a close up of the ice from the picture I already showed. This close up nicely shows how rime can form in very intricate and beautiful feathers. It is definitely one of my favorite things to take pictures of on the summit.
That is all for today. It is almost dinner time here on the summit, and I am never late for dinner!
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