Rime on the Summit!
2017-12-12 12:52:24.000 – Taylor Regan, Weather Observer
Yesterday morning, during our Facebook Live weather forecast, we were asked a question regarding rime ice, which is a type of ice commonly seen on the summit. I thought I’d take a few moments to talk about what it is, where the name came from, and what is means for us weather observers who live and work on the summit of Mt. Washington.
Figure 1. Rime ice beginning to form on the summit sign.The term rime has been used for centuries, and is attributed to the old English word hrim, meaning frost formed on cold objects. Rime icing is a phenomenon that occurs when a location or structure is in the clouds (fog) and both the location/structure and the air temperature are below freezing. Basically, the cloud (fog) is made up of millions of water droplets that are so tiny, that they are able to stay liquid well below the freezing point (and all the way down to 40°F below zero!) All these droplets need in order to turn into ice is something to come into contact with that is below freezing. This impact essentially causes the supercooled water droplet to turn into ice, and as the droplets continue to impact the structure, the ice builds up into the direction of the wind.
Figure 2. Intern Nate standing atop the parapet in heavy riming.The summit of Mount Washington spends 2 out of every 3 days in the fog, and has a yearly average temperature of 27 degrees. This means plenty of rime potential! In fact, up on the summit we can see riming rates of upwards of 9 inches an hour!
Figure 3. Summit sign with significant riming.The structure of the rime ice chances as a result of temperature. Closer to freezing, the rime is much more densely compacted, and is not only heavier, but also more translucent. When temperatures drop much below freezing, the rime becomes lighter, and much whiter, it crumbles with just the smallest force.
Figure 4. Intern Nicole standing under the rime-covered A-frame.Summit staff love rime ice because it has a tendency to coat everything in ethereal feathers that accentuate sunrise and sunset colors, and make a very familiar summit feel almost foreign for a while. Rime ice is also a form of job security; someone always needs to *carefully* remove that ice from all of our weather instrumentation!
Figure 5. Observer Taylor de-icing the parapet.
Taylor Regan, Weather Observer
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