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Soft Rime Covers Much of Region

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For those of us who woke up early enough to witness it, much of the region was treated to a winter wonderland for a short period of time Saturday morning. Of course, the snow had already tapered off and we had already seen the several inches of snow that was left behind by Friday morning’s snow event. However, freezing fog during the overnight hours Friday into Saturday led to what we call soft rime on just about every surface imaginable making our morning commute even more of a winter wonderland than the day before.

In late December, we woke up to a similar phenomenon called hoar frost, which many people often take to be the same thing. While the two may appear very much like the same thing, the processes are totally different. Here are a couple of the pictures taken of our hoar frost in December 2012.

What we saw Saturday morning, a soft rime, was caused by the condensation of water vapor into supercooled water droplets, which then, attached to all surrounding surfaces. In other words, there were high levels of humidity in the atmosphere because temperatures were cooled to the dew point of the atmosphere leading to fog. Of course, with temperatures as low as they were Saturday morning, any fog that formed froze immediately once it made contact with a surface.

The hoar frost, which we experienced back in December, was formed similarly, but not quite the same way. Instead of the atmospheric temperature cooling to the dew point of the atmosphere and forming freezing fog, high levels of humidity and temperatures below 17 degrees lead to the direct deposition of water vapor to solid ice. Most of the time, hoar frost is much thinner than soft rime, like we saw Saturday morning.

Here are a few pictures taken Saturday morning just south of Sioux Falls in rural Lincoln County. As you can see, it was certainly a sight to see and no surfaces were left untouched by the frozen fog.

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