Snow ghosts

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Snow Ghosts in Japan

The casual observer mistakenly thinks that these trees are encrusted with snow. Others erroneously called the coating hoar frost. The truth is that the enshroudment is heavy accretions of ice called rime.

Rime ice is formed when water particles in clouds and fog become supercooled. The water particles still exist in liquid form in the clouds, as they lack nuclei to freeze to. Water can exist below zero celsius and not freeze if they don't have a nuclei. The trees, lift towers, buildings, and even skiers and snowboarders if they sit still, become freezing nuclei upon which these super-cooled water droplets splash and freeze.

The resulting coating is rime ice. This process is similar to freezing rain often seen in cold moist areas near the coast (like Seattle in the USA). As this process occurs over time, a thick accretion of ice develops.

If a constant wind exists while rime develops the rime forms(mostly) on one side of the object on which it forms. Instinct suggests that the rime will form on the lee side of the object. In fact it forms on the windward side - the rime points into the prevailing wind.

Run into one of these coated trees the next time you have the opportunity and you will quickly realize that this is not just a soft coating of snow, but instead a hard, heavy accumulation of ice. A quick calculation reveals that the weight of these masses can easily be in the thousands of kilograms on larger trees. Throw in gusty winds that create swaying action back and forth and it is easy to see how extensive damage is possible. Stand back and be alert if there is a chance of breakage.

Rime is not all bad however. Light accumulations act as a barrier, protecting the tree and its needles from the abrasive action of blowing ice, and the drying of cold winds.