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Hoar in a Hole

A hole in the ground is a good place to look for large hoar crystals. Next to some rice paddies, just uphill from the tubs, lies a few small holes in the cement roadway. From eye level, the white tinge of hoar frost just inside the lip is easily overlooked. Indeed, the hole pictured below is slightly less than 3 cm across at the widest and I sometimes have to look twice before I notice the ice.

Like snow, hoar can grow as a thin plate, a thin dendrite, a long column, a sharp needle, or some intermediate form, except hoar generally grows out in only one direction, not the six directions of the snow crystal. The ground surface here can dip slightly below freezing, but a short distance below the surface lies warmer, often moister, ground. At such temperatures (i.e., within a few degrees of freezing), both snow and hoar grow into platelike, or tabular, forms. Even when nearby blades of grass or foliage have columnar hoar crystals, the hole usually has tabular, showing that a small change of temperature changes the crystal considerably. (The dendrite, the most extreme tabular crystal form, grows at temperatures even lower than the columns.) (a) shows a few long, narrow crystals with jagged edges like a sawblade. And like a sawblade, these crystals are thin, as some of the side-views in (b) show. jnelson This entry was posted on December 26th, 2009 at 09:06:36 am by Jon and is filed under Announcements.

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