Rime Ice or Hoar Frost? You Be the Judge!

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One night in eastern Nebraska*, after a couple of days of melting ice and cold nights, something quite interesting happened. A weak cold front moved through, and saturated air near the surface brought foggy but breezy conditions to our neighborhood. The following morning, I saw our trees covered in heavy frost.

Pretty isn’t it?  See? I told you Mother Nature does some really beautiful things with water! That’s a plum tree on the left, an apple tree on the right.

Upon a closer look…

…I noticed the ice was forming into super-delicate long needles, all oriented in one direction.

I had to trudge through knee-deep drifts that we still had in our front yard to get these pictures, but they’re definitely worth it.

My first thought was that this is rime icing, or rime-type frost. Defined in Wikipedia as

“a type of frost that occurs quickly, often under conditions of heavily saturated air and windy conditions. Ships traveling through Arctic seas may accumulate rime on the rigging. Unlike hoar frost, which has a feathery appearance, rime generally has an icy solid appearance. In contrast to the formation of hoar frost, in which the water vapor condenses slowly and directly into icy feathers, rime typically goes through a liquid phase where the surface is wet by condensation before freezing.”

The temperature range was right for rime ice, as were the saturation conditions and winds, which drive the direction of ice formation.

But my just-as-geeky-as-me dear husband asked if I was sure this wasn’t “hoar frost”, the phenomenon that was being compared in the quote above.

No, not “whore frost”. I don’t even want to go there.

Might this be “hoarfrost” or “hoar frost”? When hoar frost conditions have a slight breeze they can orient their formation in one particular direction. But according to the definition and pictures here, I’m less inclined to think so.

I think what we have is actually “soft rime“. What I saw and experienced fit all of these definitions, the thin, milky white needles, and the ease with which is fell off the trees in the slightest breeze.

The winds were from a northerly direction Thursday night, if there was a wind at all. So according to the definition, the needles should be pointing towards the north, and indeed they were.

So I think this is “soft rime”, but take a look at these other pictures and see what you think:

*These photos were taken in Bellevue, Nebraska in January 2010.