Hoar Frost or Rime Ice, or Something Else?

by Skip Via
skip@westvalleynaturalists.org

I woke up on December 29, after two days of dense fog, to see everything–leaves, grasses, fencing wire, pine needles–coated in a beautiful layer of hoar frost.

But wait…hoar frost occurs only in very specific conditions: namely cold, clear, and windless nights with low humidity, when the rapid radiant heat loss from surfaces causes water vapor–not liquid water droplets, as in fog–to form delicate, needle-like crystals on those surfaces. The previous two nights had been anything but cloudless with low humidity. You could feel the cold fog droplets on your face when you walked.

But it sure LOOKS like hoar frost. What’s going on here?

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Hoarfrost or Rime Ice?

by Skip Via
skip@westvalleynaturalists.org

A few days ago, driving around West Valley, I remarked on the beauty of the hoarfrost that had coated some Ponderosa pines along the roadside. That got me wondering about the nature of hoarfrost and how it forms. It turns out that some of what I have been calling hoarfrost is, in fact, not hoarfrost, but rather rime ice. The difference is not important for the casual observer, but it can be important meteorologically.

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