Hoar Frost or Rime Ice, or Something Else?

by Skip Via

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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