Photo Creative Commons License by Andrew Michaels
I awakened to a thick blanket of hoar frost covering almost everything. We also had frost 2 nights ago, but it didn't produce hoar frost. So, why did this happen?
The white ice crystals that form on the ground or exposed objects during clear, cold nights happen because heat losses into the open skies cause objects to become colder than the surrounding air. In certain conditions hoar frost can occur even when the air a few feet above an object is well above freezing, because the objects themselves are at, or below, the temperature of freezing water.
The effect here is that the hoar frost was beautiful, but the rising temperatures today have left most of my garden a thawing, brownish-green slimy mess.