Here we are, still on the January roller coaster! Sub zero to above freezing, a slick coating of ice. When most think of winter, they are usually thinking of the two most beautiful winter weather phenomenon that we have. There is the gentle snowfall, covering the ground and trees with a fluffy clean white layer, that Christmas card perfect scene. Then there is hoarfrost. That magical shiny coating that coats everything. It doesn't last long, being subject to either warming temps or winter winds, but it is a thing of dreams when it is there.
But how does it form? Why is it only there once and a while? Why doesn't it last, like the snow? Hopefully I can answer some of these questions for you.
The temperatures have to be just right for hoarfrost. It has to drop from above freezing to below, with the surface temps already below freezing before the air temps get there. There also has to be a substantial amount of moisture in the air, so days following a thaw are needed to allow that to build up. Sometimes a south wind can bring in some warmer moist air, but the ground must not warm up too much, or no hoarfrost. So we have moisture in the air, fog is the best, and ground that is frozen (below 32 degrees). Then the magic can happen.
The water vapor in the air condenses on the frozen surfaces, creating crystalline deposits...hoarfrost. This builds up as long as there is moisture in the air that is not turning into freezing rain or sleet, or rain. It can build up inches thick on objects, making for great photo opportunities when the sun comes out. If there is a wind during the build up it can affect how it is built up, forming on the downwind side of objects. A wind after it is formed, when the air is cold will quickly knock the fragile crystals to the ground.
The other nemesis would be warming temps. Sunshine warming it up just enough to turn them all to liquid again. So it is a combination of moisture, temperatures and wind. So take advantage of it when those rare times occur, especially when followed by sunshine, to try your hand at photography! It is hard to go wrong when there is magic in the air, or on the ground!