Heavy hoarfrost on leaves. (Photo by Gunnar Ries, WikiCommons.)
When I awoke yesterday morning, the long field of grass behind my house looked awfully white and sparkly. The temperatures in the North Carolina Piedmont where I live have been sliding further and further down the path to freezing the past few evenings. The night before they’d reached 30 degrees Fahrenheit. I glanced at our neighbor’s roof, which was also dusted in white and glinted in the sun. For a moment, I thought we’d had our first dusting of snow. I pulled on my boots and jeans and walked outside past the pine trees, below which there was only green grass, and into the open field. My boots crunched the greenery below. I knelt down and inspected the ground. It was not snow. A fine rime of fragile hoarfrost clung to the edge of each grass blade and each fallen pine straw. It circumscribed the outline of every fallen sweet gum and elm leaf and lent them an odd, wintry fringe. It was beautiful: a field of hoarfrost.
Hoarfrost on grass. (Photo by Dominik Unger)
Hoarfrost forms under precise conditions when water vapor in the air undergoes a process called sublimation and transitions straight to a crystalline state on a solid surface. Because it moves straight from vapor to solid, this means that it skips the watery intermediary state of liquids. Small ice crystals sublimate directly onto surfaces with fine edges, points and textures. This explains why the edges of the grass blades I saw were lined with the fine crystals, but few grew on the face of the blade itself. The leaves looked they were outlined in thick chalk, like cartoon figures.
For hoarfrost to form, the surfaces of leaves, plant stems and grass blades have to meet two criteria: they must be colder than the dew point for the ambient air, and they must be colder than the freezing point of water. When these conditions align, the water vapor condenses from the saturated air and freezes directly onto surfaces, generally those of small diameter.
There musn’t have been much wind when the frost formed, because the crystal spikes grew perpendicular from the objects they were attached to, rather than in a prevailing direction. As the sun rose, it lit the field with pale pink morning light and a million ice crystals winked back.