Someone once said that frost, ice and snow are just winter versions of summer’s dew, mist and rain.

During my walks along the Otonabee and Thompson Creek, I get to witness many versions of frost, ice, and snow. In the often clear sheet ice of the calm bay leading to the larger river, some of the most beautiful formations aren’t actually ice but frost on ice; they arise when water vapour condenses directly into hoarfrost (radiation frost) or rime when it’s cold enough during a freezing fog. Frost, particularly hoarfrost, often forms at night, when the air temperature is cold. This frost often melts quickly as the sun rises and warms the air around the frosted object. That’s why I prefer to do my first walk of the day in the early morning. Hoarfrost will form on many objects like twigs, branches, leaves, grass blades, ice and even snow.

Unlike ice that forms from liquid, frost forms from vapour. Frost forms when an outside surface cools past the dew point; the vapour attaches to a nearby twig, grass blade or other surface and turns into a solid. Frost crystals then grow into various forms and patterns. If there is enough water vapour to create large flakes of frost this becomes hoarfrost.

Frost crystals that form directly on cold surfaces such as a snow surface or an ice sheet from freezing water vapour grow larger and flower into beautiful leaf-like shapes or hexagonal snowflakes. The different frost shapes and patterns arise from the topography of the ice sheet, often specks of dust, salt, or residue, twigs and other organic material that lies on the surface. The ice crystals nucleate on the particles and branch out, forming unique fractal patterns that resemble leaves, ferns, feathers and more. This ground frost closely resembles snowflakes, given that their formation and structure are similar—only the nucleation particles for snow are dust and for frost they’re surface objects.

Glossary of Terms

ADVECTION FROST: A collection of small ice crystals in the shape of spikes that form when a cold wind blows over branches of trees, poles, and other surfaces.

FERN FROST: Frost that appears on windows and ice as moist air comes into contact with the freezing surface. The tiny water droplets freeze into patterns that resemble leaves or ferns.

FROST: Deposit of small white ice crystals formed on the ground or other surface when the temperature falls below freezing. A more extensive and larger crystalline structure of frost formed with more moisture, such as a freezing fog, is called hoarfrost. When frost forms rapidly, often during windy weather, it is called rime.

GRAUPEL: Heavily rimed snow particles or pellets, typically white, soft and crumbly.

HOARFROST: a grayish-white crystalline deposit of frozen water vapor in clear still weather on vegetation, fences, ice and snow, and other surfaces. It often resembles spiky hairs. The word ‘hoar’ comes from ‘ancient’ given it resembles an old man’s bushy, white beard. Also called radiation frost.

RIME FROST: Rime frost resembles sugar sprinkled on the edges of leaves and flower petals. It occurs whenever damp winds conspire with extremely low temperatures. Rime frost forms rapidly; the word ‘rime’ means ‘crust.’