On the last morning of the year, soft gray clouds insulate the city. The air is still and quiet except for a few American robins and goldfinches calling. In the silence of the night, fog crept in and dressed every tree branch and plant with beautiful needle-like ice crystals.
When fog or mist droplets freeze instantly on contact with the wind-facing side of a cold surface, rime ice is formed. The milky white color of rime is caused by the air pockets between the ice particles.
How is rime ice different from hoar frost? First, they differ in appearance: Hoar frost often looks feathered and lacy, in contrast with the sharp needles of rime. They also form by different processes. Hoar frost typically forms on cold, clear nights rather than during cloudy conditions. Water vapor freezes and crystallizes on surfaces without becoming water droplets first. Both rime ice and hoar frost add a magical beauty to the monochrome winter world.