Ice is common to us as something that forms flat, as low down as it can go. It sits in ice cube trays or at the bottom of skating rinks. This is because it always is a liquid before it becomes a solid, so it flows to the bottom before it freezes. But this isn’t always true in the atmosphere. The tiny droplets of water suspended in the atmosphere are often too small to flow and pool, so don’t freeze the same way. When they freeze on objects, they form either glaze ice or rime ice.
Glaze ice is clear. The water droplets are close to the freezing point, but stay liquid after initial contact with the surface. The liquid droplets cling to whatever they make contact with, group up, and then freeze. This allows all the air to escape before freezing, making it clear. It is common on aircraft flying through clouds close to freezing, or cars driving through freezing fog.
Glaze ice on the side view mirror of a car after driving through freezing fog. Though the camera flash makes it look silvery-white, it is actually clear. Photo courtesy of Scott Wolff
Rime ice is opaque and white. It forms from water droplets that are below freezing but still liquid, known as “supercooled.” The droplets won’t freeze in the air because there is nothing for the ice to make the crystal structure on. As soon as they come into contact with something solid though, they freeze. Since the freezing is instantaneous, there is no time for the water to flow or pool. Tiny air pockets form, which gives the ice it’s white color. It is common on aircraft flying through clouds below freezing, or freezing fog blowing past stationary objects.
Dramatic Rime ice on top of Mount Washington, NH. High wind is required for this kind of formation. Photo from http://www.cbsnews.com/news/supercooled-clouds-and-wind-shape-stunning-ice-formations/
Rime ice covers pine trees atop Lyon Mountain, NY. This kind of rime ice, called hoarfrost, forms with very calm wind. Photo courtesy of David Magowan
Written by Scott Wolff