Rime ice

Rime ice is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects. It is often seen on trees atop mountains and ridges in winter, when low-hanging clouds cause freezing fog. This fog freezes to the windward (wind-facing) side of tree branches, buildings, and any other solid objects.

Scientists at places like Mount Washington in New Hampshire often have to break huge chunks of rime ice off of weather equipment, in order to keep anemometers and other measuring instruments operating.