A hard, white ice that forms when water droplets in fog or low clouds freeze onto super-cooled surfaces. Rime can closely resemble hoar frost, especially in it’s early stages but the process of formation differs: in rime the water is in droplet form when it meets the surface onto which it freezes whereas hoar frost forms from condensing water vapour. Unlike hoar frost Rime grows into the wind and under the right conditions can accumulate into massive ice sculptures.
Etymology
Derived from the Old English hrim meaning “hoarforst” N.B. the term now has a meaning which is distinct [see above]