Rime ice builds up on surfaces when freezing fog droplets settle and harden in wind.

The longer version

Rime forms when supercooled fog droplets are carried by wind against surfaces that are already below freezing. On contact they freeze almost at once, and each new droplet adds to the deposit instead of merging into a smooth sheet of clear ice.

The air trapped between frozen droplets gives rime its white, granular look and its relatively low density. It builds thickest on windward edges, on branches, railings, and exposed ridges where droplets arrive most steadily. Calm, clear conditions produce frost instead, since vapor is deposited directly without any moving droplets. Rime needs wind, and it grows fastest where the wind blows hardest.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.