Rime ice forms when supercooled water droplets present in fog or low clouds strike exposed objects—tree branches, power lines, antennas, metal structures—and freeze instantly upon contact. Unlike frost, which forms by direct deposition of water vapour, rime ice requires the presence of liquid droplets at sub-zero temperatures. There are two main types: soft rime, which appears white and porous with low density, formed when very small droplets freeze trapping air between them; and hard rime (glaze ice), which is transparent and dense, produced when larger droplets spread before freezing, creating a compact and adherent layer. Under extreme mountain conditions, hard rime can accumulate layers several centimetres thick within just a few hours. Rime ice is a common phenomenon on mountain summits and passes across many mid-latitude mountain ranges during winter. It poses significant hazards to infrastructure: the weight of accumulated ice can bring down power lines and telecommunications towers, and in aviation it constitutes a serious risk by altering the aerodynamic profile of aircraft. The temperature and liquid water content of the fog determine which type of rime forms.