Hoar-frosted seaweed. Crystals of hoar frost on seaweed that has been washed up on a shoreline in winter. This seaweed is knotted wrack (Ascophyllum nodosum). Hoar frost forms from air that is supersaturated with water vapour. This can occur when moist air cools rapidly. For hoar frost to form, the temperature of the surface on which the deposition occurs has to be below the freezing point of water. This results in the formation of many needle-like ice crystals. Hoar frost forms in still, clear and cold weather. Photographed on the Isle of Skye, off the western coast of Scotland.