Frost occurs more often in valleys than higher areas because cold air is dense and flows down the valley slopes. When warm, moist air rapidly cools, tiny droplets of water may form and stay suspended in the air. This is fog. Fog is sometimes called low cloud. Warm sea breezes often produce fog when they move over colder land surfaces.
Fog is often thickest in valleys and low-lying areas.
Extension Activity Examine frost under a microscope. Describe what you observe. Frost can cause great damage to crops. Find out how frost harms plants and where it generally occurs. Fact File Jack Frost is a fictional elf-like character, part of a Scandinavian legend.