Fog is essentially a cloud that forms near the ground and, in our climate, is more likely in the cooler months than in summer. Fog is tiny droplets of water or crystals of ice that are suspended in the air due to their very small size. It usually happens when a relatively cool layer of humid air is trapped underneath a layer of warmer, drier air. This is called an inversion. If the fog is a liquid fog, it is made of liquid water droplets that leave everything damp.

Sometimes, the droplets are supercooled liquid water. This is when the temperature of the water droplets is below freezing but the droplets have been unable to freeze. Also known as freezing fog, this sort of fog usually leaves an icy glaze on almost everything and can be as dangerous to drivers as freezing drizzle. When the fog is made of tiny ice crystals, it is known as ice fog and often leaves a beautiful coating of white frost crystals on surfaces.

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