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Weather and Climate · entry 2 of 52
Fog is simply a cloud that has formed at ground level instead of high up.
The longer version
Air close to the ground can cool overnight until it reaches the point where its water vapor condenses. The result is a cloud whose base sits right at the surface, which is what people call fog. It forms most readily on clear, calm nights, in valleys, and near bodies of water.
Because the droplets hang in the air at eye level rather than high overhead, they scatter light in every direction and cut visibility sharply. Fog clears when sunlight warms the ground, when wind mixes drier air downward, or when the droplets simply evaporate as temperatures climb.
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.
More From Weather and Climate
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Read 5Cirrus clouds are thin and wispy, made of ice crystals far above the surface.
Read 6Rain begins when droplets inside a cloud grow heavy enough to fall downward.
Read 7Snow forms when water vapor turns directly into ice crystals without becoming liquid first.
Read 8Hail stones grow inside storm clouds as ice layers build up around a frozen core.
Read 9Sleet is rain that freezes into small pellets before reaching the ground below.
Read 10Freezing rain turns to ice the moment it touches a cold road or branch.
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