Clouds are condensed or frozen atmospheric moisture. Fog is a specific type of cloud, but clouds are not a type of fog. The different types of clouds are:

Other types of clouds include:

At the very basic level, clouds form when air that is relatively warm and moist cools down for some reason. In order for the water vapor to condense, condensation nuclei are required. The condensing process releases heat, which is part of how a thunderstorm is fueled. Dust and water droplets can serve as condensation nuclei. In order for the water droplets to freeze, an actual solid particulate matter of some sort is required. If it is not present, the droplets will become supercooled water - at a temperature below freezing, but still in a liquid state.

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