“Is fog essentially a cloud? Any differences?” – Robert

This is essentially a case of “a square is a rectangle, but a rectangle is not necessarily a square.” Fog is basically a cloud, just one that forms at the surface of the Earth.

But if you looked at a square, you wouldn't say "that's a rectangle," right? Just like if it were foggy outside, your first thought wouldn't be, "Oh hey, a cloud!"

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Clouds are made of tiny water droplets or ice crystals suspended in the air. 

Fog is only made of water droplets because the air is closer to the ground and not cold enough for ice crystals to form. Even freezing fog is actually water droplets that have been supercooled below freezing.

Clouds form when air is saturated and can’t hold any more water vapor (water in the gas stage). Those water molecules then condense out of the air and form water droplets (or ice crystals at high enough altitudes).

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When warm air rises, it cools, though the amount of moisture in the air stays the same. When the air cools to the dew point, it becomes saturated and the water molecules condense. This is what causes the clouds in the sky.

Fog, meanwhile, is most common when air just above the ground cools through some other process, like the standard temperature drop at night.

Fog can also form when there is a sudden surge in moisture in the air, such as after a rainstorm.

Sara Tonks is a content meteorologist with weather.com and has a bachelor’s and a master’s degree from Georgia Tech in Earth and Atmospheric Sciences, along with a master’s degree from Unity Environmental University in Marine Science.