How does fog form?

WEATHER: Like It or Not!
CHICAGO (WLS) -- Fog can conjure up images of Halloween and spooky Fall nights, but fog can form any time of the year.

Fog is basically a cloud on the ground. It is made up of tiny water droplets that are floating in the air. The density of those droplets make it hard to see.

There are two types of fog: advection fog and radiation fog. Both form through different processes.

Advection fog forms when warm, moist air moves in over cooler ground or cooler water. This process is most prevalent over Lake Michigan. When the water is cool and a warm, moist air mass comes in contact with the lake. We can get thick fog to form, and it can then drift over the city.

Radiation fog forms on a clear, calm night. The air rises and temperatures cool. The temperatures cool to the point where the air is saturated and then fog forms.


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