Fog is a cloud that's close to the ground, made of tiny water droplets floating in the air.
Fog can make it really hard to see at a distance because the water droplets can act like mirrors reflecting light back at you. That's why motorists need to use low beams instead of high beams when they're driving in fog.
There are two main types of fog: Advection and Radiation.
Advection occurs when warm, moist air moves (advects) over cooler ground or water. This causes the air to condense into water droplets, producing fog.
Radiation fog happens on calm, clear nights, when the heat the earth absorbs from the sun during the day radiates, or escapes, into the atmosphere. This cools air near the surface and when it reaches moisture saturation, fog forms.
You can see an example of advection fog at the Golden Gate Bridge in San Francisco in the video above, followed by an example of radiation fog.
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6ABC WEATHER SCHOOL