What Causes Fog to Form?


Quick Answer

Fog forms when water droplets condense to form fog close to the ground. The water droplets are formed when water vapor bonds with condensation nuclei. Water evaporation can occur from bodies of water, wind, precipitation and atmospheric heating.

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Full Answer

Weather conditions must be perfect for fog to form. The relative humidity must be near 100 percent. The air temperature and dew point temperature must be close to one another as well. As the air reaches 100 percent relative humidity and the dew point is saturated, the water vapor condenses to form fog.

There are several types of fog. Radiation fog and advection fog are the most common types of fog. Radiation fog develops in the evening with clear skies and soft winds. A thick, moist layer of air develops on the ground resulting in a dense layer of fog. Radiation fog commonly occurs in the California Central Valley. On the other hand, advection fog forms when warm, moist air moves over cool air. Advection fog is common in the San Francisco Bay Area.

Upslope fog, freezing fog, ice fog and evaporation fog are other types of fog identified by the National Weather Service.

Fog is common in Newfoundland, California, Ireland, Argentina, the Pacific Northwest, Chile and southern Europe.

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