Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earths surface. It can be considered a type of low-lying cloud usually resembling stratus, and is heavily influenced by nearby bodies of water, topography, and weather conditions. Fog appears when water vapor (water in its gaseous form) condenses. During condensation, molecules of water vapor combine to make tiny liquid water droplets that hang in the air. Fog is similar to, but less transparent than, mist. The term fog is typically distinguished from the more generic term cloud in that fog is low-lying, and the moisture in the fog is often generated locally (such as from a nearby body of water, like a lake or the ocean, or from nearby moist ground or marshes) .

There are different types of fog, including:

• Radiation fog: This type of fog forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is radiated into space. As the earth’s surface continues to cool, the air temperature drops to the dew point, and fog forms. Valley fog is a type of radiation fog.

• Advection fog: This occurs when moist air passes over a cool surface by the wind and is cooled. Advection fog typically happens when a warm front passes over an area with significant snowpack. It is even more common at sea when moist air encounters cooler waters, including areas of upswelling.

• Precipitation fog: This type of fog forms when rain falls through a layer of cool air and evaporates, adding moisture to the air. If enough moisture is added to the air to saturate it, precipitation fog starts to form.

• Upslope fog: This type of fog forms when moist air is going up the slope of a mountain or hill.

• Freezing fog: This occurs when temperatures are below freezing and is composed of droplets of supercooled water that freeze to surfaces on contact.

Fog can be hazardous when visibility is reduced to less than 1 km (0.62 mi) .