Fog is a cloud that touches the ground. Fog can be thin or thick, meaning people have difficulty seeing through it. In some conditions, fog can be so thick that it makes passing cars. Even monuments like London Bridge, in London, England, or the Golden Gate Bridge, in San Francisco, California, are almost impossible to see in thick fog.

Fog shows up when water vapor, or water in its gaseous form, condenses. During condensation, molecules of water vapor combine to make tiny liquid water droplets that hang in the air. You can see fog because of these tiny water droplets. Water vapor, a gas, is invisible.

Fog happens when it’s very, very humid. There has to be a lot of water vapor in the air for fog to form.

In order for fog to form, dust or some kind of air pollution needs to be in the air. Water vapor condenses around these microscopic solid particles. Sea fog, which shows up near bodies of salty water, is formed as water vapor condenses around bits of salt.

Depending on the humidity and temperature, fog can form very suddenly and then disappear just as quickly. This is called flash fog.

Fog is not the same thing as mist. Fog is denser than mist. This means fog is more massive and thicker than mist. There are more water molecules in the same amount of space in a fog. Fog cuts visibility down to one kilometer, meaning it will prevent you from seeing further away than one kilometer from where you’re standing. Mist can reduce visibility to between one and two kilometers.

Types of Fog

There are several different types of fog, including radiation fog, advection fog, valley fog, and freezing fog.

Radiation fog forms in the evening when heat absorbed by the Earth’s surface during the day is radiated into the air. As heat is transferred from the ground to the air, water droplets form. Sometimes people use the term “ground fog” to refer to radiation fog. Ground fog does not reach as high as any of the clouds overhead. It usually forms at night. Fog that is said to “burn off” in the morning sun is radiation fog.

Advection fog forms when warm, moist air passes over a cool surface. This process is called advection, a scientific name describing the movement of fluid. In the atmosphere, the fluid is wind. When the moist, warm air makes contact with the cooler surface air, water vapor condenses to create fog. Advection fog shows up mostly in places where warm, tropical air meets cooler ocean water. The Pacific coast of the United States, from Washington to California, is often covered in advection fog. The cold California Current, which runs along the western coast of North America, is much cooler than the warm air along the coast.

Valley fog forms in mountain valleys, usually during winter. Valley fog develops when mountains prevent the dense air from escaping. The fog is trapped in the bowl of the valley. In 1930, vapor condensed around particles of air pollution in the Meuse Valley, Belgium. More than 60 people died as a result of this deadly valley fog.

Freezing fog happens when the liquid fog droplets freeze to solid surfaces. Mountaintops that are covered by clouds are often covered in freezing fog. As the freezing fog lifts, the ground, the trees, and even objects like spider webs, are blanketed by a layer of frost. The white landscapes of freezing fog are common in places with cold, moist climates, such as Scandinavia or Antarctica.

Fog Catchers

Many ancient cultures collected water from fog by placing large pots under trees and shrubs. As the water from fog collected on these objects, the pots collected the water. This method of water collection was effective, but not as effective as collecting rainwater or other liquid water.

Today, engineers are working on more sophisticated ways to collect water from fog. The most effective way has been the development of “fog catchers.” Fog catchers are very large screens constructed in arid areas. As fog glides in, water droplets form around the thin screens and drip to the collection pools below. In one day, a single screen can collect more than a hundred gallons of water.

The village of Bellavista, Peru, relies on fog catchers. Bellavista is an area that has little access to liquid water—no rivers, lakes, or glaciers are nearby. Wells dry up quickly. Water for irrigation and human consumption is threatened. Every year, however, huge fogs blow in from the Pacific Ocean. In 2006, the community invested in a series of fog catchers outside of town. Now, the residents of Bellavista have enough water to irrigate trees and gardens, as well as provide for their own drinking and hygiene needs.

Engineers warn that fog catchers will only work in small areas. Still, engineers and politicians are working on ways to make more powerful fog catchers that will perhaps reduce the need for people to rely so much on groundwater.

fog
A tall ship disappears into a thick fog.

Pea Souper
A "pea souper" is a type of fog that forms when water condenses around microscopic particles of coal. This fog is often a brownish-yellow color, leading to the name. Pea soupers are common in areas that burn coal for energy.

The London Fog of 1952, which killed 12,000 people around the urban center of London, England, was a pea souper. The Great Fog led to legislation that regulated the coal industry and air pollution in the United Kingdom.

Grand Banks
The foggiest place in the world is Grand Banks, a spot in the Atlantic Ocean off the island of Newfoundland, Canada. The cold Labrador Current from the north and the warm Gulf Stream current from the east create prime conditions for thick fog to form almost every day.

advection
Noun

process of an air mass moving horizontally.

advection fog
Noun

water vapor that condenses as warm, moist air passes over a cool surface.

Noun

harmful chemicals in the atmosphere.

arid
Adjective

dry.

blanket
Verb

to cover entirely.

California Current
Noun

cold ocean current that flows south in the Pacific Ocean from British Columbia, Canada, to Baja California, Mexico.

climate
Noun

all weather conditions for a given location over a period of time.

Noun

visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

Noun

dark, solid fossil fuel mined from the earth.

Noun

process by which water vapor becomes liquid.

condense
Verb

to turn from gas to liquid.

dense
Adjective

having parts or molecules that are packed closely together.

Noun

tiny, dry particles of material solid enough for wind to carry.

engineer
Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

flash fog
Noun

water vapor that condenses and evaporates very quickly.

Noun

clouds at ground level.

fog catcher
Noun

device that collects water from fog.

freezing fog
Noun

water vapor that freezes to solid surfaces.

Noun

thin coat of ice covering objects when the dew point is below freezing.

gas
Noun

state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

Noun

mass of ice that moves slowly over land.

Golden Gate Bridge
Noun

(1937) suspension bridge connecting the strait between the San Francisco Bay and the Pacific Ocean.

ground fog
Noun

water vapor that condenses above land as the air cools following sunset. Also called radiation fog.

Noun

water found in an aquifer.

Gulf Stream
Noun

warm current that starts in the Gulf of Mexico and travels along the eastern coast of the U.S. and Canada before crossing the North Atlantic Ocean.

humid
Adjective

air containing a large amount of water vapor.

hygiene
Noun

science and methods of keeping clean and healthy.

industry
Noun

activity that produces goods and services.

Noun

watering land, usually for agriculture, by artificial means.

Noun

body of land surrounded by water.

Labrador Current
Noun

cold ocean current that flows south from the Arctic Ocean along the eastern coast of Canada. Also called the Arctic Current.

Noun

the geographic features of a region.

London Bridge
Noun

bridge crossing the Thames River in London, England. There has been a bridge on the site for more than 2,000 years.

microscopic
Adjective

very small.

Noun

clouds at ground-level, but with greater visibility than fog.

moist
Adjective

damp or slightly wet.

molecule
Noun

smallest physical unit of a substance, consisting of two or more atoms linked together.

particle
Noun

small piece of material.

pea souper
Noun

fog that forms as water condenses around microscopic particles of coal.

prime
Adjective

ideal or very good.

radiation fog
Noun

water vapor that condenses above land as the air cools following sunset. Also called ground fog.

rainwater
Noun

precipitation that falls as liquid water.

reduce
Verb

to lower or lessen.

salt
Noun

(sodium chloride, NaCl) crystalline mineral often used as a seasoning or preservative for food.

Scandinavia
Noun

region and name for some countries in Northern Europe: Iceland, Norway, Sweden, Finland, and Denmark.

sea fog
Noun

water vapor that condenses around particles of sea salt.

sophisticated
Adjective

knowledgeable or complex.

sunset
Noun

time when the sun descends behind the horizon.

tropical
Adjective

existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

urban center
Noun

densely populated area, usually a city and its surrounding suburbs.

valley fog
Noun

cloud that is trapped in a valley at ground level.

vapor
Noun

visible liquid suspended in the air, such as fog.

Noun

movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.