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What Is Fog and Why Does it Form?

Updated on March 14, 2013
Fog decreases the visibility to 5/8 mile or less.
Fog decreases the visibility to 5/8 mile or less. | Source
As fog becomes more dense, visibility decreases drastically.
As fog becomes more dense, visibility decreases drastically. | Source
Visibility in foggy conditions can nearly disappear to nothing.
Visibility in foggy conditions can nearly disappear to nothing. | Source
Fog clearly shows the path of light for these parking lot lamps. The lamp light has a cool effect.
Fog clearly shows the path of light for these parking lot lamps. The lamp light has a cool effect. | Source

By Joan Whetzel

"A low line of shore was visible at first on the right between the movement of the waves and fog, but when we came further it was lost sight of, and nothing could be seen but the mist curling in the rigging, and a small circle of foam." - John Millington Synge.

This quote really brings the concept of fog into sharp focus. Being lost in a fog makes things appear blurred and out of focus. At times, it can be so thick that it makes the things right in front of you seem to vanish. At other times, the wall of fog will just as suddenly part, and items that you didn't see before will emerge to surround you like the mists. What exactly is this phenomena called fog? Is it a cloud or is it a separate entity?

What Is Fog?

Fog is defined as condensed water vapor suspended in a cloudlike mass, that hugs the ground and limits visibility to less than 5/8 (0.62) of a mile (about 1 km) on land. Clouds are bodies of fine water droplets, water vapor, or ice particles suspended in the air above the ground usually at altitudes that can range up to several miles above sea level. The very basic explanation is that clouds and fog are the same thing. The only differences are (1) where the water vapor is sitting - on the ground or up in the atmosphere - and (2) whether the water vapor obstructs visibility.

Types of Fog

The more dense the fog becomes, the more it limits transportation by land and sea, even delaying or cancelling airport arrivals and departures. There are several types of fog known as radiation fog, advection fog, evaporation fog, and precipitation fog.

1. Radiation Fog occurs at night due to the radiational cooling of the earth's surface. This cooling produces a shallow layer of mist at the ground level.

2. Advection Fog occurs when warm, moist air shifts horizontally over a cold surface (advection), causing the air to cool to the same temperature as the Earth's surface.

3. Evaporation Fog develops when two unsaturated air masses mix or converge. A variation is the steam fog, which occurs when cold, dry air moves over warm water or over a patch of warm, moist land, and the warm, moist atmospheric elements mix with the cool, dry atmosphere.

4. Precipitation Fog is a form of evaporation fog occurring when warm rain or snow descends through air that is cool and nearly saturated, and evaporation from the rain or snow makes the air saturated.

The density of the fog can be broken down into 5 levels (levels 1 through 5) which are given the names of slight fog or mist, moderate fog, or thick fog. Each of these levels of fog has its own description listed in the table below.

This table illustrates the fog names and levels of fog as well as the descriptions of each.
This table illustrates the fog names and levels of fog as well as the descriptions of each. | Source

Conditions Necessary for Fog to Form

The key ingredients for any fog formation are: (a) the presence of enough condensation nuclei - which are particles of dust, pollens or other particulate matter which are necessary for water droplets to form (condense) around, and (b)an air temperature and dew point temperature that are the same. (The dew point temperature is defined as the temperature of the air at the ground level that is necessary for dew to form.) The different types of fog need the following conditions to form over land:

· Radiation fog needs calm weather conditions, a light wind, and a clear sky. It usually forms on late autumn and winter nights, usually occurring in low-lying areas since the cold, moist air becomes heavy, which causes it to sink. This fog dissipates after the sun comes up due to the sun's warming effects.

· Advection fog forms any time of the year and tends to hang around. It is most common near coastal regions due to the higher humidity levels in the air. It requires the presence of wind to stir up the moisture content of the air, and dissipates when the wind dies down.

· Evaporation fog needs the presences of moisture that evaporates from the ground surface or from the surface of any body of water - e.g. oceans, rivers, lakes, streams, swimming pools, hot tubs. The temperature of the water that is evaporating is generally warmer than the air temperature, which is what causes the fog or mist to form. It occurs most commonly in the autumn months, before the water temperatures have had a chance to cool to a range that is at or close to the air temperature.

· Precipitation fog can turn thick and be persistent, sometimes lasting for hours or even days, and can extend for miles over land and reaching out to the sea in coastal areas. This is the kind of fog one associates with areas like San Francisco. It usually occurs in conjunction with warm fronts, though it can also arise as part of a slow moving cold front or a stationary front.

As a general rule, in order for fog to form over the open ocean, it needs currents of cold water lying beneath warm air. This usually occurring during the spring when the air temperature rises faster than the water temperature. Ocean fog also needs a slow drift of air which helps to cool the lower levels of air without causing too much mixing of cool air and warm air.

Of course, these are simplified explanations of fog formation. For more detailed explanations, there are several sources available: NASA, NOAA (National Oceanic and Atmospheric Association), NWS (the National Weather Service), the Weather Channel, or the meteorologist at any local television station.

Driving in Fog

As with any weather patterns other than sunny and clear conditions, caution should be taken when driving in fog, since it can reduce visibility to 1 km (0.62 miles) or less. Visibility can be reduced to near zero, meaning drivers may not be able to see anything beyond their front bumper. This makes driving cars quite hazardous. Imagine what it does for driving trains, guiding ships along the waterways, or landing airplanes and allowing them to take off in such conditions. The denser the fog, the more likely that transportation of any kind will be brought to a grinding halt. The best advice falls along these lines:

1. If at all possible, postpone the trip until the fog clears.

2. Drive with low beams on, never the high beams, as high beams cause a glare as the light reflects off the moisture droplets and make the fog appear worse.

3. Drive slower than the posted speed limit since it will difficult to impossible to see what vehicles and road conditions (road construction, accidents, police actions, someone with a flat tire) are ahead of you on the road.

4. Turn off the radio and crack the window open just a bit, then listen to the traffic sounds around you.

5. Use the windshield wipers to keep windows clear of moisture and make use of the window de-foggers when the temperatures are cool enough to turn the fog to frost.

6. Use the right edge of the road or stripes and other painted road markers as a guide for staying in the correct lane.

7. Practice patience. Getting antsy and trying to pass around lines of traffic is a recipe for disaster. So, if you know that there are foggy conditions before you start out the door, start out the door early so you have more time to reach your destination. And remember, that all those people ahead of you in those long lines of traffic are in the exact same boat you are.

8. Don't stop in the main lanes of a freeway or any other roadway that gets heavy traffic when car problems occur. Pull over to the shoulder or into a parking lot. Then turn off the headlight and take your foot off the break. During heavy fog conditions, drivers tend to follow whatever set of lights they can see ahead of them. So if they see your brake lights glowing, those drivers behind you could accidentally mistake them for plain tail lights. If pulled over on the shoulder on the highway, get out of the car and move away from the vehicle, well onto the grassy embankment, just in case another vehicle slams into yours because the driver couldn't see clearly in the fog.


American Heritage Dictionary. Fog.

Weather Channel. Glossary.

Weather Channel. Driving in Fog.

Encyclopedia. Fog.

"Fog." World of Earth Science. 2003. 5 Nov. 2012

Lyrics Mode. Burl Ives "Foggy, Foggy Dew" Lyrics.

Good Quotes. All John Millington Synge Quotes.


What Is The Difference Between Fog, Clouds?


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    • joanwz profile image

      Joan Whetzel 5 years ago from Katy, Texas

      I'm with you. I hate driving in the fog, especially the really dense fog. It's worse when it's after dark. Really frightening.

    • joanwz profile image

      Joan Whetzel 5 years ago from Katy, Texas

      I'm not a boater, but I can imagine how scary it would be to have the fog surround you while out on the water.

    • Minnetonka Twin profile image

      Linda Rogers 5 years ago from Minnesota

      Great information on fog. I get really nervous when I get caught in the fog like your video. Great for a scary movie though :-)

    • rfmoran profile image

      Russ Moran 5 years ago from Long Island, New York

      Good hub, loaded with solid info and great pictures and videos. As a boater, I hate fog, but love it when I'm safely indoors on land.