Fog

• 1. Introduction

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  Fog is a visible aerosol made of tiny water droplets or ice crystals suspended just above the Earth's surface. It looks like a low-lying cloud, and in many ways it behaves like one. Yet fog has a character all its own. It shapes coastlines, grounded aircraft during World War II, and once helped a general escape capture by one of history's most powerful armies. How does it form? What kills it? And where in the world does it dominate the sky for as many as 200 days a year?

• 2. How Fog Is Born

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  Fog begins forming when the gap between air temperature and dew point narrows to less than 2.5 degrees Celsius. At that point, water vapor starts condensing into tiny suspended droplets. The process needs something to condense onto. Dust, ice, and salt particles in the air all serve as nuclei around which droplets can gather and grow.

  By definition, fog is any such condition that reduces visibility to less than one kilometer. Mist causes less impairment and lacks that threshold. Sea fog adds a wrinkle: it forms when water vapor condenses onto microscopic salt crystals floating above the ocean, a process that can occur even when relative humidity is as low as 70 percent. Researchers have also found that kelp seaweed plays a role along coastlines. Under stress from intense sunlight or strong evaporation, kelp releases iodine particles that can serve as condensation nuclei, producing fog that diffuses direct sunlight.

  Fog normally forms near 100 percent relative humidity, though not always. At that saturation point, air cannot hold additional moisture. Any extra water added pushes the air into a supersaturated state, and fog becomes nearly inevitable.

• 3. Fog Types and Their Triggers

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  Radiation fog is the type most people encounter. It forms after sunset when the ground loses the heat it absorbed during the day, cools the air just above it, and creates an inversion layer. Moderate to strong wind tends to prevent it; a slight breeze actually encourages it by spreading the cooled surface air into a fog layer that can reach several hundred feet in depth. Radiation fog is thickest shortly after sunrise, when rising turbulence thickens the fog before the sun grows strong enough to evaporate it. In California's Central Valley this variety is called tule fog.

  Advection fog requires wind. Moist air moving over a cooler surface gets chilled until it saturates and condenses. Along the California coastline this type is fed by several mechanisms: a cold front can push the marine layer toward shore in spring or late fall; intense inland heating can draw the dense marine layer in during summer; and a southerly surge driven by high pressure over the desert southwest can push the marine layer up the coast following a coastal heat spell.

  Frontal fog forms near weather fronts when raindrops falling from warmer air above evaporate into cooler air near the surface, raising local humidity until condensation begins. Hail fog is a rarer cousin, appearing around areas of significant hail accumulation where melt and evaporation lower the temperature and raise moisture near the ground. It can be extremely dense and arrive abruptly, forming shortly after hail falls.

  When temperatures drop to -35 degrees Celsius or below, fog's water droplets can freeze in midair to form ice fog. This condition is most common near Arctic and Antarctic regions. Urban areas see it too, because automobile exhaust and heating combustion produce water vapor that freezes in the extreme cold, creating a dense persistent haze that can last day and night until temperatures rise.

• 4. Freezing Fog and Pogonip

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  Freezing fog coats surfaces with white rime ice, a condition common on mountain tops exposed to low clouds. It is also the term applied to fog where water vapor is super-cooled, filling the air with tiny ice crystals similar to very light snow.

  In the western United States, this phenomenon carries a more evocative name: pogonip. The word comes from the Shoshone language, from the word paγi̵nappi̵h, which means "cloud". The phrase "Beware the Pogonip" appears regularly in the calendar for December in The Old Farmer's Almanac. Jack London described a deadly pogonip in his anthology Smoke Bellew, in which the fog surrounded the main characters and killed one of them.

  The Columbia Plateau is especially prone to pogonip during temperature inversions, sometimes enduring stretches lasting as long as three weeks. The fog typically starts forming near the Columbia River and can spread as far south as La Pine, Oregon, almost 150 miles from the river's edge.

• 5. Fog's Effects on Sound and Light

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  Sound behaves differently inside fog. Low-pitched tones travel farther because their long wavelengths move air slowly and interact less with the tiny water droplets suspended between them. High-pitched sounds have short wavelengths; they are reflected and refracted by those same droplets, losing energy in a process called damping. This is why foghorns are built to emit low-pitched tones.

  A temperature inversion reinforces the effect. When cold air pools at the surface and warm air sits above it, the boundary between those two air masses acts as a reflector, bouncing sound back toward the ground and extending the range at which low-frequency sounds can be heard.

  Fog also casts shadows. When light passes through gaps in a structure or a tree, the fog is dense enough to be illuminated but thin enough to let much of that light continue forward. The result is that solid objects cast visible "beams" of shadow oriented parallel to the light source. The same geometry produces crepuscular rays, which are the shadows of clouds; in fog it is buildings and trees that play that role.

• 6. Geography of Fog

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  The Grand Banks of Newfoundland sit where the cold Labrador Current from the north meets the much warmer Gulf Stream from the south, and the collision produces an average of 200 or more days in fog each year. The Grand Banks are widely considered one of the foggiest places on Earth.

  Other notably foggy locations include Hamilton in New Zealand, the Po Valley in Italy, the Swiss plateau, the coastal Atacama Desert in Chile, the Namib Desert, Cape Disappointment in Washington, Point Reyes in California, and Mistake Island in Maine. The sea area around Greenland and the Kuril Islands are also consistently foggy.

  At the other extreme, the desert areas of Arizona, California, and Nevada rank among the least foggy parts of the US mainland. San Francisco is famous for its fog, though a decline in dense fog since the 1980s has been attributed largely to reduced levels of air pollution.

  Along the coast of Chile and Peru, a variety called Garua forms when fog moving inland meets hot air and partially evaporates. The result is nearly invisible yet still causes condensation on surfaces, compelling drivers to use their windshield wipers even when they can barely perceive any fog. A similar dense fog called Camanchaca also forms in that region.

• 7. Fog as Resource and Historical Actor

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  Redwood forests in California draw approximately 30 to 40 percent of their moisture from coastal fog through a process called fog drip. Along the same coast, fog is the only source of water available to plants and animals for up to seven months of the year. In arid regions of Africa, insects depend on wet fog as a principal water source. Some coastal communities have built fog nets to harvest moisture from the atmosphere where groundwater and rainfall fall short.

  Fog has also shaped the outcomes of battles. In 1776, during the Battle of Long Island, American General George Washington and his forces used fog to conceal their escape and evade capture by the British Army. On the 6th of June 1944, Allied forces landed on the beaches of Normandy during fog conditions; both sides reported mixed results, with impaired visibility cutting both ways.

  During World War II the aviation problem became urgent enough to produce a dedicated solution. The British developed a system called Fog Investigation and Dispersal Operation, or FIDO, which burned enormous amounts of fuel alongside runways to evaporate fog and give returning fighter and bomber pilots enough visibility to land safely. The energy cost was so high that FIDO was never practical for routine peacetime use, yet it stood as a direct, fire-based answer to one of the oldest obstacles weather places in the path of human flight.