“The fog comes on little cat feet. It sits looking over harbor and city on silent haunches and then moves on.”
Carl Sandburg 1916
Whether it softly comes in on little cat feet, or rolls in like a spooky movie, everyone has experienced fog. Although there are different types of fog, essentially fog is a cloud on the ground which reduces visibility to less than 1km. The basic requirements for fog to form are moisture in the air – the closer to 100% humidity the better, and the air near the ground must be cooling to within 5 F (3 C) of dew point – this is the temperature to which air must be cooled in order for water vapor in the air to condense to liquid water. When the air near the ground cools to dew point, the water vapor in the air will become visible as fog in the air or dew on the ground.
Fogs during summer will only happen with clear skies and near 100% humidity. There must also be condensation nuclei – or something onto which the water can condense. Condensation nuclei can be dust particles, aerosols, pollutants etc. When the air is saturated, additional moisture will condense onto this nuclei. Fog so often forms after sunset because that is when the air begins to cool and condensation replaces evaporation. Air cools best with clear skies as there are no clouds to trap the heat.
Fog can form at lower humidity levels if there are a really large amount of nuclei, especially if it is something such as salt.
During the summer, if there is a storm which includes hail, a phenomenon called hail fog can form. It is usually a shallow layer of fog above the ground. It forms because of increased moisture and cold air coming in contact with the warm ground.
During fall and early winter the most common form of fog is radiation fog. This type of fog forms when the land cools after sunset by radiating the heat up into the atmosphere. The air must be calm and the skies clear, again because cloud cover will trap heat in. When the ground is cool, it will cause condensation in the air above it. The more calm the air, the lower the fog is, under ideal conditions, the fog may only be a meter deep. Air movement will produce a thicker fog. Radiation fog can occur throughout the night but rarely lasts long after sunrise.
During winter – fog forms in a different manner, almost the opposite of summer fogs. During the winter months, fog will form when humid air moves over a cold surface. Winter fog is more common around bodies of water and is sometimes called lake effect fog.
Freezing fog, or Steam fog tends to occur in localized areas where cool air passes over warm, moist land. The tiny droplets in the fog will then freeze to surfaces. This frozen “fog” is called rime ice or hoar frost. Don't confuse this with freezing rain – which is a true precipitation. Freezing fog is common at high elevations may come into contact with low clouds. An example of freezing fog is the ice which forms in old style freezers that aren't “frost-free.”
In high northern or southern latitudes, especially around urban areas Ice fog can form. Ice fog is any type of fog where the droplets freeze into very tiny crystals in midair. This type of fog forms when the air temperature is well below freezing, generally below zero, so that any vapor present almost immediately condenses. Vapor is added to the air by automobile exhaust, furnaces and industrial plant exhaust. Ice fog can be extremely dense, posing driving hazards and the fog can last all through the day and night. The minuscule ice crystals sparkle in the sunlight and are often called “diamond dust”. Diamond dust can pose a health hazard if too much is inhaled. The Shoshone tribe of Native Americans had their own name for ice fog: they called it the Pogonip, which means “white death.” again because of the hazards of breathing it in.
If it seems sometimes that fog forms really quickly you are correct. There is a condition called flash fog. Fog can dissipate just as rapidly. This condition is dependent on which side of the dew point the temperature is.
If you live near the ocean you will likely have seen sea fog. Sea fog forms when the condensation nuclei is salt. Salt is, of course, very common near the ocean where it is kicked up into the atmosphere by the breaking waves. Salt is a unique condensation nuclei in that it will allow fog to form when the humidity is as low as 70%. Typically this fog begins as a transparent mistiness which rapidly changes to thick fog. Sea fog is a common type of fog along the California cost.
Fog can sometimes be accompanied by drizzle when the humidity stays at 100%. When this happens tiny cloud droplets can coalesce into larger droplets. When the temperatures are below freezing the drizzle will freeze producing very hazardous driving conditions. Drizzle usually occurs just as the fog is lifting, and therefore cooling, or when the droplets are being compressed from the droplets above.
When wind blows moist air over a cool surface the air will cool and advection fog will form. Advection fog is very common at sea when tropical winds pass over cooler waters and on land when a warm front passes over heavy snow. It can also form in areas of upwelling, such as along the California coast. During spring or fall a cold front can propel the air layer over the land. During the summer months, a low pressure trough can be produced by intense heating inland which creates a strong pressure gradient, pulling the fog in from the water. Also during the summer, during the monsoons, a high pressure over the desert can create a southerly flow which pushes the offshore layer of air up the coastline. This type is most commonly created after a heat spell.
When winds blow up a slope they will cool as they rise, causing water vapor to condense and producing what is called upslope fog. If the slope is high enough, freezing fog will form.
Valley fog is a localized form of radiation fog and forms most often in winter when there is a temperature inversions. This is where colder, more dense air settles in the valleys with warmer air passing above. It can actually last for days if there is no wind to mix the air. In the Central Valley region of California, locals call this type of fog Tule Fog.
Most everyone has heard fog described to be “thick as pea soup,” but few folks know where this term originated from. Though it is liberally used to describe any thick fog it orginally was used to describe a dingy yellow smog from burning soft coal. This type of fog was common in Europe, especially London which is famous for its fog anyway. Such fogs occurred in London all the way up till the Clean Air Act of 1956.
Mist is often mistaken for fog and with good reason. The only difference between the two is in our definition of them. If the visibility is less then 1 km, the phenomenon is termed fog, otherwise it is mist. Mist tends to look a bit bluish from a distance. Mist is artificially created when you exhale warm breath on a cold day.
There is a fair amount of weather related folklore surrounding fog, most of it with only a marginal amount of truth.
“If fog forms on water in the autumn or spring a frost is on the way.” This one has some truth to it in that the fog indicates a lot of moisture in the air, and there are many nights cool enough for frost to form with that moisture in autumn and spring.
“A foggy morning with dew on grass indicates a clear day.” This saying is often true because fog most often forms on calm, clear nights and that naturally would lead to a clear day ahead.
Other sayings however, are purely silly and very inaccurate. “Dogs sleeping through the day indicate a coming storm or heavy fog.” Dog owners will attest that their dogs sleep through the day regardless of clouds, fog or clear. “Observe on what day the first heavy fog occurs, and expect a hard frost on the same day in October,” and “Three days of heavy morning fog, watch for bad weather in 90 days,” are about as illogical as groundhogs looking for their shadow to determine when spring will come!
Many people find fog beautiful, but no one can deny it is a visibility hazard. Before radar, fog caused many a collision. Cars and trucks however don't have radar and must use extra caution driving through fog. Localized fog is especially dangerous to unwary drivers when it appears suddenly.
Fog can wreck real havoc with airports, often grounding flights for hours at a time. There have been some attempts to disperse the fog by spraying salt particles into the air but this has been only partially successful and only when temperatures are below freezing.
If you can't get enough of fog, try visiting the foggiest place in the world. This honor goes to the Grand Banks off the island of Newfoundland, Canada. Here, one can enjoy fog over 200 days a year! Why is it so foggy here? The Grand Banks is the place where the cold Labrador Current coming in from the north, meets up with the warmer Gulf Stream coming up from the south. If you aren't a sea farer, you can still enjoy lots of foggy weather by visiting Point Reyes, California, or Argentia, Newfoundland both sites also enjoying nearly 200 foggy days a year!
Copyright © 1995 - 2007
Kathy Miles, Author, and Chuck Peters, Systems Administrator
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