Even with radar, ships occasionally run into each other when fog covers the ocean. On July 25, 1956, the worst passenger ship collision in history occurred in thick fog off the New England coast. The cruise ship Stockholm rammed into the huge Italian passenger ship Andrea Doria, which sank 11 hours later. Other ships that flocked to the scene rescued 1,606 passengers and crew, but 52 people died in the wreck.
Fog can also cause aircraft accidents near or on the ground. In fact, the deadliest crash in aviation history was the result of fog. The airport at Tenerife in Spain's Canary Islands was blanketed by fog when a KLM Boeing 747 started its takeoff run without a clearance, although the captain thought he had been cleared for takeoff. That 747 smashed into a Pan Am Boeing 747 that was taxiing across the runway, which was hidden by the fog. A total of 570 people on the two airplanes were killed.
For all of the harm it does, fog is probably the easiest meteorological phenomenon to understand. Fog forms when air near the ground reaches its dew point-the temperature at which water vapor in air begins condensing into tiny water droplets. In other words, fog is a stratus cloud that happens to be on the ground.
The most common type of fog is called radiation fog because it forms on clear nights as heat radiates away from the ground. As the ground cools, it chills the air next to it. If the air cools enough, and if it contains enough water vapor, it will eventually reach its dew point. When this happens, not only does dew begin to form on the grass, but tiny water droplets also begin to float in the air.
Fog is most likely to occur on a clear night, because clouds in the sky-even high clouds-absorb some of the heat radiating away from the earth, warm up, and then radiate heat back toward the ground. This prevents the air from cooling to its dew point.
Light wind also helps fog to form. If the wind is blowing faster than about five or 10 mph, it will stir up the air too much, bringing warm air down and taking cool air up. Lighter wind will also stir up the air, but just enough to carry cool air a little higher than it would go in still air. This movement helps to make the fog thicker.
Sometimes, any radiation fog is called ground fog, but that term should be used only for fog that's less than 20 feet thick. If the fog is less than about six feet thick, the National Weather Service reports it as shallow fog. (See the official definitions of terms used to describe fog.)
Normally, radiation fog is less than about 300 feet thick, and it's not unusual to see blue sky if you look straight up into the fog. Of course, the same is true in reverse. If you are in an airplane above the fog, looking straight down, it might not appear thick. But, as the illustration above shows, you might have trouble landing.
If you go out to the airport to fly and find that it's covered by radiation fog, someone might say, "Relax, it will burn off in a little while." The fog doesn't literally burn off, but that's not a bad description of what happens.
Fog is normally thickest at the coldest time of day, which is usually around sunrise. As the sun climbs higher into the sky, it begins to warm the ground, which in turn warms the air. As the air warms, some of the fog droplets evaporate, which allows even more sunlight through and speeds up the warming. If clouds move overhead after the fog forms, they'll delay the morning burn-off.
Since cold air is denser than warmer air, it moves downhill. This is why valleys or other low places can be foggy while hilltops are still clear. Radiation fog that forms in valleys is called valley fog. From late fall into early spring, persistent valley fog can cause day after day of poor visibility in low-lying areas, especially in the western United States.
In winter, the sun supplies less heat to evaporate fog than it does during the summer. In some valleys, such as California's Central Valley, the fog can be 2,000 feet thick or more. Sometimes enough heat will reach the ground to evaporate a thin layer of fog near the ground-maybe 400 or 500 feet thick. But the fog above remains as a low-level stratus cloud, which is sometimes called high fog.
Under the right conditions, valleys can spend days on end locked into a pattern of solid nighttime fog and daytime low clouds. Such episodes typically end when a storm brings wind strong enough to scour out the valley. While radiation fog is the result of fairly still air cooling to its dew point, moving air causes advection fog. Meteorologists use the word advection to describe the horizontal transport of an atmospheric property such as temperature or moisture by the wind. Advection fog forms when relatively warm, humid air moves over a colder surface, either the ground or water. As the humid air moves over the cold surface, it is cooled to the dew point and fog begins forming.
In the central and eastern parts of the United States, advection fog is most common in winter following a cold spell, especially when snow is left on the ground. When winds following the storm begin blowing from the south, they bring humid air up from the Gulf of Mexico or Atlantic Ocean to be cooled as it moves over the cold ground.
The West Coast has its own form of advection fog, which is more common in summer than winter. The Pacific Ocean is chilly all year. Even in late summer, the water off the Southern California Coast warms only into the high 60s, a good 10 to 20 degrees colder than the water along the Gulf of Mexico and southern Atlantic coasts. Warm winds from the southwest and south are cooled to the dew point as they blow over the cool water. This forms the fog and low clouds known locally as the marine layer. As the fog and low clouds move inland they are blocked in places by the mountains along the coast, but they flow inland through valleys or large openings such as the Golden Gate area where San Francisco Bay meets the ocean.
If you're planning to fly to a West Coast airport, especially one near the shore, it's a good idea to call an area flight school to learn what you can about the local tricks of marine layer fog. If the airport that you're planning to fly to doesn't have weather observations, you should ask if there are observations at another airport that will give you a good idea of whether your destination is likely to be fogged in.
Upslope fog could be considered a kind of advection fog because it's formed by moving air. But, instead of moving over a colder surface, the air is moving uphill over ground that could be at the same temperature as the air. As air rises, it cools at the rate of approximately 2 degrees Celsius for each 1,000 feet of elevation gained, no matter what the air temperature is to begin with. Upslope fog forms when the wind pushes humid air uphill. Large-scale upslope fog is most common in the Great Plains, where the ground gently rises from elevations of less than 1,000 feet in the Mississippi Valley to around 5,000 feet at the base of the Rocky Mountains.
Radiation, advection, and upslope fog all form when the air is cooled to its dew point. Precipitation fog and steam fog are formed when water vapor is added to the air, increasing the dew point to match the temperature.
Precipitation fog, which is also called frontal fog or rain-induced fog, forms when rain falls into cooler air. Some of the water in the rain evaporates into the air until the dewpoint increases to match the temperature. When this happens, fog and low clouds begin to form.
The name frontal fog is sometimes used because precipitation fog commonly happens ahead of an advancing warm front where warm air is moving over a layer of cooler air. The effect on pilots can be to decrease the ceiling and visibility under a layer of higher clouds in light rain.
The other kind of fog formed by adding water vapor to the air is steam fog. This is the thin layer of fog you see over rivers, ponds, and lakes early in the fall after cold air has moved in. As cold air flows over a warmer lake or river, some of the water evaporates into the air, increasing the dew point. The resulting fog is rarely a problem to pilots since it's usually only a few feet thick and over water. But, deeper and thicker steam fog can form in the Arctic and over the Great Lakes during the winter.
Even though fog often forms when the air temperature is below freezing, it consists of tiny water droplets, not ice crystals. Such fog can be reported as freezing fog since the droplets can freeze when they hit something, such as an airplane. The resulting layer of ice on an airplane might be extremely thin, but it is still dangerous because even a little ice can greatly reduce the amount of lift a wing produces.
Temperatures near 40 degrees below zero are needed to form ice fog, which consists of ice crystals instead of water droplets. Often an ice fog will form over an airport when an aircraft engine is started on an extremely cold morning. The extra moisture and tiny particles that the exhaust adds to the air are just enough to trigger the formation of ice crystals in the air.
While fog is made of tiny drops of water, haze consists of extremely small, dry particles in the air. They can be dust, sea salt, the residue from fires or volcanoes, or various kinds of pollution. Even though the individual particles are too small to be seen with the naked eye, they scatter light, producing a bluish color like that which gives the Blue Ridge Mountains their name
If the air's relative humidity is above 70 percent, water vapor begins condensing on the dry particles, creating what's known as wet haze. The wet particles scatter more light, making the haze white and reducing visibility. Wet haze is more common near the ocean, where tiny salt particles tossed into the air by waves are combined with the higher humidity found over the ocean. At night, as the air cools, more water vapor condenses onto the haze particles, further decreasing visibility.
Since most fog forms when the air's temperature cools, pilots who fly at night are more likely to be caught in the air as fog closes airports or reduces visibility to instrument conditions. If the winds are nearly calm and the temperature and dew point are within about 7 degrees of each other around sunset, fog is a real possibility that night.
Since small differences in temperature or in the amount of water vapor in the air determine whether fog forms, one airport may have less than a quarter-mile visibility in fog, while another a few miles away could be clear. This is why reports of current conditions at airports aren't always a good guide to what's happening at nearby locations.
Combining basic knowledge about how fog forms with information about the airports you're using is the best way to avoid being caught by fog.
Mist: A visible aggregate of minute water particles suspended in the atmosphere that reduces visibility to less than seven statute miles but greater than or equal to five-eighths of a statute mile.
Fog: A visible aggregate of minute water particles (droplets) which are based at the Earth's surface and reduces horizontal visibility to less than five-eighths of a statute mile. Unlike drizzle, it does not fall to the ground.
Smoke: A suspension in the air of small particles produced by combustion. A transition to haze may occur when smoke particles have traveled great distances (25 to 100 miles or more) and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere.
Haze: A suspension in the air of extremely small, dry particles invisible to the naked eye and sufficiently numerous to give the air an opalescent appearance.
Shallow Fog: The descriptor shallow shall only be used to further describe fog that has little vertical extent (less than six feet).
Partial and Patchy Fog: The descriptors partial and patches shall only be used to further describe fog that has little vertical extent (normally greater than or equal to six feet but less than 20 feet), and reduces horizontal visibility but to a lesser extent vertically. The stars may often be seen by night and the sun by day.
Freezing: When fog forms and the temperature is below 0 degrees Celsius, freezing is used to further describe the phenomenon. When drizzle and/or rain freezes upon impact and forms a glaze on the ground or other exposed objects, freezing shall be used to further describe the precipitation
The National Weather Service does not use smog as an official definition. The name is a combination of smoke and fog and originally described natural fog that was contaminated by industrial pollutants. But today the term is generally used to describe visible air pollution whether there is any natural fog or not.
Source: Federal Meteorological Handbook Number 1
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