Fog

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Fog

Radiation fog
Advection fog
Sea fog
Evaporation fog
Upslope fog
Freezing fog
Fog stratus
For More Information

Fog is a cloud that forms near the ground. Like clouds at higher altitudes, fog is formed when water vapor condenses in the air so that the moisture becomes visible. The process of water turning from vapor into liquid is called condensation. In temperate regions, fog is composed of water droplets. In polar and arctic regions, fog may be composed of ice crystals. Technically, fog is defined as condensation in the air that restricts visibility to a bit more than 0.5 mile (1.0 kilometer). If water has condensed in the low-lying air, yet visibility is greater than 0.5 mile (1.0 kilometer), the condition is defined as mist.

Fog production differs from cloud production in one significant way: While a cloud is usually formed as air rises and cools to the dew point (the temperature at which a parcel of air can no longer hold water in its vapor state), fog is formed within surface-level air. This happens in one of two ways. The air may be cooled to the dew point by contact with a cold surface. Or it is brought to the saturation point, the point at which it contains the maximum amount of water, when water vapor is added to the air by evaporation, the process by which water changes from a liquid to a gas.

On clear nights, surfaces that are open to the sky often cool faster than the air above them. When condensation takes place directly on these cooler surfaces due to contact between warmer air and the cooler surface, it is called dew or frost. Fog, on the other hand, is condensation in the air due to the air cooling below the dew point.

Radiation fog

Radiation fog is the fog that forms after sunset on clear summer nights when the air is almost, but not quite, still. The ground loses heat to space by radiation and cools to below air temperature. The cooler ground then conducts heat away from the layer of air just above the ground. This layer of air is cooled to below the dew point and fog forms. The fog layer can be anywhere from a few feet (around 1 meter) to a thousand feet (around 300 meters) thick. The layer of air above the fog layer is warmer, so the fog layer is stable, at least until the sun rises and warms the ground again.

A light wind of less than 5 miles (8 kilometers) per hour promotes the circulation of the lowest layer of air, bringing all of it in contact with the cold surface. In this way the entire layer of air loses heat and cools to the dew point. If the winds are completely calm, only the very bottom of the surface air layer will come in contact with the ground. In that case, rather than producing radiation fog, moisture will either condense in a very shallow (less than 2 feet, or .6 meter) layer of air, forming ground fog, or it will condense only on the ground and form dew or frost.

Winds also hinder the formation of radiation fog. Winds stronger than 5 miles (8 kilometers) per hour will cause mixing of surface air with warmer, drier layers of air above it. This either prevents the surface air from cooling to the dew point or, if the air does reach the dew point and fog develops, the wind rapidly disperses the fog.

Valley fog is radiation fog that forms in valleys. It may develop into a layer over 1,500 feet (450 meters) thick. Radiation fog forms in low-lying areas because of two factors. In the first place, cold air (in this case, air that has undergone radiational cooling) is heavier than warm air and flows downward into the valley. In addition, valleys often contain rivers which increase the amount of moisture in low-lying air.

In most cases, radiation fog evaporates within a few hours after sunrise, after the air and ground have warmed up. It typically reveals clear skies, since the absence of clouds was required for the formation of fog. Sometimes, however, the fog is so thick that it effectively blocks the sunlight from reaching the ground, and the ground remains cold. In such cases, which usually occur in winter, the fog may persist all day.

Radiation fog is most likely to form during long nights, when the surface air has more time to cool to its dew point. Thus, this type of fog is seen most often during winter, late fall, and early spring. Visibility in thick radiation fog may be reduced to as little as 10 feet (3 meters).

Advection fog

In contrast to radiation fog, in which a layer of air is cooled as the ground loses heat, advection fog is formed when there is horizontal movement of a warm, moist layer of air over a cold surface. The newly arrived air loses heat to the cold surface below by conduction, the process by which energy is transferred through contact between substances. Conduction lowers the air temperature. Once the air cools to the dew point, fog is formed. While it bears a resemblance to radiation fog, advection fog moves with the wind-blown warm air mass whereas radiation fog is stationary. In addition, while radiation fog usually forms at night, advection fog may form at any time.

In the spring, advection fog results when a mild breeze passes over ground that has not yet thawed. Throughout the summer, advection fog is produced by warm, moist air blowing across a lake that remains cold, such as one of the Great Lakes.

Another kind of advection fog happens in the winter, when warm air is carried northward. When this warm air encounters cold ground, it cools to the dew point and advection fog results. For instance, warm air from over the Gulf of Mexico may travel as far as the central United States before encountering a sufficiently cold surface for advection fog to form.

Experiment: Make fog in a jar

This simple experiment allows you to create fog conditions inside a glass jar. All you need for this experiment are:

a glass jar
strainer
hot water
ice cubes

First, fill up the jar with hot water and let it sit for about a minute. Then pour most of the water out, leaving about a finger's width at the bottom. Put the strainer over the top of the jar and place ice cubes on the strainer. In a little while, you will see fog in the jar. This is caused by the cold air from the ice cubes hitting the warm, moist air in the jar causing the water to condense into fog.

Advection fog is the thickest and most persistent type of fog. It sometimes reduces visibility to 650 feet (200 meters), the point at which airports are forced to close. Advection fog can form in layers that range anywhere from very shallow to 1,000 feet (300 that meters) deep.

Sea fog

Sea fog is a special form of advection fog that only occurs at sea and in coastal areas. It is produced by the interaction of two adjacent ocean currents, the major routes through which ocean water travels, of different temperatures. A prime example of sea fog occurs in the Atlantic Ocean off the coast of Newfoundland, in Canada. There the warm Gulf Stream flows northward, parallel to the cold, southward-flowing Labrador Current. When the air that is warmed by the Gulf Stream travels over the iceberg-filled Labrador Current, it is cooled and moisture condenses, forming fog. For two out of every three summer days, the coastal region off Newfoundland is shrouded by the famous "Grand Banks fog."

This process is also responsible for the thick fog that is common in parts of the British Isles. In that case, the warm air comes from above the Gulf Stream and blows across cooler British coastal waters.

WORDS TO KNOW

advection:: the horizontal movement of a mass such as air or an ocean current.
condensation:: the process by which water changes from a gas to a liquid.
conduction:: the transfer of heat from one molecule to another.
dew point:: the temperature at which a given parcel of air reaches its saturation point and can no longer hold water in the vapor state.
evaporation:: the process by which water changes from a liquid to a gas.
radiational cooling:: the loss of heat from the ground upward into the atmosphere.
relative humidity:: a measure of humidity as a percentage of the total moisture a given volume of air, at a particular temparature, can hold.
saturation point:: the point at which a given volume of air contains the maximum possible amount of water vapor.
supercooled water:: water that remains in a liquid state below the freezing point.

Sea fog is also well-known to the residents of San Francisco, California, and other West Coast communities. It forms in the summer when warm air from over the Pacific Ocean is carried shoreward by westerly winds. As the air moves over the colder, coastal surface water, the air cools and fog forms. The fog is then blown inland by a sea breeze. This explains the fog that can often be seen rolling in past the Golden Gate Bridge on a breezy summer day. Conversely, if a land breeze is blowing out to sea, it will take the fog out with it.

As sea fog moves farther inland, it crosses over warmer ground and dissipates. The bottom of the fog layer evaporates first, revealing a low layer of gray clouds. As the surface temperature increases, ever-higher layers of the fog evaporate until it has completely disappeared. Fog also dissipates at a given location as the air becomes progressively warmer throughout the day. For this reason, coastal areas often have fog in the morning and at night, with clear skies in the afternoon.

Sea fog is extremely important to the vegetation of northern coastal California. Throughout the dry summers, the fog condenses on objects in its path. It provides moisture to the redwood trees; the redwoods absorb moisture through their needles. Some water drips off the trees and onto the ground, where it is absorbed by the trees' shallow roots.

Evaporation fog

Evaporation fog is formed in a completely different manner than the other types of fog discussed so far. Rather than being formed when warm air cools to the dew point, evaporation fog is formed when water evaporates into cool air and brings that cool air to its saturation point. The water may evaporate from a wide range of sources, such as a puddle, a lake, a river, an ocean, or even exhaled breath.

Weather report: Smothered by sea fog

The town of Argentia, situated on the Avalon Peninsula of southeastern Newfoundland, averages 206 days per year of thick fog. This makes it the foggiest place in Canada and among the foggiest places in the world. Argentia's fog is sea fog, generated by the passage of warm air over cold North Atlantic ocean water.

A similar process produces the fog at Cape Disappointment, Washington, one of the foggiest places in the United States. Cape Disappointment is situated at the point where the Columbia River flows into the Pacific Ocean. It is blanketed by sea fog 29 percent of the time, or about 106 days each year.

The reason why water molecules evaporate into unsaturated air (air which has less than 100 percent relative humidity) has to do with equilibrium. Water molecules are continually entering and leaving the surface of a body of water. If the relative humidity is less than 100 percent, water molecules will leave the surface faster than they enter. Thus the number of water molecules in the air will increase. At 100 percent relative humidity, the rates of entering and leaving are equal. The water vapor in the air is at equilibrium with the liquid water.

There are two main types of evaporation fog: steam fog, which occurs over a body of water, and frontal fog, which accompanies the passage of a cold or a warm front (the line behind which a cold or a warm air mass is advancing).

Steam fog

The production of steam fog requires cold air to travel over a warmer body of water. Steam fog can often be seen over inland lakes and rivers in the fall, when the air is cool but the water is still warm. Water evaporates from the lake or river, saturates the cold air, and condenses into a shallow (less than 2 feet, or .6 meter) layer of steam fog.

Steam fog occurs all year long over the thermal ponds (hot springs) in Yellowstone National Park. The reason for this pattern is that the temperature of the water is always greater than that of the air. In the winter, it is common to see steam fog over large bodies of water, such as the Great Lakes, which can take several weeks to cool to air temperature. The fog there sometimes forms in dense, rising, swirling columns known as steam devils, because of their similarity to dust devils in the American Southwest.

In arctic regions, where the air is always extremely cold and dry, steam fog occurs over unfrozen waters on a large scale. Commonly called arctic sea smoke, this form of steam fog is patchy and wispy in appearance.

Frontal fog

Frontal fog, sometimes called precipitation fog, is a type of evaporation fog that forms when a layer of warm air rises over a shallow layer of colder surface air. The uplift of warm air forms clouds that often yield precipitation, or falling water particles. The precipitation, usually rain, is warmer than the cold air beneath it and evaporates into the air. This raises the cold air to its saturation point and fog is produced.

This type of fog is called frontal fog because the conditions that give rise to it occur just before the arrival of a warm front or just after the passage of a cold front.

Did you know? Driving in fog

Most people, when driving along poorly lit roads at night or in other conditions of reduced visibility, instinctively turn on their high-beam headlights. However, this is not a wise strategy for driving in fog. Light is scattered by fog droplets and is reflected into the driver's eyes. High-beam lights merely illuminate the fog directly ahead of the vehicle and make it difficult to see anything beyond. Close to the ground, the fog is usually less dense. Low-beam headlights point lower toward the ground than do high-beam headlights and are thus more appropriate for foggy, nighttime driving. Fog lamps are often installed just above or below the front bumper. This low mounting position reduces the amount of light scattered back to the driver's eyes.

Upslope fog

Upslope fog is formed by the slow passage of a moist parcel of air up the side of a hill or mountain. As the air rises, it expands adiabatically; that is, with no heat transfer. When air expands, it cools. Once the air cools to the dew point, condensation occurs. Upslope fog generally covers a large area, sometimes hundreds of miles, and may persist for days.

Upslope fog is common in all mountain ranges. It is prevalent on the eastern slopes of the Rocky Mountains in the winter and spring. It occurs when cold air, following in the wake of a cold front, drifts westward from the Great Plains and rides up the gentle slopes. A similar phenomenon occurs in eastern Australia. There, moist air from the Tasman Sea is blown to shore and travels along the eastern slopes of the Great Dividing Range to a level where fog forms.

Freezing fog

Freezing fog is the term used to describe fog that develops in air that has a temperature below freezing. In most cases, freezing fog is comprised of supercooled water droplets. Supercooled water is water that exists in the liquid state below its freezing point. Freezing fog freezes onto any solid surface it comes in contact with, such as trees, telephone poles, cars, and roadways.

When freezing fog encounters a surface, it deposits a layer of frost called rime. Rime is not crystalline like true frost, which is called hoar frost. Rather, it is ice that contains trapped air, giving it a whitish appearance. Rime often persists long after the fog has cleared. It creates extremely hazardous driving conditions and is nearly impossible to walk on without slipping.

In very cold air, at temperatures below ?22°F (?30°C), water droplets in freezing fog will freeze into ice crystals, becoming ice fog. One way in which ice fog is formed is from the water vapor released by the breathing of a herd of caribou or reindeer. Another way it is formed is from the passage of moist, marine air over an icy surface. Ice fog is the least dense type of fog. It glitters in the sun, earning it the nickname diamond dust.

Fog stratus

Fog stratus, also called high fog, is a layer of fog that does not reach all the way to the ground. Rather, it hovers a short distance above ground. Fog stratus represents the intermediate stage through which a layer of fog (most commonly valley fog) passes as it dissipates.

Typically, fog forms at night. When the sun rises the next morning it begins to warm the ground. This, in turn, warms the lowest layer of air, from which the fog evaporates.

Sometimes the process of fog evaporation proceeds smoothly, from the bottom to the top of the layer, in a relatively short time. Other times, however, the fog is so thick that little sunlight penetrates it, and the air remains cold enough that fog stratus persists. Fog stratus also requires calm conditions, since winds promote mixing of the air layers and speed up evaporation.

Fog stratus usually clears by late morning. Occasionally, however, when clouds have moved in and inhibit the sunlight's warming of the ground, fog stratus may persist all day. A thick enough layer of fog stratus may even bring drizzle or snow flurries.

[See AlsoClouds; Weather: An Introduction ]