What causes obstructions to visibility in flight?

What can cause an obstruction to visibility in flight? Weather and obstructions to visibility include: fog, mist, haze, smoke, precipitation, blowing snow, dust storm, sandstorm, and volcanic ash. We will discuss each one in detail.

1. Fog

Fog is a visible aggregate of minute water droplets that are based at the Earth’s surface, and it reduces horizontal visibility to less than 5/8 sm (1 km); unlike drizzle, it does not fall to the ground. Fog differs from a cloud only in that its base must be at the Earth’s surface, while clouds are above the surface. Cloud droplets can remain liquid even when the air temperature is below freezing. Fog composed of water droplets and occurring with temperatures at or below freezing is termed freezing fog. When fog is composed of ice crystals, it is termed ice fog. If fog is so shallow that it is not an obstruction to vision at a height of 6 ft (2 m) above the surface, it is called shallow (ground) fog. Fog forms when the temperature and dewpoint of the air become identical (or nearly so). This may occur through cooling of the air to its dewpoint (producing radiation fog, advection fog, or upslope fog), or by adding moisture and thereby elevating the dewpoint (producing frontal fog or steam fog). Fog seldom forms when the temperature-dewpoint spread is greater than 2 °C (4 °F).

Different Types of Fog

Fog types are named according to their formation mechanism.

Radiation Fog

Radiation fog is a common type of fog, produced over a land area when radiational cooling reduces the air temperature to or below its dewpoint. Thus, radiation fog is generally a nighttime occurrence and often does not dissipate until after sunrise.

Radiation fog is relatively shallow fog. It may be dense enough to hide the entire sky, or it may conceal only part of the sky. Ground fog is a form of radiation fog that is confined to near ground level.

Factors favoring the formation of radiation fog are:

1) a shallow surface layer of relatively moist air beneath a dry layer

2) clear skies

3) light surface winds

Terrestrial radiation cools the ground; in turn, the ground cools the air in contact with it. When the air is cooled to its dewpoint, fog forms. When rain soaks the ground, followed by clearing skies, radiation fog is not uncommon the following morning.

Radiation fog is restricted to land because water surfaces cool little from nighttime radiation. It is shallow when wind is calm. Winds up to about 5 kt mix the air slightly and tend to deepen the fog by spreading the cooling through a deeper layer. Stronger winds disperse the fog or mix the air through a still deeper layer, with stratus clouds forming at the top of the mixing layer.

Ground fog usually burns off rather rapidly after sunrise. Other radiation fog generally clears before noon unless clouds move in over the fog. It can be difficult at times to differentiate between this and other types of fog, especially since nighttime cooling intensifies all fogs.

Mountain/Valley Fog

Mountain tops jutting skyward out of the fog can be a beautiful sight, but it can also be dangerous. There are two ingredients that add to the formation of mountain/valley fog in areas of variable terrain. First, overnight, the ground cools as the heat that was gathered from the Sun’s rays during the day is released back into the air near the ground level. The denser, cooler air on mountaintops sinks into valleys and collects there. Second, over the course of the night, the valley begins to fill from the bottom with cold layers of air. This phenomenon is known as “cold air drainage.” This cooler air lowers the surrounding air temperatures closer to the dewpoint and subsequently saturation. If there is sufficient moisture in the air, fog will begin to form in these valleys as the night progresses. This type of fog is most commonly observed in the autumn and spring months, and is densest around sunrise when surface temperatures are often lowest.

Advection Fog

Advection fog forms when moist air moves over a colder surface and the subsequent cooling of that air to below its dewpoint. It is most common along coastal areas, but often moves deep into continental areas. At sea, it is called sea fog. Advection fog deepens as wind speed increases up to about 15 kt. Wind much stronger than 15 kt lifts the fog into a layer of low stratus or stratocumulus clouds. The west coast of the United States is quite vulnerable to advection fog. This fog frequently forms offshore as a result of cold water and then is carried inland by the wind. It can remain over the water for weeks, advancing over the land during night and retreating back over the water the next morning. During the winter, advection fog over the central and eastern United States results when moist air from the Gulf of Mexico spreads northward over cold ground. The fog may extend as far north as the Great Lakes. Water areas in northern latitudes have frequent dense sea fog in summer as a result of warm, moist, tropical air flowing northward over colder Arctic waters. A pilot will notice little difference between flying over advection fog and over radiation fog. Also, advection fog is usually more extensive and much more persistent than radiation fog. Advection fog can move in rapidly regardless of the time of day or night.

Upslope Fog

Upslope fog forms as a result of moist, stable air being adiabatically cooled to or below its dewpoint as it moves up sloping terrain (see Figure 18-7). Winds speeds of 5 to 15 kt are most favorable since stronger winds tend to lift the fog into a layer of low stratus clouds. Unlike radiation fog, it can form under cloudy skies. Upslope fog is common along the eastern slopes of the Rocky Mountains, and somewhat less frequent east of the Appalachian Mountains. Upslope fog is often quite dense and extends to high altitudes.

Frontal Fog

When warm, moist air is lifted over a front, clouds and precipitation may form. If the cold air below is near its dewpoint, evaporation (or sublimation) from the precipitation may saturate the cold air and form fog (see Figure 18-8). A fog formed in this manner is called frontal (or precipitation-induced) fog. The result is a more or less continuous zone of condensed water droplets reaching from the ground up through the clouds. Frontal fog can become quite dense and continue for an extended period of time. This fog may extend over large areas, completely suspending air operations. It is most commonly associated with warm fronts, but can occur with other fronts as well

Steam Fog

When very cold air moves across relatively warm water, enough moisture may evaporate from the water surface to produce saturation. As the rising water vapor meets the cold air, it immediately re-condenses and rises with the air that is being warmed from below. Because the air is destabilized, fog appears as rising filaments or streamers that resemble steam. This phenomenon is called steam fog. It is commonly observed over lakes and streams on cold autumn mornings, and over the ocean during the winter when cold air masses move off the continents and ice shelves. Steam fog is often very shallow, for as the steam rises, it reevaporates in the unsaturated air above. However, it can be dense and extend over large areas. Steam fog is associated with a shallow layer of unstable air; thus, pilots can expect convective turbulence flying through it. On occasion, columns of condensed vapor rise from the fog layer, forming whirling steam devils, which appear similar to the dust devils on land.

Freezing Fog

Freezing fog occurs when the temperature falls to 32 °F (0 °C) or below. Tiny, supercooled liquid water droplets in fog can freeze instantly on exposed surfaces when surface temperatures are at or below freezing. Some surfaces that these droplets may freeze on include tree branches, stairs and rails, sidewalks, roads, and vehicles. For those flying, or even taxiing, a layer of ice can form on the aircraft, making flight very dangerous unless the aircraft is treated or has effective deicing equipment.

2. Mist

Mist is a visible aggregate of minute water droplets or ice crystals suspended in the atmosphere that reduces visibility to less than 7 sm (11 km), but greater than, or equal to, 5/8 sm (1 km). Mist forms a thin grayish veil that covers the landscape. It is similar to fog, but does not obstruct visibility to the same extent. Mist may be considered an intermediate between fog and haze. It has lower relative humidity (95 to 99 percent) than fog and does not obstruct visibility to the same extent. However, there is no distinct line between any of these categories.

3. Haze

Haze is a suspension in the air of extremely small particles invisible to the naked eye and sufficiently numerous to give the air an opalescent appearance. It reduces visibility by scattering the shorter wavelengths of light. Haze produces a bluish color when viewed against a dark background and a yellowish veil when viewed against a light background. Haze may be distinguished by this same effect from mist, which yields only a gray obscuration. Certain haze particles increase in size with increasing relative humidity, drastically decreasing visibility. While visibility is a measure of how far one can see, including the ability to see the textures and colors therein, haze is the inability to view a similar scene with equal clarity. Haze occurs in stable air and is usually only a few thousand feet thick, but may extend upwards to 15,000 ft (4,600 m). A haze layer has a definite ceiling above which in-flight (air-to-air) visibility is unrestricted. At or below this level, the slant range (air-to-ground) visibility is poor. Visibility in haze varies greatly, depending on whether the pilot is facing into or away from the Sun.

4. Smoke

Smoke is a suspension in the air of small particles produced by combustion due to fires, industrial burning, or other sources. It may transition to haze when the particles travel 25 to 100 mi (40 to 160 km) or more, and the larger particles have settled and others become widely scattered through the atmosphere. Not only can smoke reduce visibility to zero, but many of its compounds are highly toxic and/or irritating. The most dangerous is carbon monoxide, which can lead to carbon monoxide poisoning, sometimes with supporting effects of hydrogen cyanide and phosgene. When skies are clear above a surface-based layer of haze or smoke, visibility generally improves during the day. Heating during the day may cause convective mixing, spreading the smoke or haze to a higher altitude, and decreasing the concentration near the surface. However, the improvement is slower than the clearing of fog. Fog evaporates, but haze and smoke must be dispersed by the movement of air. A thick layer of clouds above haze or smoke may block sunlight, preventing dissipation. Visibility will improve little, if any, during the day.

5. Precipitation

Precipitation is any of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. Snow, rain, and drizzle are types of precipitation. Heavy snow may reduce visibility to zero. Rain seldom reduces surface visibility below 1 mi except in brief, heavy showers. Drizzle usually restricts visibility to a greater degree than rain. It forms in stable air, falls from stratiform clouds, and is typically accompanied by fog. When drizzle changes to light rain, visibility usually improves because the droplet size increases, meaning there are fewer droplets per unit area.

6. Blowing Snow

Blowing snow is snow lifted from the surface of the Earth by the wind to a height of 6 ft (2 m) or more above the ground, and blown about in such quantities that the reported horizontal visibility is reduced to less than 7 sm (11 km). Light, dry powder snow is most prone to being blown by the wind. When strong winds keep the snow suspended up to 50 ft (15 m) or so, obscuring the sky, and reducing surface visibility to near zero, it is called a whiteout. Visibility improves rapidly when the wind subsides.

7. Dust Storm

A dust storm is a severe weather condition characterized by strong winds and dust-filled air over an extensive area. Dust storms originate over regions when fine-grained soils, rich in clay and silt, are exposed to strong winds and lofted airborne. Fine-grained soils are commonly found in dry lake beds (called playas), river flood plains, ocean sediments, and glacial deposits. Most of the dust originates from a number of discrete point sources. Intense dust storms reduce visibility to near zero in and near source regions, with visibility improving away from the source. A dust storm is favored with extreme daytime heating of barren ground and a turbulent, unstable air mass that allows the dust to be lofted. Surface winds need to be 15 kt or greater to mobilize dust. A speed of 35 kt may be needed over a desert surface that is covered with closely packed rock fragments called desert pavement. The average height of a dust storm is 3,000 to 6,000 ft (about 1 km); however, they can frequently extend up to 15,000 ft (4,600 m). Strong cooling after sunset quickly stabilizes the lowest atmosphere, forming a temperature inversion and settling the dust. Without turbulence, dust generally settles at a rate of 1,000 ft (300 m) per hour. It can take many hours (or days) for the dust to completely settle. However, precipitation will very effectively remove dust from the atmosphere. Aircraft operation in a dust storm can be very hazardous. Visibility can drop to zero in a matter of seconds. Dust can also clog the air intake of engines, damage electro-optical systems, and cause problems with human health. From a pilot’s point of view, it is important to recognize that slant range (air-to-ground) visibility in dust is generally reduced compared to reported surface (horizontal) visibility. Therefore, it may not be possible to pick out an airfield from above, even when reported surface visibility is 3 mi or more.

8. Sandstorm

A sandstorm is particles of sand carried aloft by a strong wind. The sand particles are mostly confined to the lowest 10 ft (3.5 m), and rarely rise more than 50 ft (15 m) above the ground. Sandstorms are similar to dust storms, but occur on a localized scale. This is because sand particles are larger and heavier than dust particles. Sandstorms are best developed in desert regions where there is loose sand, often in dunes, without much admixture of dust.

9. Volcanic Ash

Volcanic ash is made up of fine particles of rock powder that originate from a volcano and that may remain suspended in the atmosphere for long periods. Severe volcanic eruptions that send ash into the upper atmosphere occur somewhere around the world several times per year. The explosive eruption from the volcano in Tonga, South Pacific Ocean in January 2022 sent an ash cloud into the mesosphere. Weather satellites estimated the ash cloud reached an altitude of 190,000 ft, which was the highest ash cloud ever observed. Visible ash is what an observer or aircrew member sees with their eyes. The lower limit of visible ash ranges from an ash concentration of approximately 0.01 milligrams per cubic meter (mg/m3 ) to 10 mg/m3 , depending on many factors such as time of day, sky background, and position of the sun to the observer (pilot), as well as the angle from which the ash cloud is viewed (e.g., viewed from the side). Discernible ash is what a satellite or other remote sensing instrument detects. Forecasters at the world’s nine VAACs (see Section 26.5.1) use discernible ash from satellites to define the observed area of the ash cloud in the VAA product (see Section 26.5). The lower limit of discernible ash from satellites is approximately 0.1 to 0.2 mg/m3, depending on the satellite and other factors. The ash cloud may not be visible, especially at night or in instrument meteorological conditions (IMC). Even if visible, it is difficult to distinguish visually between an ash cloud and an ordinary cloud. Radar may be able to detect heavy concentrations of airborne ash near the volcano, but is not able to detect fine airborne ash, and is not likely to detect the ash cloud as it spreads downwind of the volcano. Flying into a volcanic ash cloud can be hazardous. Volcanic ash is composed of silica (glass). When ash is ingested into a jet engine, it melts to produce a soft, sticky molten product that adheres to the compressor turbine blades and fuel injectors/igniters. With no air going into the engine, the fuel cannot ignite, the engine comes to a slow spinning stop by spooling down, and a flameout occurs. As the aircraft exits the ash cloud and enters colder temperatures, the cooled, hardened silicas on the turbine blades become dislodged, again. Piston-powered aircraft are less likely to lose power, but severe engine damage is likely after an encounter with a volcanic ash cloud that is only a few hours old. Volcanic ash also causes abrasive damage to aircraft flying through it at hundreds of miles per hour. Particles impacting the windshield can sandblast the surface into a frosted finish that obscures the pilot’s view. The sandblasting can also remove paint and pit metal on the nose and leading edges of wings and navigation equipment. Ash contaminates aircraft ventilation, hydraulic, instrument, electronic, and air data systems. Ash covering a runway can cover its markings and cause aircraft to lose traction during takeoffs and landings.

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