Chapter 18, Weather and Obstructions to Visibility 18-7
Figure 18-8. Frontal Fog Formation
18.1.1.1.5 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 recondenses and
rises with the air that is being warmed from bel ow. Because the air is destabilized, fog appears as rising
filaments or streamers that resemble steam. This phenomenon is called steam fog (see Figure 18-9). 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 unsa turated 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.
Chapter 18, Weather and Obstructions to Visibility 18-8
Figure 18-9. Steam Fog Formation
18.1.1.1.6 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.
Surfaces that these droplets may freeze on include tree branches, stairs and rails, sidewalks, roads , and
vehicles (see Figure 18-10). 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.
Figure 18-10. Freezing Fog
Chapter 18, Weather and Obstructions to Visibility 18-9
18.1.2 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.
18.1.3 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 relativ e
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.
18.1.4 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 –100 mi (40–160 km) or more, 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 impr ove
little, if any, during the day.
18.1.5 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.
For more information on precipitation, see Chapter 14, Precipitation.
Chapter 18, Weather and Obstructions to Visibility 18-10
18.1.6 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.
18.1.7 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 3,000–6,000 ft (about 1 km); however, they can frequently
extend up to 15,000 ft (4,600 m).
Strong cooling after sunset quickly stabilizes t he 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
(see Figure 24-3) 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.
18.1.8 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.
18.1.8.1 Haboob
A haboob (see Figure 18-11) is a dust storm or sandstorm that forms as cold downdrafts from a thunderstorm
turbulently lift dust and sand into the air. While haboobs are often short -lived, they can be quite intense.
The dust wall may extend horizonta lly for more than 60 mi (100 km) and rise vertically to the base of the
thunderstorm. Spinning whirlwinds of dust frequently form along the turbulent cold air outflow, giving rise
to huge dust/sand whirls.
Chapter 18, Weather and Obstructions to Visibility 18-11
Figure 18-11. Haboob
18.1.9 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 seve ral 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 e ver
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/m 3) 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 it 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,
Chapter 18, Weather and Obstructions to Visibility 18-12
allowing the fan blades to rotate, allowing for an engine relight as the air moves through the engine 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 c an 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.
18.2 Low Ceiling and Mountain Obscuration
18.2.1 Low Ceiling
Stratus is the most frequent cloud associated with low ceilings. Stratus clouds, like fog, are composed of
extremely small water droplets or ice crystals suspended in air. An observer on a mountain in a stratus layer
would call it fog. Stratus and fog frequently exist together. In many cases, there is no real line of distinction
between the fog and stratus; rather, one gradually merges into the other. Flight visibility may approach zero
in stratus clouds. Stratus over land tends to be lowest during night and early morning, lifting or dissipating
due to solar heating by late morning or early afternoon. Low stratus clouds often occur when moist air
mixes with a colder air mass or in any situation where the temperature-dewpoint spread is small.
18.2.2 Mountain Obscuration
A mountain obscuration is a condition in which mountains or mountain ridges are obscured due to clouds,
precipitation, smoke, or other obscurations.
Flight can be especially hazardous over mountain routes when the mountains are obscured. The large
elevation variations around mountains can cause surface weather observations to mislead. For example, a
weather station located in a valley could report a VFR cloud ceiling, while a hiker in the mountains sees fog.