16-4 (see Figure 16-4). 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.
Figure 16-4. Frontal Fog Formation
16.1.1.1.5 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 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.
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16.1.2 Mist. Mist is a visible aggregate of minute water droplets or ice crystals suspended in the atmosphere that reduces visibility to less than 7 statute miles (11 kilometers), but greater than, or equal to, 5/8 statute mile (1 kilometer). 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 (95-99 percent) than fog and does not obstruct visibility to the same extent.
However, there is no distinct line between any of these categories.
16.1.3 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 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 feet (4,600 meters). 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.
16.1.4 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 miles (40-160 kilometers) or more, and the larger particles have settled and others become widely scattered through the atmosphere.
Not only can smoke reduce visibility to zero, 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.
16.1.5 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 mile except in brief, heavy showers.
16-6 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.
16.1.6 Blowing Snow. Blowing snow is snow lifted from the surface of the Earth by the wind to a height of 6 feet (2 meters) or more above the ground, and blown about in such quantities that the reported horizontal visibility is reduced to less than 7 statute miles (11 kilometers). Light, dry powder snow is most prone to being blown by the wind.
Strong winds keep the snow suspended up to 50 feet (15 meters) or so, obscuring the sky, and reducing surface visibility to near zero (called a whiteout). Visibility improves rapidly when the wind subsides.
16.1.7 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 knots or greater to mobilize dust. A speed of 35 knots may be required 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 feet (~1 kilometer). However, they can frequently extend up to 15,000 feet (4,600 meters).
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 feet (300 meters) per hour. It can take many hours (or days) for the dust to 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 three miles or more.
16.1.8 Sandstorm. A sandstorm is particles of sand carried aloft by a strong wind. The sand particles are mostly confined to the lowest 10 feet (3.5 meters), and rarely rise more than 50 feet (15 meters) 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
16-7 particles. Sandstorms are best developed in desert regions where there is loose sand, often in dunes, without much admixture of dust.
16.1.8.1 Haboob. A haboob (see Figure 16-5) 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 horizontally for more than 60 miles (100 kilometers) 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.
Figure 16-5. Haboob
16.1.9 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 several times per year. The ash plume 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. Ash cannot be detected by air traffic control (ATC) radar. However, it may be detected by weather radar, particularly during the early stages of a volcanic eruption when the ash is more concentrated.
Flying into a volcanic ash cloud can be exceedingly dangerous. 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
16-8 spinning stop by spooling down, and a flameout occurs. As the aircraft exits the ash cloud and into colder temperatures, the cooled, hardened silicas on the turbine blades become dislodged, allowing the fan blades to start rotating, and allow for an engine relight as the air starts moving through the engine again. Piston-powered aircraft are less likely to lose power, but severe damage is almost certain to ensue 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.
16.2 Low Ceiling and Mountain Obscuration.
16.2.1 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 temperature-dewpoint spread is small.
Not all ceilings are equally hazardous to a pilot. An indefinite ceiling is more hazardous than an equal ceiling caused by a layer aloft. Once a pilot descends below a ceiling caused by a layer aloft, the pilot can see both the ground below and the runway ahead.
However, an indefinite ceiling restricts the pilot’s slant range (air-to-ground) visibility.
Thus, the pilot may not see the runway ahead after he descends below the indefinite ceiling (see Figure 16-6).
16-9 Figure 16-6. Layer Aloft Ceiling Versus Indefinite Ceiling
16.2.2 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 visual flight rules (VFR) cloud ceiling, while a hiker in the mountains sees fog.
17-1 CHAPTER 17. TURBULENCE
17.1 Introduction. Aircraft turbulence is irregular motion of an aircraft in flight, especially when characterized by rapid up-and-down motion caused by a rapid variation of atmospheric wind velocities. Turbulence varies from annoying bumpiness to severe jolts which cause structural damage to aircraft and/or injury to its passengers. Turbulence intensities and their associated aircraft reactions are described in the Aeronautical Information Manual (AIM).
17.2 Causes of Turbulence. Turbulence is caused by convective currents (called convective turbulence), obstructions in the wind flow (called mechanical turbulence), and wind shear.
17.2.1 Convective Turbulence. Convective turbulence is turbulent vertical motions that result from convective currents and the subsequent rising and sinking of air. For every rising current, there is a compensating downward current. The downward currents frequently occur over broader areas than do the upward currents; therefore, they have a slower vertical speed than do the rising currents.
Convective currents are most active on warm summer afternoons when winds are light.
Heated air at the surface creates a shallow, absolutely unstable layer within which bubbles of warm air rise upward. Convection increases in strength and to greater heights as surface heating increases. Barren surfaces such as sandy or rocky wastelands and plowed fields become hotter than open water or ground covered by vegetation. Thus, air at and near the surface heats unevenly. Because of uneven heating, the strength of convective currents can vary considerably within short distances.
As air moves upward, it cools by expansion. A convective current continues upward until it reaches a level where its temperature cools to the same as that of the surrounding air.
If it cools to saturation, a cumuliform cloud forms.
Billowy cumuliform clouds, usually seen over land during sunny afternoons, are signposts in the sky indicating convective turbulence. The cloud top usually marks the approximate upper limit of the convective current. A pilot can expect to encounter turbulence beneath or in the clouds, while above the clouds, air generally is smooth (see Figure 17-1). When convection extends to great heights, it develops larger towering cumulus clouds and cumulonimbus with anvil-like tops. The cumulonimbus gives visual warning of violent convective turbulence.
17-2 Figure 17-1. Convective Turbulence
When the air is too dry for cumuliform clouds to form, convective currents can still be active. This is called dry convection, or thermals (see Figure 17-2). A pilot has little or no indication of their presence until encountering the turbulence.
Figure 17-2. Thermals
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17.2.2 Mechanical Turbulence. Mechanical turbulence is turbulence caused by obstructions to the wind flow, such as trees, buildings, mountains, and so on. Obstructions to the wind flow disrupt smooth wind flow into a complex snarl of eddies (see Figure 17-3).
An aircraft flying through these eddies experiences mechanical turbulence.
Figure 17-3. Mechanical Turbulence
The intensity of mechanical turbulence depends on wind speed and roughness of the obstructions. The higher the speed and/or the rougher the surface, the greater the turbulence.
The wind carries the turbulent eddies downstream. How far depends on wind speed and stability of the air. Unstable air allows larger eddies to form than those that form in stable air; but the instability breaks up the eddies quickly, while in stable air they dissipate slowly.
17.2.2.1 Mountain Waves. A mountain wave (see Figure 17-4) is an atmospheric wave disturbance formed when stable air flow passes over a mountain or mountain ridge. Mountain waves are a form of mechanical turbulence which develop above and downwind of mountains. The waves remain nearly stationary while the wind blows rapidly through them. The waves may extend 600 miles (1,000 kilometers) or more downwind from the mountain range.
Mountain waves frequently produce severe to extreme turbulence. Location and intensity varies with wave characteristics. Incredibly, vertically propagating mountain waves have been documented up to 200,000 feet (60,000 meters) and higher.
17-4 Figure 17-4. Mountain Waves
Mountain waves often produce violent downdrafts on the immediate leeward side of the mountain barrier. Sometimes the downward speed exceeds the maximum climb rate of an aircraft and may drive the aircraft into the mountainside.
A mountain wave cloud is a cloud that forms in the rising branches of mountain waves and occupies the crests of the waves. The most distinctive are the sharp-edged, lens-, or almond-shaped lenticular clouds. When sufficient moisture is present in the upstream flow, mountain waves produce interesting cloud formations (see Figure 17-5) including: cap clouds, cirrocumulus standing lenticular (CCSL), Altocumulus Standing Lenticular (ACSL), and rotor clouds. These clouds provide visual proof that mountain waves exist. However, these clouds may be absent if the air is too dry.
For additional information on hazardous mountain waves, refer to the current edition of AC 00-57, Hazardous Mountain Winds and their Visual Indicators.
17-5 Figure 17-5. Mountain Wave Clouds
17.2.3 Wind Shear Turbulence. Wind shear is the rate of change in wind direction and/or speed per unit distance. Wind shear generates turbulence between two wind currents of different directions and/or speeds (see Figure 17-6). Wind shear may be associated with either a wind shift or a wind speed gradient at any level in the atmosphere.
Figure 17-6. Wind Shear Turbulence
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17.2.3.1 Temperature Inversion. A temperature inversion is a layer of the atmosphere in which temperature increases with altitude. Inversions commonly occur within the lowest few thousand feet above ground due to nighttime radiational cooling, along frontal zones, and when cold air is trapped in a valley. Strong wind shears often occur across temperature inversion layers, which can generate turbulence (see Figure 17-7).
Figure 17-7. Wind Shear Turbulence Associated with a Temperature Inversion
17.2.3.2 Clear Air Turbulence (CAT). Clear Air Turbulence (CAT) is a higher altitude (~20,000 to 50,000 feet) turbulence phenomenon occurring in cloud-free regions associated with wind shear, particularly between the core of a jet stream and the surrounding air. It can often affect an aircraft without warning. CAT frequency and intensity are maximized during winter when jet streams are strongest.
For additional information on CAT, refer to the current edition of AC 00-30, Clear Air Turbulence Avoidance.
17.3 Turbulence Factors. How an aircraft will respond to turbulence varies with: intensity of the turbulence, aircraft size, wing loading, airspeed, and aircraft altitude. When an aircraft travels rapidly from one current to another, it undergoes abrupt changes in acceleration.