14-2 Figure 14-1. The Collision-Coalescence or Warm Rain Process
Most cloud droplets are too small and light to fall to the ground as precipitation. However, the larger cloud droplets fall more rapidly and are able to sweep up the smaller ones in their path and grow.
The other process is the ice crystal process. This occurs in colder clouds when both ice crystals and water droplets are present. In this situation, it is easier for water vapor to deposit directly onto the ice crystals so the ice crystals grow at the expense of the water droplets. The crystals eventually become heavy enough to fall. If it is cold near the surface, it may snow; otherwise, the snowflakes may melt to rain. This is thought to be the primary growth process in mid- and high-latitudes.
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14.4 Precipitation Types. The vertical distribution of temperature will often determine the type of precipitation that occurs at the surface. Snow occurs when the temperature remains below freezing throughout the entire depth of the atmosphere (see Figure 14-2).
Figure 14-2. Snow Temperature Environment
Ice pellets (sleet) occur when there is a shallow layer aloft with above freezing temperatures and with a deep layer of below freezing air based at the surface. As snow falls into the shallow warm layer, the snowflakes partially melt. As the precipitation reenters air that is below freezing, it refreezes into ice pellets (see Figure 14-3).
Figure 14-3. Ice Pellets Temperature Environment
14-4 Freezing rain occurs when there is a deep layer aloft with above freezing temperatures and with a shallow layer of below freezing air at the surface. It can begin as either rain and/or snow, but becomes all rain in the warm layer. The rain falls back into below freezing air, but since the depth is shallow, the rain does not have time to freeze into ice pellets (see Figure 14-4). The drops freeze on contact with the ground or exposed objects.
Figure 14-4. Freezing Rain Temperature Environment
Rain occurs when there is a deep layer of above freezing air based at the surface (see Figure 14-5).
Figure 14-5. Rain Temperature Environment
15-1 CHAPTER 15. ADVERSE WIND
15.1 Introduction. Adverse wind is a category of hazardous weather that is responsible for many weather-related accidents. Adverse winds include: crosswinds, gusts, tailwind, variable wind, and a sudden wind shift. Takeoff and landing are the most critical periods of any flight and are most susceptible to the effects of adverse wind. The most at-risk group is General Aviation (GA) pilots flying aircraft with lower crosswind and tailwind threshold values.
15.2 Crosswind. A crosswind is a wind that has a component directed perpendicularly to the heading of an aircraft. The potential of drift produced by crosswind is critical to air navigation, and can have its biggest impact during takeoff and landing. Airplanes take off and land more efficiently when oriented into the wind. The aircraft’s groundspeed is minimized, a shorter runway is required to achieve lift-off, and the pilot has more time to make adjustments necessary for a smooth landing. As the wind turns more perpendicular to the runway to become a crosswind, the airplane directional control is affected. If a pilot does not correctly compensate for the crosswind, the aircraft may drift off the side of the runway or sideload on landing gear might occur. In extreme cases, the landing gear may collapse (see Figure 15-1 below).
Figure 15-1. Crosswind
15.3 Gust. A gust is a fluctuation of wind speed with variations of 10 knots or more between peaks and lulls.
Even if the airplane is oriented into the wind, gusts during takeoff and landing cause airspeed fluctuations which can cause problems for pilots. A gust increases airspeed, which increases lift, and may cause an aircraft to briefly balloon up. Once the gust ends, a sudden decrease of airspeed occurs, which decreases lift and causes the aircraft to sink.
Gusty winds at the point of touchdown provide significant challenges to a safe landing.
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15.4 Tailwind. A tailwind is a wind with a component of motion from behind the aircraft.
A tailwind can be hazardous during both takeoff and landing. A longer takeoff roll is necessary because a higher groundspeed is required to generate sufficient lift, and the aircraft may roll off the end of the runway before lift-off. Also, a smaller initial climb gradient occurs during takeoff, which may be insufficient to clear obstacles at the end of the runway. During a landing, a longer landing roll is required because the aircraft will touch down at a higher groundspeed. Wind should always be considered in takeoff performance planning.
15.5 Variable Wind/Sudden Wind Shift. A variable wind is a wind that changes direction frequently, while a sudden wind shift is a line or narrow zone along which there is an abrupt change of wind direction. Both, even at low wind speeds, can make takeoffs and landings difficult. A headwind can quickly become a crosswind or tailwind.
15.6 Wind Shear. Wind shear is the change in wind speed and/or direction, usually in the vertical. The characteristics of the wind shear profile are of critical importance in determining the impact for an aircraft on takeoff or landing.
Please refer to the current edition of AC 00-54, Pilot Windshear Guide, for additional information.
16-1 CHAPTER 16. WEATHER, OBSTRUCTIONS TO VISIBILITY, LOW CEILING,
AND MOUNTAIN OBSCURATION
16.1 Weather and Obstructions to Visibility. Weather and obstructions to visibility include:
fog, mist, haze, smoke, precipitation, blowing snow, dust storm, sandstorm, and volcanic ash.
16.1.1 Fog. Fog is a visible aggregate of minute water droplets that are based at the Earth’s surface and reduces horizontal visibility to less than 5/8 statute mile (1 kilometer); unlike drizzle, it does not fall to the ground. Fog differs from 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 feet (2 meters) above the surface, it is called simply 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 a little beyond 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).
16.1.1.1 Fog Types. Fog types are named according to their formation mechanism.
16.1.1.1.1 Radiation Fog. Radiation fog (see Figure 16-1) 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.
Figure 16-1. Radiation Fog
16-2 Radiation fog is relatively shallow fog. It may be dense enough to hide the entire sky or 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, and 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 knots 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.
16.1.1.1.2 Advection Fog. Advection fog (see Figure 16-2) forms when moist air moves over a colder surface (see Figure 16-3), and the subsequent cooling of that air to below its dewpoint. It is most common along coastal areas, but often moves deep in continental areas. At sea, it is called sea fog. Advection fog deepens as wind speed increases up to about 15 knots. Wind much stronger than 15 knots lifts the fog into a layer of low stratus or stratocumulus clouds.
Figure 16-2. Advection Fog
16-3 Figure 16-3. Advection Fog Formation
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.
16.1.1.1.3 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.
Winds speeds of 5 to 15 knots 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 Rockies, and somewhat less frequent east of the Appalachians. Upslope fog is often quite dense and extends to high altitudes.
16.1.1.1.4 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
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