13-9 Figure 13-7. Thick Altostratus (As) or Nimbostratus (Ns)
Thick altostratus is denser and of a darker gray or bluish gray than thin altostratus with the greater part sufficiently dense to completely mask the sun or moon. With further thickening of the altostratus and a lowering of its base, the cloud may begin to produce precipitation, at which point it is called nimbostratus. Some cloud charts will depict nimbostratus as a low-level cloud. This is because oftentimes, during continuously falling precipitation, the base of nimbostratus clouds decrease into the low level. But officially and historically, nimbostratus is classified as a mid-level cloud.
13.4.3 Low Clouds. Cumulus (Cu), Towering cumulus (TCu), Stratocumulus (Sc), Stratus (St), and Cumulonimbus (Cb) are low clouds composed of water droplets. However, they can also be composed of supercooled liquid water droplets and/or ice crystals when temperatures are below freezing. Cumulus and Cumulonimbus usually have bases in the low level, but their vertical extent is often so great that their tops may reach into the middle and high levels.
13.4.3.1 Cumulus (Cu) and Towering Cumulus (TCu). Cumulus is a cloud type in the form of individual, detached elements that are generally dense and possess sharp, nonfibrous outlines. These elements develop vertically, appearing as rising mounds, domes, or towers, the upper parts of which often resemble a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their bases are relatively dark and nearly horizontal. Near the horizon, the vertical development of cumulus often causes the individual clouds to appear merged. If precipitation occurs, it is usually of a showery nature.
Various effects of wind, illumination, etc., may modify many of the above characteristics.
13-10 Cumulus is composed of a great density of small water droplets, frequently supercooled. Within the cloud, larger water drops are formed that may, as the cloud develops, fall from the base as rain or virga.5 Ice crystal formation will occur within the cloud at sufficiently low temperatures, particularly in upper portions as the cloud grows vertically.
For cumulus with little vertical development, pilots can expect some turbulence and no significant icing. However, for towering cumulus
(i.e., cumulus of moderate/strong development) pilots can expect very strong turbulence and some clear icing above the freezing level (where temperatures are negative). Towering cumulus is also referred to as the first stage of a thunderstorm.
Figure 13-8. Cumulus (Cu) with Little Vertical Development
Cumulus formation is often preceded by hazy spots out of which the clouds evolve. When completely formed, the clouds have clear-cut horizontal bases and flattened or slightly rounded tops. At this stage of development, they are known as fair weather cumulus. Over land, on clear mornings, cumulus may form as the sun rapidly heats the ground. Near coasts, cumulus may form over the land by day in a sea breeze and over the sea during the night in a land breeze.
5 Virga are wisps or streaks of water or ice particles falling out of a cloud, but vaporizing before reaching the Earth’s surface as precipitation.
13-11 Figure 13-9. Towering Cumulus (TCu)
13.4.3.2 Stratocumulus (Sc). Stratocumulus (Sc) (see Figure 13-10) is a cloud type, predominantly stratiform, in the form of a gray and/or whitish layer or patch, which nearly always has dark parts and is nonfibrous (except for virga).
Its elements are tessellated, rounded, roll-shaped, etc.; they may or may not be merged, and usually are arranged in orderly groups, lines, or undulations, giving the appearance of a simple (or occasionally a cross pattern) wave system. These elements are generally flat-topped, smooth, and large; observed at an angle of more than 30° above the horizon, the individual stratocumulus element subtends an angle of greater than 5°. When a layer is continuous, the elemental structure is revealed in true relief on its undersurface.
Stratocumulus is composed of small water droplets, sometimes accompanied by larger droplets, soft hail, and (rarely) by snowflakes. Under ordinary conditions, ice crystals are too sparse even to give the cloud a fibrous aspect;
however, in extremely cold weather, ice crystals may be numerous enough to produce abundant virga, and sometimes even halo phenomena. The highest liquid water contents are in the tops of these clouds where the icing threat is the greatest, if cold enough. Virga may form under the cloud, particularly at very low temperatures. Precipitation rarely occurs with stratocumulus.
Pilots can expect some turbulence and possible icing at subfreezing temperatures. Ceiling and visibility are usually better than with low stratus.
13-12 Figure 13-10. Stratocumulus (Sc)
This stratocumulus occurs in patches or layers, composed of rounded masses or rolls, at one or more levels.
The clouds are gray or whitish and always have dark parts. Sometimes the elements lie in parallel bands.
Due to perspective these may appear to converge towards the horizon. It may also occur in the shape of lenses or almonds, although this is fairly rare.
13.4.3.3 Stratus (St). Stratus (St) (see Figure 13-11 and Figure 13-12) is a cloud type in the form of a gray layer with a fairly uniform base. Stratus does not usually produce precipitation, but when it does occur, it is in the form of minute particles, such as drizzle, ice crystals, or snow grains. Stratus often occurs in the form of ragged patches or cloud fragments, in which case rapid transformation is a common characteristic. When the sun is seen through the cloud, its outline is clearly discernible. In the immediate area of the solar disk, stratus may appear very white. Away from the sun, and at times when the cloud is sufficiently thick to obscure it, stratus gives off a weak, uniform luminance.
The particulate composition of stratus is quite uniform, usually of fairly widely dispersed water droplets and, at lower temperatures, of ice crystals (although this is much less common). Halo phenomena may occur with this latter composition.
Stratus produces little or no turbulence, but temperatures near or below freezing can create hazardous icing conditions. When stratus is associated with fog or precipitation, the combination can become troublesome for visual flying.
13-13 Figure 13-11. Stratus (St)
Stratus most commonly occurs as a gray, fairly uniform, and featureless single layer of low cloud. Occasionally it can be dark or threatening, although at most it can only produce weak precipitation. This feature makes it fairly easy to distinguish it from nimbostratus (Ns), which nearly always produces rain, snow, or ice pellets.
Fog will often lift into a layer of stratus by an increase in wind or a rise in temperature. Stratus is sometimes comparatively thin, and the disk of the sun or moon may be seen with a clear outline.
13-14 Figure 13-12. Stratus Fractus (StFra) and/or Cumulus Fractus (CuFra) of Bad Weather
These ragged shreds of low cloud always appear in association with other clouds for a short time before, during, and a short time after precipitation. They often form beneath lowering altostratus (As) or nimbostratus (Ns).
They also occur beneath cumulonimbus (Cb) and precipitating cumulus (Cu) and are collectively known as scud clouds.
13.4.3.4 Cumulonimbus (Cb). A cumulonimbus (Cb) (see Figure 13-13 and Figure 13-14) is a cloud type, exceptionally dense and vertically developed, occurring either as isolated clouds or as a line or wall of clouds with separated upper portions. These clouds appear as mountains or huge towers, at least a part of the upper portions of which are usually smooth, fibrous, or striated, and almost flattened as it approaches the tropopause. This part often spreads out in the form of an anvil or vast plume. Under the base of cumulonimbus, which is often very dark, there frequently exist virga, precipitation, and low, ragged clouds, either merged with it or not. Its precipitation is often heavy and always of a showery nature. The usual occurrence of lightning and thunder within or from this cloud leads to its popular appellations: thundercloud, thunderhead (the latter usually refers only to the upper portion of the cloud), and thunderstorm.
Cumulonimbus is composed of water droplets and ice crystals, the latter almost entirely in its upper portions. It also contains large water drops, snowflakes, snow pellets, and sometimes hail. The liquid water forms may be substantially supercooled. Cumulonimbus contains nearly the entire spectrum of flying hazards, including extreme turbulence.
13-15 Figure 13-13. Cumulonimbus (Cb) Without Anvil
No part of the cloud top has acquired a fibrous appearance or any anvil development. The protuberances tend to form a whitish mass without striations. Showers or thunderstorms may occur. The presence of lightning/thunder differentiate between this cloud and towering cumulus. Cumulus, stratocumulus, or stratus may also be present.
13-16 Figure 13-14. Cumulonimbus (Cb) with Anvil
The characteristic shape of these clouds can only be seen as a whole when viewed from a distance. The tops of these massive clouds show a fibrous or striated structure that frequently resembles an anvil, plume, or huge mass of hair. They may occur as an isolated cloud or an extensive wall and squalls, hail, and/or thunder often accompany them. Underneath the base, which is often very dark, stratus fractus frequently form and, in storms, these may be only a few hundred feet above the Earth’s surface, and they can merge to form a continuous layer.
Mammatus may form, especially on the underside of the projecting anvil, and may appear particularly prominent when the sun is low in the sky. A whole variety of other clouds, such as dense cirrus, altocumulus, altostratus, stratocumulus, cumulus, and stratus may also be present.
14-1 CHAPTER 14. PRECIPITATION
14.1 Introduction. Precipitation is any of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The precipitation types are: drizzle, rain, snow, snow grains, ice crystals, ice pellets, hail, and small hail and/or snow pellets.
14.2 Necessary Ingredients for Formation. Precipitation formation requires three ingredients: water vapor, sufficient lift to condense the water vapor into clouds, and a growth process that allows cloud droplets to grow large and heavy enough to fall as precipitation. Significant precipitation usually requires clouds to be at least 4,000 feet thick. The heavier the precipitation, the thicker the clouds are likely to be. When arriving or departing from an airport reporting precipitation of light or greater intensity, expect clouds to be more than 4,000 feet thick.
14.3 Growth Process. All clouds contain water, but only some produce precipitation. This is because cloud droplets and/or ice crystals are too small and light to fall to the ground as precipitation. Because of their microscopic size, the rate at which cloud droplets fall is incredibly slow. An average cloud droplet falling from a cloud base at 3,300 feet (1,000 meters) would require about 48 hours to reach the ground. It would never complete this journey because it would evaporate within minutes after falling below the cloud base. Two growth processes exist which allow cloud droplets (or ice crystals) to grow large enough to reach the ground as precipitation before they evaporate (or sublimate). One process is called the collision-coalescence, or warm rain process (see Figure 14-1). In this process, collisions occur between cloud droplets of varying size and different fall speeds, sticking together or coalescing to form larger drops. Finally, the drops become too large to be suspended in the air, and they fall to the ground as rain.
This is thought to be the primary growth process in warm, tropical air masses where the freezing level is very high.
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.
14-3
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