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Figure 4-16. Stratus clouds.
Figure 4-17. Stratocumulus clouds.
Figure 4-18. Altocumulus clouds.Low clouds are those that form near the Earth’s surface. The low cloud group consists of stratus and stratocumulus clouds. [Figures 4-16 and 4-17] Clouds in this family create low ceilings, hamper visibility, and can change rapidly. Because of this, they influence flight planning and can make visual flight rules (VFR) flight impossible. The bases of these clouds can start near the surface, with the top extending to 6,500 feet or more above the terrain. Low clouds are of great importance to the balloon pilot, as they can create low ceilings and poor visibility. The heights of the cloud bases may change rapidly. If low clouds form below 50 feet, they are classified as fog, and may completely blanket landmarks and landing fields.Middle clouds form around 6,500 feet above ground level (AGL) and extend up to 20,000 feet AGL. They are composed of water, ice crystals, and supercooled water droplets. The middle cloud group consists of altocumulus [Figure 4-18], altostratus, and nimbostratus [Figure 4-19] clouds. Altocumulus clouds, which usually form when altostratus clouds are breaking apart, also may contain light turbulence and icing. Altostratus clouds can produce turbulence and may contain moderate icing. The altocumulus has many variations in appearance and formation, whereas the altostratus varies mostly in thickness, from very thin to several thousand feet. Bases of the middle clouds start as low as 6,500 feet and tops can range as high as 20,000 feet above the terrain. These clouds may be composed of ice crystals or water droplets (which may be supercooled). Altocumulus rarely produces precipitation, but altostratus usually indicates the proximity of unfavorable flying weather and precipitation.High clouds form above 20,000 feet AGL and usually form only in stable air. The high cloud group consists of cirrus, cirrocumulus, and cirrostratus clouds. The mean base level of these three cloud types starts at 18,000 feet or higher above terrain. Cirrus clouds [Figure 4-20] may give indications of approaching weather changes. Cirriform clouds are composed of ice crystals, are generally thin, and the outline of the sun or moon may sometimes be seen through them, producing a halo or corona effect. High clouds are generally of no interest to the balloon pilot, other than they may indicate future conditions.
4-17
Figure 4-19. Nimbostratus clouds.
Figure 4-20. Cirrus clouds.
Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus clouds, often developing into thunderstorms. The bases of these clouds form in the low to middle cloud region, but can extend into high altitude cloud levels. Towering cumulus clouds indicate areas of instability in the atmosphere, and the air around and inside them is turbulent. These clouds generally have their bases below 6,500 feet above the terrain and tops sometimes extend above 60,000 feet. Clouds with extensive vertical development are caused by lifting action, such as convective currents, orographic lift, or frontal lift.
Scattered cumulus or isolated cumulonimbus clouds seldom present a flight problem, since these clouds can usually be circumnavigated without difficulty. However, these clouds may rapidly develop in groups or lines of cumulonimbus. They may also become embedded and hidden in stratiform clouds, resulting in hazardous flight conditions.
Within the high, middle, and low cloud groups are two main subdivisions. These are:
• Clouds formed when localized vertical currents carry moist air upward to the condensation level. These vertical development clouds are characterized by their lumpy or billowy appearance, and are designated cumuliform type clouds, meaning “accumulation” or “heap.” Turbulent flying conditions usually exist in, below, around, and above cumuliform clouds.
• Clouds formed when complete layers of air are cooled until condensation takes place. These clouds are stratiform type clouds, meaning “layered out,” since they lie mostly in horizontal layers or sheets. Flight in stratiform cloud conditions is usually smooth.
In addition to the two main subdivisions discussed above, is the word nimbus, meaning “rain cloud.” These clouds normally produce heavy precipitation, either liquid or solid. For example, a stratiform cloud producing precipitation is referred to as nimbostratus, and a heavy, swelling cumulus cloud that has grown into a thunderstorm is referred to as cumulonimbus. Cumulonimbus clouds contain large amounts of moisture and unstable air, and usually produce hazardous weather phenomena such as lightning, hail, tornadoes, gusty winds, and wind shear. These extensive vertical clouds can be obscured by other cloud formations and are not always visible from the ground or while in flight. When this happens, these clouds are said to be embedded, hence the term, embedded thunderstorms.
　
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