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: 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. To pilots, the cumulonimbus cloud is perhaps the most  dangerous cloud type. It appears individually or in groups and  is known as either an air mass or orographic thunderstorm.  Heating of the air near the Earth’s surface creates an air mass  thunderstorm; the upslope motion of air in the mountainous  regions causes orographic thunderstorms. Cumulonimbus  clouds that form in a continuous line are nonfrontal bands  of thunderstorms or squall lines. 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.  Knowledge of principal cloud types and the factors that affect  them helps the pilot visualize expected weather conditions,  and to recognize potential weather hazards. 4-17 Figure 4-20.  Cirrus clouds. Figure 4-19.  Nimbostratus clouds. •  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.