Figure 12-22.  Basic cloud types.  Middle clouds form around 6,500 feet AGL and extend up to  20,000 feet AGL. They are composed of water, ice crystals,  and supercooled water droplets. Typical middle-level clouds  include altostratus and altocumulus. These types of clouds  may be encountered on cross-country flights at higher  altitudes. Altostratus clouds can produce turbulence and may  contain moderate icing. Altocumulus clouds, which usually  form when altostratus clouds are breaking apart, also may  contain light turbulence and icing.  High clouds form above 20,000 feet AGL and usually form  only in stable air. They are made up of ice crystals and pose  no real threat of turbulence or aircraft icing. Typical high  level clouds are cirrus, cirrostratus, and cirrocumulus.  Clouds with extensive vertical development are cumulus  clouds that build vertically into towering cumulus or  cumulonimbus clouds. The bases of these clouds form in  the low to middle cloud base 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 types of clouds often develop into  cumulonimbus clouds or thunderstorms. Cumulonimbus  clouds contain large amounts of moisture and unstable air  Since rising air currents cause cumulonimbus clouds, they  are extremely turbulent and pose a significant hazard to flight  safety. For example, if an aircraft enters a thunderstorm,  the aircraft could experience updrafts and downdrafts that  exceed 3,000 fpm. In addition, thunderstorms can produce  large hailstones, damaging lightning, tornadoes, and large  quantities of water, all of which are potentially hazardous  to aircraft.  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.  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.  12-16  20,000 AGL  6,500 AGL  Low clouds  Middle clouds  High clouds  Nimbostratus  Altocumulus  Cirrostratus  Cirrocumulus  Cirrus  Altostratus  Stratus  Stratocumulus  Cumulonimbus  Clouds with vertical development  Towering Cumulus 