Weather Theory

20,000 AGL 6,500 AGL 12-16 sduolc hgiH sduolc elddiM sduolc woL Cirrocumulus Cirrostratus Cumulonimbus Cirrus Altostratus Altocumulus Clouds with vertical development Stratocumulus Towering Cumulus Stratus Nimbostratus Figure 12-22. Basic cloud types. Middle clouds form around 6,500 feet AGL and extend up to and usually produce hazardous weather phenomena, such 20,000 feet AGL. They are composed of water, ice crystals, as lightning, hail, tornadoes, gusty winds, and wind shear. and supercooled water droplets. Typical middle-level clouds These extensive vertical clouds can be obscured by other include altostratus and altocumulus. These types of clouds cloud formations and are not always visible from the ground may be encountered on cross-country flights at higher or while in flight. When this happens, these clouds are said altitudes. Altostratus clouds can produce turbulence and may to be embedded, hence the term, embedded thunderstorms. contain moderate icing. Altocumulus clouds, which usually form when altostratus clouds are breaking apart, also may To pilots, the cumulonimbus cloud is perhaps the most contain light turbulence and icing. dangerous cloud type. It appears individually or in groups and is known as either an air mass or orographic thunderstorm. High clouds form above 20,000 feet AGL and usually form Heating of the air near the Earth’s surface creates an air mass only in stable air. They are made up of ice crystals and pose thunderstorm; the upslope motion of air in the mountainous no real threat of turbulence or aircraft icing. Typical high regions causes orographic thunderstorms. Cumulonimbus level clouds are cirrus, cirrostratus, and cirrocumulus. 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 Since rising air currents cause cumulonimbus clouds, they cumulonimbus clouds. The bases of these clouds form in are extremely turbulent and pose a significant hazard to flight the low to middle cloud base region but can extend into high safety. For example, if an aircraft enters a thunderstorm, altitude cloud levels. Towering cumulus clouds indicate areas the aircraft could experience updrafts and downdrafts that of instability in the atmosphere, and the air around and inside exceed 3,000 fpm. In addition, thunderstorms can produce them is turbulent. These types of clouds often develop into large hailstones, damaging lightning, tornadoes, and large cumulonimbus clouds or thunderstorms. Cumulonimbus quantities of water, all of which are potentially hazardous clouds contain large amounts of moisture and unstable air to aircraft.