are classified as fog, and may completely blanket landmarks  and landing fields. Middle clouds form around 6,500 feet above ground level  (AGL)andextendupto20,000feetAGL.Theyarecomposedof  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.  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  4-16 Figure 4-18.  Altocumulus clouds. Figure 4-17.  Stratocumulus clouds. Figure 4-16.  Stratus clouds.