sunny weather. A hill or mountaintop may be warmed by  the sun, which in turn heats the air just above it. As that air  rises through convection, it creates a low pressure region,  into which the air at the bottom of the slope flows, and  causes winds.  Katabatic winds are the reverse of anabatic winds; that is,  they flow down slope, and most frequently at night. They are  created by the effect of the air near the ground losing heat  thru radiational cooling at a faster rate than air at a similar  altitude over the surrounding land mass. Clouds are weather signposts in the sky. They provide the  balloon pilot with visible evidence of the atmospheric motion,  water content, and degree of stability. In this sense, clouds  are of significant importance to the aeronaut. However, when  they become too numerous or widespread, form at low levels,  or show extensive vertical development, they present weather  hazards to ballooning. Clouds are visible condensed moisture, consisting of droplets  of water or crystals of ice. They are supported and transported  by air movements as slow as one-tenth of a mile per hour.  Cloud formation is the direct result of saturation producing  processes which take place in the atmosphere. A pilot should  be able to identify cloud formations that are associated with  weather hazards. Knowledge of cloud types will also assist  the pilot in interpreting weather conditions from weather  reports and existing weather. Clouds are often indicative of future weather. For clouds to  form, there must be adequate water vapor and condensation  nuclei (miniscule particles of matter like dust, salt, and  smoke), as well as a method by which the air can be cooled.  When the air cools and reaches its saturation point, the  invisible water vapor changes into a visible state. Through  the processes of sublimation and condensation, moisture  condenses or sublimates onto condensation nuclei. The nuclei  are important because they provide a means for the moisture  to change from one state to another. Cloud type is determined by its height, shape, and behavior.  They are classified according to the height of their bases as  low, middle, or high clouds, as well as clouds with vertical  development. The International Cloud Classification is  designed to provide a uniform cloud classification system.  [Figure 4-15]  Within this system, cloud types are usually  divided into four major groups and further classified in terms  of their forms and appearance.  Cloud classification can be further broken down into specific  cloud types according to the outward appearance and cloud  composition. Knowing these terms can help identify visible  clouds. The following is a list of cloud classifications: Cloud Formation Clouds Katabatic Winds  International Cloud Classification Abbreviations and Weather Map Symbols Base Altitude Cloud Type Abbreviation Symbol Bases of high clouds usually above 18,000 feet  Cirrus  CI  Cirrocumulus  CC  Cirrostratus  CS  Altocumulus  AC  Altostratus  AS * Cumulus  CU * Cumulonimbus  Cb  Nimbostratus  NS  Stratocumulus  SC  Stratus  St Bases of middle clouds range from 6,500 feet to 18,000 feet Bases of low clouds range from surface to 6,500 feet * Cumulus and cumulonimbus are clouds with vertical  development. Their bases are usually below 6,500 feet, but  may be slightly higher. The tops of the cumulonimbus some  times exceed 60,000 feet. 18,000 FT 6,500 FT Surface Figure 4-15.  Cloud classification per international agreement. The four major groups are: •  Low clouds, •  Middle clouds, •  High clouds, and •  Clouds with vertical development. •  Cumulus—heaped or piled clouds •  Stratus—formed in layers •  Cirrus—ringlets, fibrous clouds, also high-level clouds  above 20,000 feet •  Castellanus—common base with separate vertical  development, castle-like •  Lenticularus—lens shaped, formed over mountains in  strong winds •  Nimbus—rain-bearing clouds •  Fracto—ragged or broken •  Alto—meaning high, also middle-level clouds existing  at 5,000 to 20,000 feet 4-15