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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 

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 

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. 

## Cloud Formation 

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. 

<img src='content_image/566524.jpg'>

The four major groups are: 

• Low clouds, 

• Middle clouds, 

• High clouds, and 

• Clouds with vertical development. 

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: 

• 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 

