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Characteristic cloud forms peculiar to wave action provide the best means of identification. Lenticular clouds, formed by mountain waves, are smooth in contour. [Figure 4-23] These clouds may occur alone or in layers at heights above 20,000 feet MSL, and be quite ragged when the airflow at that level is turbulent. The roll cloud forms at a lower level, generally near the height of the mountain ridge. The cap cloud must always be avoided in flight because of turbulence, concealed mountain peaks, and strong downdrafts on the lee slope. The lenticulars, like the roll and cap clouds, are stationary. They are constantly forming on the windward side and dissipating on the lee side of the mountain wave. 

## Rotors 

Rotors or eddies can also be found embedded in mountain waves. Formation of rotors can also occur as a result of down slope winds. Their formation usually occurs where wind speeds change in a wave or where friction slows the wind near the ground. These rotors are often experienced as gusts or wind shear. Clouds may also form within a rotor. 

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Research on mountain waves and rotors or eddies continues, but there is no doubt that pilots need to be aware of these phenomena and take appropriate precautions. Although mountain wave activity is normally forecast, many local factors may affect the formation of rotors and eddies. When planning a flight, a pilot should take note of the winds and terrain to assess the likelihood of waves and rotors. There may be telltale signs in flight, including disturbances on water or wheat fields and the formation of clouds, provided there is sufficient humidity to allow cloud formation. 

## Thunderstorms 

A thunderstorm is a local storm produced by a cumulonimbus cloud and accompanied by lightning and thunder. Thunderstorms and cumulonimbus clouds contain many of the most severe atmospheric hazards for the balloon pilot. They are almost always accompanied by strong gusty winds, severe turbulence, heavy rain showers, and lightning. These hazards may extend well away from the central core of the thunderstorm mass, sometimes as much as 30 miles or more. 

Ceilings and visibility in the precipitation areas under the thunderstorms are normally poor. Because of the heavy precipitation, the ceiling reported is at best an estimate of where the pilot may break out into visual contact with the surface. The weather observer determines the vertical visibility into the precipitation, which may be significantly different from the slant-range of the pilot. 

Potentially hazardous turbulence, as well as many other hazards associated with thunderstorms, make a thunderstorm a dangerous weather formation. The safe balloon pilot avoids any conditions that may expose him or her to potential thunderstorm activity. He or she exercises discretion and good judgment when potential thunderstorm conditions exist. 

