Figure 4-22.  Characteristics of a mountain wave. 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.  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. 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. 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. 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. $_{Figure 4-23.}$  Multiple lenticular clouds over Mount Shasta,  California. 4-24 Thunderstorms Rotors Rotor turbulence Breaking wave