THUNDERSTORM SURFACE PHENOMENA

Figure 5-1.-Location of electric charges inside a typical thunderstorm cell. conditions listed below are generally representative of many (but not necessarily all) thunderstorms. . The chance of severe or extreme turbulence within thunderstorms is greatest at higher altitudes, with most cases of severe and extreme turbulence about 8,000 to 15,000 feet above ground level (AGL). The least turbulence may be expected when flying at or just below the base of the main thunderstorm cloud. (The latter rule would not be true over rough terrain or in mountainous areas where strong eddy currents produced by strong surface winds would extend the turbulence up to a higher level.) l The heaviest turbulence is closely associated with the areas of heaviest rain. . The strongest updrafts are found at heights of about 10,000 feet AGL or more; in extreme cases, updrafts in excess of 65 feet per second occur. 5-3 Downdrafts are less severe, but downdrafts on the order of 20 feet per second are quite common. l The probability of lightning strikes occurring is greatest near or slightly above the freezing level. Because of the potential hazards of flying in a thunderstorm, it is obviously nothing short of folly for pilots to attempt to fly in thunderstorms, unless operationally necessary. THUNDERSTORM SURFACE PHENOMENA The rapid change in wind direction and speed immediately before thunderstorm passage is a significant surface hazard associated with thunderstorm activity. The strong winds that accompany thunderstorm passage are the result of horizontal spreading of downdraft currents from within the storm as it approaches the ground.