Turbulence  Anvil  Storm movement  First gust  Roll cloud  Wind shear turbulence  Dust  Wind shear turbulance  Figure 12-29.  Movement and turbulence of a maturing thunderstorm.  Tornadoes occur with both isolated and squall line  thunderstorms. Reports for forecasts of tornadoes indicate  that atmospheric conditions are favorable for violent  turbulence. An aircraft entering a tornado vortex is almost  certain to suffer loss of control and structural damage. Since  the vortex extends well into the cloud, any pilot inadvertently  caught on instruments in a severe thunderstorm could  encounter a hidden vortex.  of an approaching storm. Advisory Circular (AC) 00-54, Pilot  Windshear Guide, explains gust front hazards associated with  thunderstorms. Figure 2 in the AC shows a cross section of a  mature stage thunderstorm with a gust front area where very  serious turbulence may be encountered.  Families of tornadoes have been observed as appendages of  the main cloud extending several miles outward from the area  of lightning and precipitation. Thus, any cloud connected to  a severe thunderstorm carries a threat of violence.  Updrafts in a thunderstorm support abundant liquid water  with relatively large droplet sizes. When carried above  the freezing level, the water becomes supercooled. When  temperature in the upward current cools to about –15 °C,  much of the remaining water vapor sublimates as ice crystals.  Above this level, at lower temperatures, the amount of  supercooled water decreases.  Potentially hazardous turbulence is present in all  thunderstorms, and a severe thunderstorm can destroy an  aircraft. Strongest turbulence within the cloud occurs with  shear between updrafts and downdrafts. Outside the cloud,  shear turbulence has been encountered several thousand feet  above and 20 miles laterally from a severe storm. A low-level  turbulent area is the shear zone associated with the gust front.  Often, a “roll cloud” on the leading edge of a storm marks the  top of the eddies in this shear, and it signifies an extremely  turbulent zone. Gust fronts often move far ahead (up to 15  miles) of associated precipitation. The gust front causes a  rapid, and sometimes drastic, change in surface wind ahead  Thunderstorms are not the only area where pilots could  encounter icing conditions. Pilots should be alert for icing  anytime the temperature approaches 0 °C and visible moisture  is present.  Supercooled water freezes on impact with an aircraft. Clear  icing can occur at any altitude above the freezing level, but at  high levels, icing from smaller droplets may be rime or mixed  rime and clear ice. The abundance of large, supercooled  water droplets makes clear icing very rapid between 0 °C and  –15 °C and encounters can be frequent in a cluster of cells.  Thunderstorm icing can be extremely hazardous.  12-24  Turbulence  Icing 