To avoid turbulence associated with strong thunderstorms,  circumnavigate cells by at least 20 miles. Turbulence may  also be present in the clear air above a thunderstorm. To  avoid this, fly at least 1,000 feet above the top for every 10  knots of wind at that level, or fly around the storm. Finally,  do not underestimate the turbulence beneath a thunderstorm.  Never attempt to fly under a thunderstorm. The possible  results of turbulence and wind shear under the storm could  be disastrous. When moderate to severe turbulence is encountered, aircraft  control is difficult, and a great deal of concentration is  required to maintain an instrument scan.  [Figure 10-14] Pilots should immediately reduce power and slow the  aircraft to the recommended turbulence penetration speed  as described in the POH/AFM. To minimize the load factor  imposed on the aircraft, the wings should be kept level and the  aircraft’s pitch attitude should be held constant. The aircraft  is allowed to fluctuate up and down because maneuvering  to maintain a constant altitude only increases the stress on  the aircraft. If necessary, the pilot should advise ATC of  the fluctuations and request a block altitude clearance. In  addition, the power should remain constant at a setting that  maintains the recommended turbulence penetration airspeed. The best source of information on the location and intensity  of turbulence are PIREPs. Therefore, pilots are encouraged to  familiarize themselves with the turbulence reporting criteria  Rime ice forms if the droplets are small and freeze  immediately when contacting the aircraft surface. This type  of ice usually forms on areas such as the leading edges of  wings or struts. It has a somewhat rough-looking appearance  and a milky-white color. Structural icing refers to the accumulation of ice on the  exterior of the aircraft and is broken down into three  classifications: rime ice, clear ice, and mixed ice. For ice  to form, there must be moisture present in the air, and the  air must be cooled to a temperature of 0 °C (32 °F) or less.  Aerodynamic cooling can lower the surface temperature of  an airfoil and cause ice to form on the airframe even though  the ambient temperature is slightly above freezing. The very nature of flight in instrument meteorological  conditions (IMC) means operating in visible moisture such  as clouds. At the right temperatures, this moisture can  freeze on the aircraft, causing increased weight, degraded  performance, and unpredictable aerodynamic characteristics.  Understanding avoidance and early recognition followed  by prompt action are the keys to avoiding this potentially  hazardous situation. found in the AIM, which also describes the procedure for  volunteering PIREPs relating to turbulence. Structural Icing Figure 10-14 .  Maintaining an instrument scan in severe turbulence can be difficult. 10-23