Clear ice is usually formed from larger water droplets or  freezing rain that can spread over a surface. This is the most  dangerous type of ice since it is clear, hard to see, and can  change the shape of the airfoil. Mixed ice is a mixture of clear ice and rime ice. It has the  bad characteristics of both types and can form rapidly. Ice  particles become embedded in clear ice, building a very  rough accumulation. The table in  Figure 10-15 lists the  temperatures at which the various types of ice form. Volcanic eruptions create volcanic ash clouds containing  an abrasive dust that poses a serious safety threat to flight  operations. Adding to the danger is the fact that these ash  clouds are not easily discernible from ordinary clouds when  encountered at some distance from the volcanic eruption. When an aircraft enters a volcanic ash cloud, dust particles  and smoke may become evident in the cabin, often along with  the odor of an electrical fire. Inside the volcanic ash cloud,  the aircraft may also experience lightning and St. Elmo’s fire  on the windscreen. The abrasive nature of the volcanic ash  can pit the windscreens, thus reducing or eliminating forward  visibility. The pitot-static system may become clogged,  causing instrument failure. Severe engine damage is probable  in both piston and jet-powered aircraft. Every effort must be made to avoid volcanic ash. Since  volcanic ash clouds are carried by the wind, pilots should plan  their flights to remain upwind of the ash-producing volcano.  Visual detection and airborne radar are not considered  a reliable means of avoiding volcanic ash clouds. Pilots  witnessing volcanic eruptions or encountering volcanic ash  should immediately pass this information along in the form of  a pilot report. The National Weather Service (NWS) monitors  volcanic eruptions and estimates ash trajectories. This  information is passed along to pilots in the form of SIGMETs. Structural icing is a condition that can only get worse.  Therefore, during an inadvertent icing encounter, it is  important the pilot act to prevent additional ice accumulation.  Regardless of the level of anti-ice or deice protection offered  by the aircraft, the first course of action should be to leave  the area of visible moisture. This might mean descending  to an altitude below the cloud bases, climbing to an altitude  that is above the cloud tops, or turning to a different course.  If this is not possible, then the pilot must move to an altitude  where the temperature is above freezing. Pilots should report  icing conditions to ATC and request new routing or altitude  if icing will be a hazard. Refer to the AIM for information  on reporting icing intensities. As for many other hazards to flight, the best source of  volcanic information comes from PIREPs. Pilots who  witness a volcanic eruption or encounter volcanic ash in flight  should immediately inform the nearest agency. Volcanic  Ash Forecast Transport and Dispersion (VAFTAD) charts  are also available; these depict volcanic ash cloud locations  in the atmosphere following an eruption and also forecast  dispersion of the ash concentrations over 6- and 12-hour  time intervals. See AC 00-45, Aviation Weather Services. A thunderstorm packs just about every weather hazard known  to aviation into one vicious bundle. Turbulence, hail, rain,  snow, lightning, sustained updrafts and downdrafts, and  icing conditions are all present in thunderstorms. Do not  take off in the face of an approaching thunderstorm or fly an  aircraft that is not equipped with thunderstorm detection in  clouds or at night in areas of suspected thunderstorm activity.  [Figure 10-16] Instrument pilots must learn to anticipate conditions leading  to the formation of fog and take appropriate action early in  the progress of the flight. Before a flight, close examination  of current and forecast weather should alert the pilot to the  possibility of fog formation. When fog is a consideration,  pilots should plan adequate fuel reserves and alternate landing  sites. En route, the pilot must stay alert for fog formation  through weather updates from EFAS, ATIS, and ASOS/ AWOS sites. Two conditions lead to the formation of fog. Either the air  is cooled to saturation, or sufficient moisture is added to the  air until saturation occurs. In either case, fog can form when  the temperature/dewpoint spread is 5° or less. Pilots planning  to arrive at their destination near dusk with decreasing  temperatures should be particularly concerned about the  possibility of fog formation. There is no useful correlation between the external visual  appearance of thunderstorms and the severity or amount of  turbulence or hail within them. All thunderstorms should be  considered hazardous, and thunderstorms with tops above  35,000 feet should be considered extremely hazardous. Volcanic Ash Thunderstorms Fog 0 °C to –10 °C –10 °C to –15 °C –15 °C to –20°C Clear  Mixed clear and rime  Rime  Figure 10-15 .  Temperature ranges for ice formation. 10-24