2.  From  point  B  in  figure  5-19,  follow  the  dry adiabat to the surface level. The temperature of the dry adiabat at the surface level is the surface temperature required for the dissipation of the stratus layer to be complete.  This  is  point  D. Determining Time of Dissipation After determination of the temperatures necessary for stratus dissipation to begin and to be completed, a forecast  of  the  time  these  temperatures  will  be  reached must be made. Estimate the length of time for the required amount of heating to take place; and on the basis of this estimate, the time of dissipation may be forecasted. Remember to take into consideration the absence or the presence of cloud layers above the stratus deck. In addition, consider the trajectory of the air over the station. If the trajectory is from a water surface, temperatures will beheld down for a longer than normal period of time. One  rule  of  thumb  used  widely  in  forecasting  the dissipation  of  the  stratus  layer  is  to  estimate  the thickness of the layer; and if no significant cloud layers are  present  above  and  normal  heating  is  expected, forecast the dissipation of the layer with an average of 360 feet per hour of heating. In this way an estimate can be made of the number of hours required to dissipate the layer. AIRCRAFT  ICING LEARNING  OBJECTIVES:   Recall  factors conducive to aircraft icing. Be familiar with icing  hazards  at  the  surface.  Analyze  aircraft icing forecasts by using synoptic data. Prepare aircraft  icing  forecasts  by  using  the  –8D method. Aircraft icing is another of the weather hazards to aviation. It is important that the pilot be advised of icing because of the serious effects it may have on aircraft performance. Ice on the airframe decreases lift and increases weight, drag, and stalling speed. In addition, the accumulation of ice on exterior movable surfaces affects the control of the aircraft. If ice begins to form on the blades of the propeller, the propeller’s efficiency is decreased, and still further power is demanded of the engine to maintain flight.    Today,  most  aircraft  have sufficient reserve power to fly with a heavy load of ice; airframe icing is still a serious problem because it results in  greatly  increased  fuel  consumption  and  decreased range.  Further,  the  possibility  always  exists  that engine-system icing may result in loss of power. The  total  effects  of  aircraft  icing  are  a  loss  of aerodynamic efficiency; loss of engine power; loss of proper operation of control surfaces, brakes, and landing gear;  loss  of  aircrew’s  outside  vision;  false  flight instrument   indications;   and   loss   of   radio communication. For these reasons, it is important that you, the forecaster, be alert and aware of the conditions conducive to ice formation. It is also important that you accurately  forecast  icing  conditions  during  flight weather  briefings. This  chapter  will  cover  icing  intensities,  icing hazards near the ground, operational aspects of aircraft icing, and icing forecasts. For a discussion of the types of icing, physical factors affecting aircraft icing, and the distribution of icing in the atmosphere, refer to the AG2  TRAMAN,  volume  2,  unit  6,  as  well  as Atmospheric   Turbulence   and   Icing   Criteria, NAVMETOCCOMINST  3140.4,  which  discusses associated  phenomena,  as  well  as  a  common  set  of criteria for the reporting of icing. SUPERCOOLED WATER IN RELATION TO ICING Two basic conditions must be met for ice to form on an airframe in significant amounts. First, the aircraft surface temperature must be colder than 0°C. Second, supercooled water droplets, or liquid water droplets at subfreezing  temperatures,  must  be  present.  Water droplets in the free air, unlike bulk water, do not freeze at  0°C.  Instead,  their  freezing  temperature  varies  from an upper limit near –10°C to a lower limit near —40°C. The smaller and purer the droplets, the lower their freezing  point.  When  a  supercooled  droplet  strikes  an object, such as the surface of an aircraft, the impact destroys the internal stability of the droplet and raises its  freezing  temperature.  In  general,  the  possibility  of icing  must  be  anticipated  in  any  flight  through supercooled   clouds   or   liquid   precipitation   at temperatures  below  freezing.  In  addition,  frost sometimes forms on an aircraft in clear, humid air if both the aircraft and air are at subfreezing temperatures. PROCESS OF ICE FORMATION ON AIRCRAFT The  first  step  in  ice  formation  is  when  the supercooled droplets strike the surface of the aircraft. As the droplet, or portion of it, freezes, it liberates the 5-28