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 canbe made of the number of hours required to dissipate the layer.

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 propellers 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 beause 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 aircrews 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 0C. 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 0C. Instead, their freezing temperature varies from an upper limit near 10C to a lower limit near 40C. 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.

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