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Icing Forecasts Using the 8D Method
TURBULENCE - 14010_120

Aerographers Mate 1 & C
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from the Skew T Log P Diagram by using the following 7 steps: 1.  Plot  the  temperature  against  pressure  as determined  from  a  RAOB  sounding. 2. Record the temperature and dewpoint in degrees and tenths to the left of each plotted point. 3.  Determine  the  difference  (in  degrees  and  tenths) between the temperature and dewpoint for each level. This  difference  is  D,  the  dewpoint  deficit;  it  is  always taken  to  be  positive. 4. Multiply D by –8 and plot the product (which is in  degrees  Celsius)  opposite  the  corresponding temperature point at the appropriate place. 5.  Connect  the  points  plotted  by  step  4  with  a dashed line in the manner illustrated in figure 5-21. 6. The icing layer is outlined by the area enclosed by the temperature curve on the left and the –8D curve on the right. In this outlined area, supersaturation with respect to ice exists. This is the hatched area, as shown in figure 5-21. 7. The intensity of icing is indicated by the size of the area enclosed by the temperature curve and the –8D curve. In addition, the factors given in the following section should be considered when formulating the icing forecast. The cloud type and the precipitation observed at the RAOB time or the forecast time maybe used to determine whether icing is rime or glaze. Conclusions arrived at by using the-SD method for forecasting  icing: . When the temperature and dewpoint coincide in the RAOB sounding, the –8D curve must fall along the 0°C isotherm. In a subfreezing layer, the air would be saturated with respect to water and supersaturated with respect  to  ice.  Light  rime  icing  would  occur  in  the altostratus/nimbostratus  clouds  in  such  a  region,  and moderate  rime  icing  would  occur  in  cumulonimbus clouds in such a region. Severe clear ice would occur in the stratocumulus virga, cumulus virga, and stratus. .  When  the  temperature  and  dewpoint  do  not coincide but the temperature curve lies to the left of the –8D  curve  in  the  subfreezing  layer,  the  layer  is supersaturated with respect to cloud droplets. If the clouds  in  this  layer  are  altostratus,  altocumulus, cumulogenitus, or altocumulus virga, only light rime will   be   encountered. If  the  clouds  are  cirrus, cirrocumulus, or cirrostratus, only light hoarfrost will be sublimated  on  the  aircraft.  In  cloudless  regions,  there Figure 5-21.-The –8D ice forecast method. 5-33





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