will be no supercooled droplets, but hoarfrost will form on  the  aircraft  through  direct  sublimation  of  water  vapor. .  When  the  temperature  curve  lies  to  the  right  of the  –8D  curve  in  a  subfreezing  layer,  the  layer  is subsaturated with respect to both ice and water surface. No icing will occur in this region. Modification of the Icing Forecast The final phase is to modify your icing forecast. This is essentially a subjective process. The forecaster should   consider   the   following   items:   probable intensification or weakening of synoptic features, such as low-pressure centers, fronts, and squall lines during the  time  interval  between  the  latest  analysis  and  the forecast;  local  influences,  such  as  geographic  location, terrain  features,  and  proximity  to  ocean  coastlines  or lake shores, and radar weather observations and pilot reports of icing. The forecaster should be cautious in either  underforecasting  or  overforecasting  the  amount and intensity of icing. An overforecast results in a reduced  payload  for  the  aircraft  due  to  increased  fuel load, while an underforecast may result in an operational emergency. TURBULENCE LEARNING   OBJECTIVES: R e c a ll characteristics  associated  with  turbulence. Determine  the  four  intensities  of  turbulence. Forecast  surface,  in-cloud,  and  clear  air turbulence (CAT). Recognize the advantages of  the  use  of  Doppler  radar  in  turbulence forecasting. Turbulence is of major importance to pilots of all types of aircraft; therefore, it is also of importance to the forecaster, whose duty it is to recognize situations where turbulence may exist, and to forecast both the areas and intensity of the turbulence. The following text discusses the  classification  and  intensity  of  turbulence,  the forecasting  of  turbulence  near  the  ground,  the forecasting of turbulence in convective clouds, and the forecasting of clear air turbulence (CAT). Refer to the  AG2 TRAMAN, volume 2, unit 6, for a  discussion  of  the  types  and  properties  of  turbulence. TURBULENCE  CHARACTERISTICS Turbulence  may  be  defined  as  irregular  and instantaneous motions of air that are made up of a number of small eddies that travel in the general air current.  Atmospheric  turbulence  is  caused  by  random fluctuations  in  the  windflow.  Given  an  analyzed  wind field  with  both  streamlines  and  isotachs  smoothly drawn, any difference between an actual wind and this smooth field is attributed to turbulence. To an aircraft in flight, the atmosphere is considered turbulent when irregular whirls or eddies of air affect the motion of the aircraft, and a series of abrupt jolts or bumps is felt by the pilot. Although a large range of sizes of eddies exists in the atmosphere, those causing bumpiness are roughly of the same size as the aircraft dimensions,  and  usually  occur  in  an  irregular  sequence imparting sharp translation or angular motions to the aircraft. The intensity of the disturbances to the aircraft varies not only with the intensity of the irregular motions of the atmosphere but also with aircraft characteristics, such as flight speed, weight stability, and size. Wind Shear and CAT A relatively tight gradient, either horizontal or vertical, produces churning motions (eddies), which result in turbulence. The greater the change of wind speed  and/or  direction,  the  more  severe  the  turbulence. Turbulent  flight  conditions  are  often  found  in  the vicinity of the jetstream, where large shears in the horizontal and vertical are found. Since this type of turbulence may occur without any visual warning, it is often referred to as CAT. The term clear air turbulence  is  misleading  because not   all   high-level   turbulence   included   in   this classification occurs in clear air. However, the majority (75 percent) is found in a cloud-free atmosphere. CAT is not necessarily limited to the vicinity of the jetstream; it may occur in isolated regions of the atmosphere. Most frequently, CAT is associated with the jetstream or mountain waves. However, it may also be associated with a closed low aloft, a sharp trough aloft, or an advancing cirrus shield. A narrow zone of wind shear, with   its   accompanying   turbulence,   is   sometimes encountered by aircraft as it climbs or descends through a temperature inversion. Moderate turbulence may also be  encountered  momentarily  when  passing  through  the wake of another aircraft. 5-34