thunderstorm  conditions.  If  the  data  are  old  and inaccurate, an aircraft mishap could result. THUNDERSTORM  FORECASTING The  standard  method  of  forecasting  air  mass thunderstorms  has  long  consisted  primarily  of  an analysis of rawinsonde data with particular emphasis on the so-called “positive areas.” Many   times   conditions   are   favorable   for thunderstorm development, with a large positive energy area showing up on the sounding, with no ensuing thunderstorm  activity.  At  other  times,  thunderstorms may  occur  when  they  are  not  forecasted.  Clearly, factors other than instability are important, and, at times, of  overriding  importance. A number of thunderstorm forecasting methods have been developed, but many of these are beyond the scope of this manual.    The forecasting of convective clouds by using variations of the parcel method are covered  in  this  section.  Further,  a  method  for  the prediction of these storms that enables the forecaster to arrive at a fairly accurate, reasonably objective forecast will be discussed. For a more detailed discussion of the determinations of instability, stability, the convective condensation level (CCL), the level of free convection (LFC), and the lifting  condensation  level  (LCL),  refer  to  the  AG2 TRAMAN, volume 2. THE PARCEL METHOD The temperature of a minute parcel of air is assumed to  change  adiabatically  as  the  parcel  is  displaced vertically from its original position. If, after vertical displacement,  the  parcel  has  a  higher  virtual  temperature than the surrounding atmosphere, the parcel is subjected to  a  positive  buoyancy  force  and  will  be  further accelerated   upwards;   conversely,   if   its   virtual temperature  has  become  lower  than  that  of  the surrounding  air,  the  parcel  will  be  denser,  and  eventually return to its initial or equilibrium position, Formation of Clouds by Heating From Below The  first  step  is  to  determine  the  convection temperature  or  the  surface  temperature  that  must  be reached  to  start  the  formation  of  convection  clouds  by solar heating of the surface air layer. The procedure is to first determine the CCL on the plotted sounding and, from the CCL point on the T curve, proceed downward along the dry adiabat to the surface pressure isobar. The temperature read at this intersection is the convection temperature. Figure  5-3  shows  an  illustration  of  forecasting afternoon  convective  cloudiness  from  a  plotted sounding. The dewpoint curve was not plotted to avoid confusion.  The  dashed  line  with  arrowheads  indicates the path the parcel of air would follow under these conditions.  You  can  see  that  the  sounding  was  modified at various times during the day. To determine the possibility of thunderstorms by the use of this method and from an analysis of the sounding, the following conditions must exist: . Sufficient heating must occur. l The positive area must exceed the negative area. The greater the excess, the greater the possibility of thunderstorms. l  The  parcel  must  rise  to  the  ice  crystal  level. Generally, this level should be –10°C and below. . There must be sufficient moisture in the lower troposphere.  This  is  the  most  important  single  factor  in thunderstorm  formation. l  Climatic  and  seasonal  conditions  should  be favorable. .  A  weak  inversion  (or  none  at  all)  should  be present in the lower levels. l An approximate height of the cloud top may be determined by assuming that the top of the cloud will extend beyond the top of the positive area by a distance equal to one-third of the height of the positive area. Formation  of  Clouds  by  Mechanical  Lifting When using this method, it is assumed that the type lifting will be either orographic or frontal, Here we will be  concerned  with  the  Lifting  Condensation  Level (LCL)  and  the  Level  of  Free  Convection  (LFC). The LCL is the height at which a parcel of air becomes  saturated  when  it  is  lifted  dry  adiabatically. The LCL for a surface parcel is always found at, or below, the CCL. The LFC is the height at which a parcel of air is lifted dry adiabatically until saturated, and thereafter   would   first   become   warmer   than   the surrounding  air.  The  parcel  will  then  continue  to  rise freely above this level until it becomes colder than the surrounding air. 5-5