Figure 4-4.-Convergence and divergence in waves. Convergence  and  divergence  are  not  quite  so  easily identified in wave-shaped flow patterns because the wave  speed  of  movement  is  often  the  factor  that determines  the  distribution.  The  most  common distribution  for  waves  moving  toward  the  east  is illustrated  in  figure  4-4.  There  is  relatively  little divergence  at  the  trough  and  ridge  lines,  with convergence  to  the  west  and  divergence  to  the  east  of the trough lines. This  chapter  devotes  more  time  to  a  discussion  of convergence   because   it   is   the   most   difficult characteristic to assess. Its extent ranges from the extremely  local  convergence  of  thunderstorm  cells and tornadoes to the large-scale convergence of the broad   and   deep   currents   of   poleward-   and equatorward-moving  air  masses. The amount, type, and intensity of the weather phenomena resulting from any of the lifting processes described in this chapter depend on the stability or convective stability of the air being lifted. All  of  the  lifting  mechanisms  (orographic,  frontal, vertical stretching) can occur in any particular weather situation.  Any  combination,  or  all  three,  are  possible, and  even  probable.  For  instance,  an  occluded  cyclone of maritime origin moving onto a mountainous west coast of a continent could easily have associated with it warm frontal lifting, cold frontal lifting, orographic lifting, lateral convergence, and convergence in the southerly flow. All fronts have a degree of convergence associated  with  them. WEATHER DISSIPATION PROCESSES LEARNING OBJECTIVES: Identify  processes leading  to  the  dissipation  of  weather. Each of the processes described in the preceding text has its counterpart among the condensation-preventing   or   weather-dissipating processes. Downslope  flow  on  the  lee  side  of orographic barriers results in adiabatic warming. If the air mass above and in advance of a frontal surface is moving  with  a  relative  component  away  from  the  front, downslope  motion  with  adiabatic  warming  will  occur. Divergence of air from an area must be compensated for by  subsiding  air  above  the  layer,  which  is  warmed adiabatically. These mechanisms have the common effect of increasing the temperature of the air, thus preventing   condensation. Likewise,  these  processes  occur  in  combination with  one  another,  and  they  may  also  occur  in combination   with   the   condensation-producing processes. This may lead to situations that require careful analysis. For instance, a current of air moving equatorward on a straight or anticyclonically curved path  (divergence  indicated)  encounters  an  orographic barrier;  if  the  slope  of  this  orographic  barrier  is sufficiently  steep  or  the  air  is  sufficiently  moist, precipitation  will  occur  in  spite  of  divergence  and subsidence  associated  with  the  flow  pattern.  The  dry, sometimes even cloudless, cold front that moves rapidly from west to east in winter is an example of upper level, downslope  motion,  which  prevents  the  air  being  lifted by the front from reaching the condensation level. The  precipitation  process  itself  opposes  the mechanism  that  produces  it,  both  by  contributing  the latent heat of vaporization and by exhausting the supply of  water  vapor. FORECASTING  FRONTAL CLOUDS AND WEATHER LEARNING  OBJECTIVES:  Evaluate  surface and upper level synoptic data in the analysis of frontal clouds and weather. Cloud  and  weather  regimes  most  difficult  to forecast are those associated with cyclogenesis. It is well known that falling pressure, precipitation, and an expanding shield of middle clouds indicate that the cyclogenetic  process  is  occurring  and,  by  following these  indications,  successful  forecasts  can  often  be made for 6 to 48 hours in advance. Most of the winter precipitation of the lowlands in the middle latitudes is chiefly  cyclonic  or  frontal  in  origin,  though  convection is involved when the displaced air mass is unstable. 4-3