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Figure 4-18.-Example of lnferring clouds from a RAOB
THREE-DIMENSIONAL HUMIDITY ANALYSISTHE  MOIST  LAYER

Aerographers Mate 1 & C
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Figure 4-19.-Percent probability of existence of cloud layer bases for different values of dewpoint depression (degrees C). Solid lines represent probability of clear or scattered conditions; dashed lines, the probability of broken or overcast conditions with the cloud layer bases between 1,000-hPa and 600-hPa. scattered   conditions   as   a   function   of   the   dewpoint depression;  the  other  curve  shows  that  of  broken  or overcast conditions. Separate graphs are based on 1,027 observations, which are enough to indicate the order of magnitude  of  the  dewpoint  depressions  at  the  base  of winter  cloud  layers.  Minor  irregularities  in  the  curves were  not  smoothed  out  because  it  is  not  certain  that they  are  all  due  to  insufficient  data.  The  graphs  are applicable  without  reference  to  the  synoptic  situation. For a given winter sounding, you can estimate from the graph  the  probability  of  different  sky  cover  conditions with  cloud  bases  between  1,000-hPa  and  600-hPa  for layers of given minimum dewpoint depressions. HUMIDITY FIELD IN THE VICINITY OF FRONTAL SYSTEMS Studies  of  the  humidity  field  throughout  frontal zones  indicate  there  is  a  tongue  of  dry  air  extending downward in the vicinity of the front, and sloping in the same direction as the front. One study found that such a  dry  tongue  was  more  or  less  well  developed  for  all frontal  zones  investigated.  This  dry  tongue  was  best developed   near   warm   fronts;   it   extended,   on   the average, down to 700-hPa in cold fronts and to 800-hPa in warm fronts. In about half the fronts, the driest air was  found  within  the  frontal  zone  itself  on  occasion  it was found on both the cold and warm sides of the zone. About half the flights through this area showed a sharp transition from moist to dry air, and the change in frost point on these flights averaged about 20°C in 35 miles. Some  flights  gave  changes  of  more  than  20°C  in  20 miles. As   a   frontal   cloud   deck   is   approached,   the dewpoint  or  frost  point  depression  starts  diminishing rapidly. At distances beyond 10 to 15 nm, this variation is  much  less.  You  should  keep  this  fact  in  mind  when attempting  to  locate  the  edge  of  a  cloud  deck  from rawinsonde data. Linear extrapolation or interpolation of  dewpoint  depressions  cannot  be  expected  to  yield good  results.  For  instance,  when  one  station  shows  a dewpoint   depression   of   10°C   and   the   neighboring station  shows  saturation,  the  frontal  cloud  may  be anywhere between them, except within about 10 nm of the driest station. Since the frontal cloud masses at midtropospheric levels is usually surrounded by relatively dry air, it  is possible  to  locate  the  edge  of  the  cloud  mass  from humidity  data  on  constant  pressure  charts.  This  is  so because  the  typical  change  in  dewpoint  depression  in going   from   the   cloud   edge   into   cloudfree   air   is considerably  greater  than  the  average  error  in  the reported dewpoint depression. 500-hPa ANALYSIS OF DEWPOINT DEPRESSION Figure  4-20   shows   an   analysis   of   the   500-hPa dewpoint    depression    field    superimposed    upon    an analysis  of  areas  of  continuous  precipitation,  and  of areas of overcast middle clouds. The 500-hPa dewpoint depression  isopleths  were  drawn  independently  of  the surface data. The analysis shows the following: 1.    The  regions  of  high  humidity  at  the  500-hPa level coincide well with the areas of middle clouds and the areas of precipitation. 2.    The  regions  of  high  humidity  at  the  500-hPa level are separated from the extensive dry regions by strong humidity gradients. These gradients are, in all probability, much stronger than those shown on this analysis. 3.   A   dewpoint   depression   of   4°C   or   less   is characteristic  of  the  larger  areas  of  continuous 4-17





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