precipitation  accompany  it.  View  A  of  figure  4-27 shows the height of the inversion in two different parts of a frontal zone, and view B of figure 4-27 shows a strong  frontal  inversion  with  a  consequent  dew-point inversion. A cold front generally shows a stronger inversion than   a   warm   front,   and   the   inversion   appears   at successively  higher  levels  as  the  front  moves  past  a station. The reverse is true of warm fronts. Occluded fronts generally show a double inversion. However, as the  occlusion  process  continues,  mixing  of  the  air masses takes place, and the inversions are wiped out or fuse into one inversion. It is very important in raob analysis not to confuse the subsidence inversion of polar and arctic air masses with   frontal   inversions.   Extremely   cold   continental arctic   air,   for   instance,   has   a   strong   inversion   that extends to the 700-mb level.  Sometimes it is difficult to find an inversion on a particular sounding, though it is known  that  a  front  intersects  the  column  of  air  over  a given   station.   This   may   be   because   of   adiabatic warming  of  the  descending  cold  air  just  under  the frontal surface or excessive local vertical mixing in the vicinity   of   the   frontal   zone.   Under   conditions   of subsidence of the cold air beneath the frontal surface, the  subsidence  inversion  within  the  cold  air  may  be more marked than the frontal zone itself. Sometimes fronts on a raob sounding, which might show a strong inversion, often are accompanied by little weather  activity.  This  is  because  of  subsidence  in  the warm air, which strengthens the inversion. The weather activity  at  a  front  increases  only  when  there  is  a  net upward vertical motion of the warm air mass. Wind Since winds near Earth’s surface flow mainly along the isobars with a slight drift toward lower pressure, it follows that the wind direction in the vicinity of a front must  conform  with  the  isobars.  The  arrows  in  figure 4-28 indicate the winds that correspond to the pressure distribution. 4-28 ATMOSPHERIC  SOUNDINGS IN  THE  COLD  AIR  MASS FRONTAL  SURFACE WARM  AIR  MASS COLD  AIR  MASS SURFACE POSITION OF  FRONT WARM  AIR  MASS DEW  POINT TEMPERATURE ACCUMULATION OF  MOISTURE HERE A.   HEIGHT  AND  THICKNESS OF  INVERSION  INDICATES  SLOPE OF  FRONT  AND  INTENSITY B.   FRONTAL  INVERSION AG5f0427 Figure 4-27.—Inversions. HIGH AG5f0428 HIGH LOW 1004 1008 A HIGH HIGH LOW B 1000 1004 HIGH HIGH LOW C 1000 1004 HIGH HIGH LOW D 996 1000 Figure 4-28.—Types of isobars associated with fronts.