The    weather    conditions    during    the    summer monsoon   consist   of   cloudy   weather   with   almost continuous rain and widespread shower activity. High temperatures  and  high  humidities  further  add  to  the discomfort. AIR MASSES OVER EUROPE Although,  in  general,  the  characteristics  of  air masses over Europe are much the same as those found over North America, certain differences do exist. One reason for this is that an open ocean extends between Europe  and  North  America  toward  the  Arctic.  This allows an influx of mA air to reach Europe. This type of air   is   not   encountered   over   North   America.   The location of an extensive mountain range in an east-west direction   across   southern   Europe   is   an   additional influence not present over North America, where the prevailing    ranges    are    oriented    in    a    north-south direction. If the trajectory of the air is observed carefully and the modifying influences of the underlying surface are known, it is easy to understand the weather and flying conditions that occur in an air mass over any continent or ocean. Maritime Arctic (mA) Air in Winter Maritime   arctic   air   is   observed   primarily   over western Europe. Strong outbreaks of this air, originating   in   the   Arctic   between   Greenland   and Spitsbergen,  usually  follow  a  cyclonic  trajectory  into western Europe. Because  of  their  moisture  content  and  instability, cumulus  and  cumulonimbus  clouds  are  typical  of  this air mass, frequently producing widespread showers and squalls.  Visibility  is  generally  good,  but  moderate  to severe icing often affects aircraft operations. With the presence of a secondary cyclonic system over  France  or  Belgium,  mA  air  occasionally  sweeps southward across France to the Mediterranean, giving rise to the severe mistral winds of the Rhone Valley and the  Gulf  of  Lyons.  Heavy  shower  and  thunderstorm conditions are typical in this situation. Maritime Arctic (mA) Air in Summer In  summer,  this  air  is  so  shallow  that  in  moving southward  from  its  source  region,  it  modifies  to  the point  where  it  can  no  longer  be  identified  and  is  then indicated as mP air. Maritime Polar (mP) Air in Winter Maritime  polar  air  observed  over  Europe  usually originates in the form of cP air over North America. It reaches the west coast of Europe by various trajectories and  is  found  in  different  stages  of  modification;  it produces weather similar to mP air over the west coast of North America. Maritime Polar (mP) Air in Summer Maritime polar air observed over Europe is similar to mP air observed on the west coast of North America. The  weather  conditions  associated  with  this  air  are generally   good.   Occasionally,   because   of   surface heating,  a  shower  or  thunderstorm  is  observed  in  the daytime over land. Continental Arctic (cA) and Continental Polar (cP) Air in Winter The source region for cA and cP air is over northern Russia,   Finland,   and   Lapland.   The   cA   and   cP   air masses are observed over Europe in connection with an anticyclone centered over northern Russia and Finland. Occasionally  they  reach  the  British  Isles  and  at  times extend southward to the Mediterranean. Because of the dryness of cA and cP air, clouds are usually absent over the continent. Fair-weather cumulus are the typical clouds when cA and cP air are observed over the British Isles. Over the Mediterranean,  cA  and  cP  air  soon  become  unstable and  give  rise  to  cumulus  and  cumulonimbus  clouds with showers. Occasionally these air masses initiate the development of deep cyclonic systems over the central Mediterranean. Visibility  is  usually  good;  however, after this type becomes modified, haze layers form and reduce the visibility. Continental Arctic (cA) and Continental Polar (cP) Air in Summer The source region for cA and cP air is the same as for its counterpart in winter. It is a predominantly dry air mass and produces generally fair weather over the continent and the British Isles. The visibility is usually reduced  because  of  haze  and  smoke  in  the  surface layers.  As  cA  and  cP  air  stream  southward,  the  lower layers   become   unstable;   and   eventually   convective clouds and showers develop in the later stages of their life cycles. 4-18