WIND SPEED (VELOCITY) SHEAR Convergence is occurring when wind speeds are decreasing downstream; that is, mass is accumulating upstream. Conversely, divergence is occurring when wind speeds are increasing downstream; that is, mass is being  depleted  upstream. DIRECTIONAL AND SPEED WIND SHEAR Wind speed in relation to the wind direction is also a valuable indicator. For example, on a streamline analysis chart we can analyze both wind direction and wind  speed,  variations  in  wind  speed  along  the streamlines, or the convergence or divergence of the streamlines. The  following  are  some  of  the  combinations  or variations of wind speed and direction: . In a field of parallel streamlines (wind flow), if the wind speed is decreasing downstream (producing a net inflow of air for the layer), convergence is taking place.  If  the  flow  is  increasing  downstream  (a  net outflow of air from the layer), divergence is occurring. .  In  an  area  of  uniform  wind  speed  along  the streamlines,  if  the  streamlines  diverge  (fan  out), divergence is occurring; if the streamlines converge (come together), convergence is taking place. .   Normally,   the   convergence   and   divergence components are combined. The fact that streamlines converge  or  diverge  does  not  necessarily  indicate convergence  or  divergence.  We  must  also  consider  the wind   speeds— whether   they   are   increasing   or decreasing  downstream  in  relation  to  whether  the streamlines are spreading out or coming together. . If, when looking downstream on the streamlines, the wind speed increases and the streamlines diverge, divergence is taking place. On the other hand, if the wind  speed  decreases  downstream  and  the  streamlines come together, convergence is taking place. There are other situations where it is more difficult to determine whether divergence or convergence is occurring,  such  as  when  the  wind  speed  decreases downstream and the wind flow diverges, as well as when wind  speed  increases  downstream  and  the  wind  flow converges. A special evaluation then must be made to determine  the  net  inflow  or  outflow. DIVERGENCE  AND  CONVERGENCE (COMPLEX MOTIONS) In  this  section  we  will  be  discussing  high-level convergence and divergence in relation to downstream contour  patterns  and  the  associated  advection  patterns. Low tropospheric advection (and also stratospheric advection) certainly play a large role in pressure change mechanisms. Since  the  term  divergence   is  meant  to  denote depletion of mass, while  convergence is meant to denote accumulation of mass, the forecaster is concerned with the mass divergence or mass convergence in estimating pressure or height changes. Mass divergence in the entire column of air produces pressure or height falls, while mass convergence in the entire column of air produces pressure or height rises at the base of the column. Mass  Divergence  and  Mass  Convergence Mass  divergence  and  mass  convergence  involve  the density field as well as the velocity field. However, the mass  divergence  and  mass  convergence  of  the atmosphere are believed to be largely stratified into two layers as follows: l Below about 600 hPa, velocity divergence and convergence occur chiefly in the friction layer, which is about one-eighth of the weight of the 1,000-to 600-hPa advection  stratum,  and  may  be  disregarded  in comparison with density transport in estimating the contribution to the pressure change by the advection stratum. .   Above   600   hPa,   mass   divergence   and convergence  largely  result  from  horizontal  divergence and  convergence  of  velocity.  However,  on  occasion, stratospheric advection of density may be a modifying factor. The  stratum  below  the  400-hPa  level  may  be regarded  as  the  ADVECTION  stratum,  while  the stratum above the 400-mb level maybe regarded as the significant   horizontal   divergence   or   convergence stratum. Also, the advection stratum maybe thought of as the zone in which compensation of the dynamic effects  of  the  upper  stratum  occurs. The Isopycnic Level At  about  8km  (26,000  ft)  the  density  is  nearly constant. This level, which is near the 350-hPa pressure surface, is called the isopycnic level. This level is the 1-2