location of constant density, with mass variations above and below. Since the density at 200 hPa is only four-sevenths the density at the isopycnic level, the height change at 200 hPa would have to be twice that at the isopycnic level (350 hPa) for the same pressure/height change to occur.  Thus,  height  changes  in  the  lower  stratosphere tend to be a maximum even though pressure changes are a maximum at the isopycnic level. Pressure changes occur at the isopycnic level, and in order to maintain constant density a corresponding temperature change must also occur. Since the density is nearly constant at this level, the required temperature variations  must  result  from  vertical  motions.  When  the pressures are rising at this level, the temperature must also rise to keep the density constant. A temperature rise can  be  produced  by  descending  motion. Similarly falling   pressures   at   this   level   require   falling temperatures to keep the density constant. Falling temperatures  in  the  absence  of  advection  can  be produced  by  ascent  through  this  level. Thus,  rising  heights  at  the  isopycnic  level  are associated with subsidence, and falling heights at the isopycnic  level  are  associated  with  convection. The 350-hPa to 200-hPa Stratum Subsidence at 350 hPa can result from horizontal convergence above this level, while convection here would result from horizontal divergence above this level. Since rising heights in the upper troposphere result in a rising of the tropopause and the lower stratosphere, the  maximum  horizontal  convergence  must  occur between the isopycnic level (350 hPa) and the average level of the tropopause (about 250 hPa). This is due to the  reversal  of  the  vertical  motion  between  the tropopause and the isopycnic level. Thus, the level of maximum  horizontal  velocity  convergence  must  be between  300  hPa  and  200  hPa  and  is  the  primary mechanism for pressure or height rises in the upper air. Similarly, upper height falls are produced by horizontal velocity divergence with a maximum at the same level. The maximum divergence occurs near or slightly above the tropopause and closer to 200 hPa than to 300 hPa. Therefore,  it  is  more  realistic  to  define  a  layer  of maximum divergence and convergence as occurring between  the  300-  and  200-hPa  pressure  surfaces.  The 300- to 200-hPa stratum is also the layer in which the core of the jet stream is usually located. It is also at this level  that  the  cumulative  effects  of  the  mean  temperature field of the troposphere produce the sharpest horizontal contrasts in the wind field. The   level   best   suited   for   determination   of convergence  and  divergence  is  the  300-hPa  level. Because  of  the  sparsity  of  reports  at  the  300-hPa level, it is frequently advantageous to determine the presence  of  convergence  and  divergence  at  the  500-hPa level. Divergence/Convergence  and  Surface Pressure Systems The   usual   distribution   of   divergence   and convergence relative to moving pressure systems is as follows: l In advance of the low, convergence occurs at low levels and divergence occurs aloft, with the level of nondivergence  at  about  600  hPa. l  In  the  rear  of  the  low,  there  is  usually convergence aloft and divergence near the surface. The low-level convergence ahead of the low occurs usually in the stratum of strongest warm advection, and the low-level divergence in the rear of the low occurs in the  stratum  of  strongest  cold  advection.  The  low-level divergence  occurs  primarily  in  the  friction  layer (approximately 3,000 ft) and is thought to be of minor importance  in  the  modification  of  thickness  advection compared with heating and cooling from the underlying surfaces. Divergence/Convergence  Features  Aloft In advance of the low, the air rises in response to the low-level convergence, with the maximum ascending motion   at   the   level   of   nondivergence   eventually becoming  zero  at  the  level  of  maximum  horizontal divergence  (approximately  300  hPa).  Above  this  level, descending motion is occurring. In the rear of the low, the reverse is true; that is, descending motion in the surface stratum and ascending motion in the upper troposphere above the level of maximum horizontal convergence. In deepening systems, the convergence aloft to the rear of the low is small or may even be negative    (divergence). In  filling  systems,  the divergence aloft in advance of the low is small or even negative   (convergence). Thus,  in  the  development  and  movement  of  surface highs and surface lows, two vertical circulations are involved,  one  below  and  one  above  the  300-hPa  level. The lower vertical circulation is upward in the cyclone 1-3