location of constant density, with mass variations above
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.
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
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
Divergence/Convergence and Surface
The usual distribution of divergence and
convergence relative to moving pressure systems is as
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
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
In filling systems, the
divergence aloft in advance of the low is small or even
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