Changes in Stability
When convergence or divergence occurs, whether
on a large or small scale, it may have a very pronounced
effect on the stability of the air. For example, when
convection is induced by convergence, air is forced to
rise without the addition of heat. If this air is
unsaturated, it cools first at the dry adiabatic rate; or if
saturated at the moist rate. The end result is that the air
is cooled, which will increase the instability y of that air
column due to a net release of heat. Clouds and weather
often result from this process.
Conversely, if air subsides, and this process is
produced by convergence or divergence, the sinking air
will heat at the dry adiabatic lapse rate due to
compression. The warming at the top of an air column
will increase the stability of that air column by reducing
the lapse rate. Such warming often dissipates existing
clouds or prevents the formation of new clouds. If
sufficient warming due to the downward motion takes
place, a subsidence inversion is produced.
Effect on Weather
The most important application of vertical motion
is the prediction of rainfall probability and rainfall
amount. In addition, vertical motion affects practically
all meteorological properties, such as temperature,
humidity, wind distribution, and particularly stability.
In the following section the distribution of large-scale
and small-scale vertical motions are considered.
Since cold air has a tendency to sink, subsidence is
likely to be found to the west of upper tropospheric
troughs, and rising air to the east of the troughs. Thus,
there is a good relation between upper air meridional
flow and vertical flow.
In the neighborhood of a straight Northern
Hemisphere jet stream, convergence is found to the
north of the stream behind centers of maximum speed
as well as to the south and ahead of such centers.
Divergence exists in the other two quadrants. Below the
regions of divergence the air rises; below those of
convergence there is subsidence.
These general rules of thumb are not perfect, and
only yield a very crude idea about distribution of vertical
motion in the horizontal. Particularly over land in
summer, there exists little relation between large-scrale
weather patterns and vertical motion. Rather, vertical
motion is influenced by local features and shows strong
diurnal variations. Large-scale vertical motion is of
small magnitude at the ground (zero if the ground is flat).
Above ground level, it increases in magnitude to at least
500 hPa and decreases in the neighborhood of the
tropopause. There have been several studies of the
relation between frontal precipitation and large-scale
vertical velocities, computed by various techniques. In
all cases, the probability of precipitation is considerably
higher in the 6 hours following an updraft than following
subsidence. Clear skies are most likely with
downdrafts. On the other hand, it is not obvious that
large-scale vertical motion is related to showers and
thunderstorms caused during the daytime by heating.
However, squall lines, which are formed along lines of
horizontal convergence, show that large-scale vertical
motion may also play an important part in convective
Vertical Velocity Charts
Vertical velocity charts are currently being
transmitted over the facsimile network and are
computed by numerical weather prediction methods.
The charts have plus signs indicating upward motion
and minus signs indicating downward motion. The
figures indicate vertical velocity in centimeters per
second (cm/sec). With the larger values of upward
motions (plus values) the likelihood of clouds and
precipitation increases. However, an evaluation of the
moisture and vertical velocity should be made to get
optimum results. Obviously, upward motion in dry air
is not as likely to produce precipitation as upward
motion in moist air.
Studies have shown that surface cyclones and
anticyclones are not independent of developments in the
upper atmosphere, rather, they work in tandem with one
The relationship of the cyclone to the
large-scale flow patterns aloft must therefore be a part
of the daily forecast routine.
Many forecasters have a tendency to shy away from
the subject of vorticity, as they consider it too complex
a subject to be mastered. By not considering vorticity
and its effects, the forecaster is neglecting an important
forecasting tool. The principles of vorticity are no more
complicated than most of the principles of physics, and
can be understood just as readily. In the following
section we will discuss the definition of vorticity, its
evaluation, and its relationships to other meteorological