Figure 4-4.-Convergence and divergence in waves.
Convergence and divergence are not quite so easily
identified in wave-shaped flow patterns because the
wave speed of movement is often the factor that
determines the distribution. The most common
distribution for waves moving toward the east is
illustrated in figure 4-4. There is relatively little
divergence at the trough and ridge lines, with
convergence to the west and divergence to the east of
the trough lines.
This chapter devotes more time to a discussion of
convergence because it is the most difficult
characteristic to assess. Its extent ranges from the
extremely local convergence of thunderstorm cells
and tornadoes to the large-scale convergence of the
broad and deep currents of poleward- and
equatorward-moving air masses.
The amount, type, and intensity of the weather
phenomena resulting from any of the lifting processes
described in this chapter depend on the stability or
convective stability of the air being lifted.
All of the lifting mechanisms (orographic, frontal,
vertical stretching) can occur in any particular weather
situation. Any combination, or all three, are possible,
and even probable. For instance, an occluded cyclone
of maritime origin moving onto a mountainous west
coast of a continent could easily have associated with it
warm frontal lifting, cold frontal lifting, orographic
lifting, lateral convergence, and convergence in the
southerly flow. All fronts have a degree of convergence
associated with them.
LEARNING OBJECTIVES: Identify processes
leading to the dissipation of weather.
Each of the processes described in the preceding
condensation-preventing or weather-dissipating
Downslope flow on the lee side of
orographic barriers results in adiabatic warming. If the
air mass above and in advance of a frontal surface is
moving with a relative component away from the front,
downslope motion with adiabatic warming will occur.
Divergence of air from an area must be compensated for
by subsiding air above the layer, which is warmed
adiabatically. These mechanisms have the common
effect of increasing the temperature of the air, thus
Likewise, these processes occur in combination
with one another, and they may also occur in
combination with the condensation-producing
processes. This may lead to situations that require
careful analysis. For instance, a current of air moving
equatorward on a straight or anticyclonically curved
path (divergence indicated) encounters an orographic
barrier; if the slope of this orographic barrier is
sufficiently steep or the air is sufficiently moist,
precipitation will occur in spite of divergence and
subsidence associated with the flow pattern. The dry,
sometimes even cloudless, cold front that moves rapidly
from west to east in winter is an example of upper level,
downslope motion, which prevents the air being lifted
by the front from reaching the condensation level.
The precipitation process itself opposes the
mechanism that produces it, both by contributing the
latent heat of vaporization and by exhausting the supply
of water vapor.
CLOUDS AND WEATHER
LEARNING OBJECTIVES: Evaluate surface
and upper level synoptic data in the analysis of
frontal clouds and weather.
Cloud and weather regimes most difficult to
forecast are those associated with cyclogenesis. It is
well known that falling pressure, precipitation, and an
expanding shield of middle clouds indicate that the
cyclogenetic process is occurring and, by following
these indications, successful forecasts can often be
made for 6 to 48 hours in advance. Most of the winter
precipitation of the lowlands in the middle latitudes is
chiefly cyclonic or frontal in origin, though convection
is involved when the displaced air mass is unstable.