4.2 Weather dissipation processes
LEARNING OBJECTIVES:
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Identify processes leading to the dissipation of weather.
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Each of the processes described in the preceding text has its counterpart
among the condensation-preventing or weather-dissipating processes. 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 preventing condensation.
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.
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