WEATHER DISSIPATION PROCESSES

LEARNING OBJECTIVES: Identify processes leading to the dissipation of weather.

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.

FORECASTING FRONTAL 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. Cyclones are important generators of precipitation in the Tropics as well as in midlatitudes.

Factors to be considered in arriving at an accurate forecast are listed below; these factors are not listed in any order of importance:

The source region of the parent air mass.

Nature of the underlying surface.

The type and slope of the front(s).

Wind and contour patterns aloft.

Past speed and direction of movement of the low or front(s).

Familiarization with the normal weather patterns.

As pointed out earlier, a thorough understanding of the physical processes by which precipitation develops and spreads is essential to an accurate forecast.

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