Figure 5-14.-Saturation curve.
An estimate of the formation time of fog, and
possibly stratus, can be aided greatly if some type of
saturation time chart, such as that illustrated in figure
5-17, can be constructed on which the forecasted
temperature versus the forecasted dewpoint can be
plotted. To use this diagram, note the maximum
temperature and consider the general sky condition from
the surface chart, forecasts, or sequence reports, By
projecting the temperature and dewpoint temperature,
an estimated time of fog formation can be forecast. If
smoke is observed in the area, fog will normally form
about 1 hour earlier than the formation line indicates on
the charts because of the abundance of condensation
nuclei.
Air-mass Trajectories
The trajectory of an air mass during the forecast
period can be another important factor in fog formation.
Warm air moving over a colder surface is a primary fog
producer. This can happen when a station is in the warm
sector, following a warm frontal passage. Cooling of
the air mass takes place, allowing condensation and
widespread fog or low stratus to form. To determine the
probabilities of condensation behind a warm front,
compare the temperature ahead of the front with the
dewpoint behind the front. If the temperature ahead of
the warm front is lower than the dewpoint behind the
front, the air mass behind the front will cool to a
temperature near the
causing condensation
stratus.
temperature ahead of the front,
and the formation of fog or low
Over water areas, warm air passing over cold water
may cause enough cooling to allow condensation and
the production of low clouds.
Another instance in which trajectory is important is
when cold air moves over a warmer water surface,
marsh land, or swamp, producing steam fog. In
addition, air passing over a wet surface will evaporate a
portion of the surface moisture, causing an increase in
the dewpoint.
Whenever there is a moisture source
present, air will evaporate a portion of this moisture,
unless the vapor pressure of the air is as great, or greater,
than the vapor pressure of the water. A dewpoint
increase may be enough to allow large eddy currents,
nocturnal cooling, or terrain lifting to complete the
saturation process and allow condensation to occur.
CONDITIONS FAVORABLE FOR GROUND
OR RADIATION FOG
For the formation of ground or radiation fog, ideally,
the air mass should be stable, moist in the lower layers,
dry aloft, and under a cloud cover during the day, with
clear skies at night. Winds should be light, nights long,
and the underlying surface wet.
A stationary, subsiding, high-pressure area
furnishes the best requirements for light winds, clear
skies, stability, and dry air aloft. If the air in the high
has been moving over a body of water, or if it lies over
ground previously moistened by an active precipitating
front, the wet surface will cause an increase in the
dewpoint of the lowest layers of the air. In addition, long
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