Figure 5-17.-Saturation time chart. (0’s indicate actual temperature and dewpoint observations. Straight line is forecast
temperature-dewpoint trends.)
In late fall and winter, when continental temperature
gradients have intensified, and the land temperature has
become colder than the adjacent water, poleward
moving air is cooled by advection over colder ground,
as well as by radiation. If the air is sufficiently moist,
fog or stratus may form. During daytime, heating may
dissipate the fog or stratus entirely. If not, the heating,
together with the wind, which is advecting the air, sets
up a turbulence inversion and stratus or stratocumulus
layers form at the base of the inversion. At night if the
air is cooled again and the surface pressure gradient is
weak, a surface inversion may replace the turbulence
inversion, and fog again occurs at the surface. However,
if the pressure gradient is strong, cooling will intensify
the inversion. Under these conditions stratus or
stratocumulus clouds occur just as in the daytime,
except with lower cloud bases.
Late fall and winter advection-radiation fogs cart
occur any place over the continent that can be reached
by maritime air or modified returning continental air.
Mainly, this occurs over the eastern half of the United
States. However, since tropical air masses do not reach
as high a latitude in winter as in summer, the frequency
of such fogs are much less in the northern regions of the
country. With large, slow-moving, continental warm
highs over the eastern half of the country, however, the
fogs may extend all the way from the Gulf of Mexico to
Canada.
CONDITIONS FAVORABLE FOR
UPSLOPE FOG AND STRATUS
Upslope fog and stratus occur in those regions in
which the land slopes gradually upward, and those areas
accessible to humid, stable air masses. In North
America, the areas best meeting these conditions are the
Great Plains of the United States and Canada and the
Piedmont region east of the Appalachians.
The synoptic conditions necessary for formation of
this type of fog or stratus are the presence of humid air
and a wind with an upslope component. The stratus is
not advected over the station as a solid sheet. It forms
gradually overhead. The length of time between the first
signs of stratus and a ceiling usually ranges from 1/2
hour to 2 hours; although at times, the stratus may not
form a ceiling at all. A useful procedure is to check the
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