stratocumulus clouds will form in the area. See figure.
5-13.
Saturation of the Air Mass
The saturation curve in figure 5-14 shows the
amount of moisture in grams per kilogram the air will
hold at various temperatures.
The air along the curve is saturated and is at its
dewpoint. Any further cooling will yield water as a
result of condensation; hence, fog or low ceiling clouds
(depending upon the wind velocity) will form.
Nocturnal Cooling
Nocturnal cooling begins after the temperature
reaches its maximum during the day. Cooling will
continue until sunrise, or shortly thereafter. This
cooling affects only the lower limits of the atmosphere.
If nocturnal cooling reduces the temperature to a value
near the dewpoint, fog or low clouds will develop. The
wind velocity and terrain roughness will control the
depth of the cooled air. Calm winds will allow a patchy
type of ground fog or a shallow, continuous ground fog
to form. Winds of 5 to 10 knots will usually allow the
fog to thicken vertically. Winds greater than 10 knots
will usually cause low stratus or stratocumulus to form.
See figures 5-15 and 5-16 for examples of fog and
stratus formation.
The amount of cooling at night is dependent on soil
composition, vegetation, cloud cover, ceiling, and other
factors. Cloud cover based below 10,000 feet has a
greenhouse effect on surface temperatures, absorbing
some terrestrial radiation and reradiating a portion of
this heat energy back to be absorbed by the land. This
causes a reduction in nocturnal cooling. Nocturnal
cooling between 1530 local and sunrise will vary from
as little as 5° to 10° (with an overcast sky condition
based around 1,000 feet), to 25° or 30°F with a clear sky
or a cloud layer above 10,000 feet. Other factors and
exceptions must also be considered. If a front is
expected to pass the station during the night, or onshore
winds are expected to occur during the night, the amount
of cooling expected would have to be modified in light
of these developments.
Figure 5-13.-How wind velocity can cause a low cloud layer.
5-20
