the expected cooling is greater than the late afternoon
spread, either fog or low clouds should be expected.
Wind velocity will determine which of the two
conditions will form.
FORECASTING STRATUS FORMATION
AND DISSIPATION
Fog and stratus forecasting are so closely tied
together that many of the fog forecasting rules and
conditions previously mentioned also apply to the
forecasting of stratus clouds.
Determining the Base and Top
of a Stratus Layer
One of the first steps in forecasting the dissipation
of stratus is to determine the thickness of the stratus
layer. The procedure is an follows:
1. Determine a representative mixing ratio
between the surface and the base of the inversion.
2. Project this mixing ratio line upward through the
sounding.
3. The intersection of the average mixing ratio line
with the temperature curve gives the approximate base
and maximum top of the stratus. Point A in figure 5-19
is the base of the stratus layer, and point B is the
maximum top of the layer. Point A is the initial base of
the layer; but as heating occurs during the morning, the
base will lift. Point B represents the maximum top of
the stratus layer; although in the very early morning, it
might lie closer to the base of the inversion. However,
as heating occurs during the day, the top of the stratus
layer will also rise and will be approximated by point B.
If the temperature and the dewpoint are the same at the
top of the inversion, the stratus will extend to this level.
To determine the height of the base and the top of
the stratus layer, use either the method previously
outlined for fog, or the pressure altitude scale.
Determining Dissipation Temperatures
To determine the temperature necessary for the
dissipation of a stratus layer, the following steps are
provided:
1. From point A in figure 5-19, follow the dry
adiabat to the surface level. The temperature of the dry
adiabat at the surface level is the temperature required
to be reached for stratus dissipation to begin. This is
point C.
Figure 5-19.-Sounding showing the base and the top of stratus layers. Also note temperature at which dissipation begins and
temperature when dissipation is complete.
5-27
