Figure 4-18.-Example of lnferring clouds from a RAOB
showing layer clouds with their Intermediate clear layers
not showing in the humidity trace.
18,300 and 20,000 feet. The second reported cloud
layer is indicated by a decrease in dewpoint depression,
but the humidity element is obviously slow in
responding. The dewpoint depression at the base of the
cloud at 21,000 feet is 14°C and at 400-hPa; after about
a 3-minute climb through the cloud, it is still 10°C.
From the sounding, clouds should have been inferred to
be from about 4,500 feet (base of the rapid humidity
increase) to 500-hPa and a second layer from 20,000 feet
up. In view of the rapid falling of the cloud free gap
between 15,000 and 21,000 feet that followed as the
warm front approached, the agreement between
reported and inferred conditions is good.
Figure 4-17 shows a middle cloud layer with no
precipitation reaching the surface. This is a case of a
cloud in the 500-hPa surface with no precipitation
reaching the surface; the nearest rain reaching the
surface was in Tennessee. The evidence from the
sounding for placing the cloud base at 12,200 feet is
strong, yet the base is inexplicably reported at 15,700
feet. The reported cloud base of 15,000 feet was
probably not representative, since altostratus, with bases
11,000 to 14,000 feet, was reported for most stations
over Ohio and West Virginia.
Figure 4-18 shows layered clouds with their
intermediate clear layers not showing in the humidity
trace. There is good agreement between the sounding
and the aircraft report. The clear layer between 6,000
and 6,500 feet is not indicated on the sounding. Thin,
clear layers, as well as thin cloud layers, usually cannot
be recognized on the humidity trace.
Comparisons between soundings and cloud reports
provide us with the following rules:
1. A cloud base is almost always found in a layer,
indicated by the sounding, where the dewpoint
depression decreases.
2. You should not always associate a cloud with a
layer of decreasing dewpoint, but only when the
decrease leads to minimum dewpoint depressions from
6°C to 0°C. However, at temperatures below -25°C,
dewpoint depressions in clouds are often higher than
6°C.
3. The dewpoint depression in a cloud is, on the
average, smaller in clouds that have higher
temperatures. typical dewpoint depressions are 1°C to
2°C at temperatures of 0°C and above, and 4°C between
-10°C and -20°C.
4. The base of a cloud should be located at the base
of the layer of decreasing dewpoint depression, if the
decrease is sharp.
5. If a layer of decreasing dewpoint depression is
followed by a layer of a stronger decrease, the cloud
base should be associated with the base of the strongest
decrease.
6. The top of a cloud layer is usually indicated by
an increase in dewpoint depression. Once a cloud base
is determined, the cloud is extended up to a level where
a significant increase in dewpoint depression starts. The
gradual increase of dewpoint depression with height in
a cloud is not significant.
In addition to the above analysis, another study was
made to determine how reliable the dewpoint depression
is as an indicator of clouds. The results are summarized
in figure 4-19.
Each graph shows the percent
probability of the existence of a cloud layer in January
for different values of dewpoint depression. On each
graph one curve shows the probability of clear or
4-16
