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feet). Since the parcel is saturated above this level, it
no longer cools at the DALR, but at the SALR. Next,
from the CCL, draw a line parallel to a saturated adiabat
until it intersects the original sounding temperature
curve. This shows the maximum cumulus height, at
about 670 mb (11,000 feet). [Figure 9-12]
The above analysis leads to a rule of thumb for estimating
the CCL. The temperature and dew point
converge at about 4.4°F per 1,000 feet of altitude
gain. This is the same as saying for every degree of
surface temperature and dew point spread in
Fahrenheit, multiply by 225 feet to obtain the base of
the convective cloud (if any). Since aviation surface
reports are reported in degrees Centigrade, convert
the data by multiplying every degree of surface temperature
and dew point spread in degrees Centigrade
by 400 feet. For example, if the reported temperature
is 28ºC and the reported dew point is 15°C, we
would estimate cloud base as (28 – 15) x 400 = 5200
feet AGL.
Notice that in Figure 9-12, the dew point curve shows
a rapid decrease with height from the surface value. As
thermals form and mixing begins, it is likely that the
drier air just above the surface will be mixed in with
the moister surface air. Amore accurate estimate of the
CCL is found by using an average dew point value in
the first 50 mb rather than the actual surface value.
This refinement can change the analyzed CCL by as
much as 1,000 feet.
The second example, Figure 9-11, would only produce
dry thermals, even at this day’s maximum temperature
of 32°C. Following a line parallel to a mixing ratio line
from the surface dew point, the height of any cumulus
would be almost 12,000 feet AGL, while at 32°C, thermals
should only reach 9,000 feet AGL. The elevated
inversion at 9,000 to 10,000 feet AGL effectively caps
thermal activity there.
12 16 20
500
600
700
800
900
1000 360
3200
6400
9900
13800
18300
1010
2020
3030
07 09 10 12 16
Figure 9-11. Skew-T from an actual sounding.
12 16 20
500
600
700
800
900
1000
2020
3030
Figure 9-12. Skew-T from an actual sounding.
9-11
It is also important to recognize the limitations of a
sounding analysis. The sounding is a single snapshot
of the atmosphere, taken at one time in one location.
(This is not absolutely true since the sounding balloon
rises at about 1,000 feet per minute (fpm), so it takes
about 30 minutes to reach 30,000 feet, during which
time it has also drifted with the winds aloft from the
launch point). The analysis is limited by how well the
sounding is representative of the greater area. This may
or may not be a factor depending on the larger-scale
weather situation, and in any case, tends to be less
valid in regions of mountainous terrain. In addition,
the upper air patterns can change during the day due
to passing fronts or smaller-scale, upper-air features.
For example, local circulation patterns near mountains
can alter the air aloft over nearby valleys during the
day. A temperature change aloft of only a few degrees
also can make a large difference. Despite these limitations,
the sounding analysis is still an excellent tool
for soaring pilots.
In recent years, with the advent of the Internet, soundings
from numerical weather model forecasts have
become available in graphical form, like the Skew-T.
Thus, forecast soundings are available for a variety of
locations (far more numerous than the observational
sounding network) and at many intervals over the forecast
cycle. The advantage of using model forecast
soundings is a dramatic increase in both space and time
resolution. For instance, maps of the predicted thermal
tops can be made over a large (e.g., multi-state) area
from model data spaced every 10 miles or closer. Great
detail in the forecast distribution of thermals is available.
In addition, model output can be produced far
more frequently than every 12 hours. For instance,
hourly model soundings can be produced for a location.
This is a tremendous potential aide to planning
both local and cross-country flights. The disadvantage
is that these forecasts are not real data. They are a
model forecast of what the real atmosphere should do.
Model forecasts of critical items, such as temperatures
at the surface and aloft, are often inaccurate. Thus, the
model-forecast soundings are only as good as the
model forecast. Fortunately, models show continual
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Glider Flying Handbook(112)
