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entering the thermal, centering the thermal, and finally
leaving the thermal. Keep in mind that every thermal is
unique in terms of size, shape, and strength.
In the last chapter, we learned that if the air is moist
enough and thermals rise high enough, cumulus clouds,
or Cu (pronounced ‘q’) form. Glider pilots seek Cu in
their developing stage, while the cloud is still being
built by a thermal underneath it. The base of the Cu
should be sharp and well defined. Clouds that have a
fuzzy appearance are likely well past their prime and
will probably have little lift left or even sink as the
cloud dissipates. [Figure 10-1]
Judging which clouds have the best chance for a good
thermal takes practice. On any given day, the lifetime
of an individual Cu can differ from previous days, so it
becomes important to observe their lifecycle on a particular
day. Agood looking Cu may already be dissipating
by the time you reach it. Soaring pilots refer to such
Cu as rapid or quick cycling, meaning they form,
mature, and dissipate in a short time. The lifetime of
Cu often varies during a given day as well; quick
cycling Cu early in the day will often become well
formed and longer lived as the day develops.
Figure 10-1. Photographs of (A) mature cumulus likely producing good lift, and (B) dissipating cumulus.
A B
Courtesy of NCAR
10-2
Sometimes Cu cover enough of the sky that seeing the
cloud tops becomes difficult. Hence, glider pilots
should learn to read the bases of Cu. Generally, a dark
area under the cloud base indicates a deeper cloud;
therefore, a higher likelihood of a thermal underneath.
Also, several thermals can feed one cloud, and it is
often well worth the deviation to those darker areas
under the cloud. At times, an otherwise flat cloud base
under an individual Cu has wisps or tendrils of cloud
hanging down from it, producing a particularly active
area. Cloud hanging below the general base of a Cu
indicate that that air is more moist, and hence more
buoyant. Note the importance of distinguishing features
under Cu that indicate potential lift from virga.
Virga is rain or snow from the cloud base that is not yet
reaching the ground and often signals that the friendly
Cu has grown to cumulus congestus or thunderstorms.
[Figure 10-2] Another indicator that one area of Cu
may provide better lift is a concave region under an
otherwise flat cloud base. This indicates air that is
especially warm, and hence more buoyant, which
means stronger lift. This can cause problems for the
unwary pilot, since the lift near cloud base often dramatically
increases, for instance from 400 to 1,000
(fpm). When trying to leave the strong lift in the concave
area under the cloud, pilots can find themselves
climbing rapidly with cloud all around—another good
reason to abide by required cloud clearances.
After a thermal rises from the surface and reaches the
Convective Condensation Level (CCL), a cloud
begins to form. At first, only a few wisps form. Then
the cloud grows to a cauliflower shape. The initial
wisps of Cu in an otherwise blue (cloudless) sky indicate
where an active thermal is beginning to build a
cloud. When crossing a blue hole (a region anywhere
from a few miles to several dozen miles of cloud-free
sky in an otherwise Cu-filled sky), diverting to an initial
wisp of Cu is often worthwhile. On some days,
when only a few thermals are reaching the CCL, the
initial wisps may be the only cloud markers around.
The trick is to get to the wisp when it first forms, in
order to catch the thermal underneath.
Lack of Cu does not necessarily mean lack of thermals.
If the air aloft is cool enough and the surface temperature
warms sufficiently, thermals will form whether or
not enough moisture exists for cumulus formation.
These blue or dry thermals, as they are called, can be
just as strong as their Cu-topped counterparts. Glider
pilots can find blue thermals, without Cu markers, by
gliding along until stumbling upon a thermal. With any
luck, other blue thermal indicators exist, making the
search less random.
One indicator of a thermal is another circling glider.
Often the glint of the sun on wings is all you will see,
so finding other gliders thermalling requires keeping a
good lookout, which glider pilots should be doing anyway.
Circling birds are also good indicators of thermal
activity. Thermals tend to transport various aerosols,
such as dust, upward with them. When a thermal rises
to an inversion it disturbs the stable air above it and
spreads out horizontally, thus depositing some of the
　
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