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aerosols at that level. Depending on the sun angle and
the pilot’s sunglasses, haze domes can indicate dry
thermals. If the air contains enough moisture, haze
domes often form just before the first wisp of Cu.
On blue, cloudless days, gliders and other airborne
indicators are not around to mark thermals. In such
cases, you must pay attention to clues on the ground.
First, think about your previous flight experiences. It is
worth noting where thermals have been found previously
since certain areas tend to be consistent thermal
sources. Remember that weather is fickle, so there is
never a guarantee that a thermal will exist in the same
place. In addition, if a thermal has recently formed, it
will take time for the sun to reheat the area before the
next thermal is triggered. Glider pilots new to a soaring
location should ask the local pilots about favored
spots—doing so might save the cost of a tow. Glider
pilots talk about house thermals, which are simply
thermals that seem to form over and over in the same
spot or in the same area.
Stay alert for other indicators, as well. In drier climates,
dust devils mark thermals triggering from the ground.
In hilly or mountainous terrain, look for sun-facing
slopes. Unless the sun is directly overhead, the heating
of a sun-facing slope is more intense than that over
Figure 10-2. Photographs of (A) cumulus congestus, (B) cumulonimbus (Cb), (C) virga.
A B C
10-3
streets, a joyous sight when they lie along a cross-country
course line. [Figure 10-4]
In lighter wind conditions, consideration of thermal
drift is still important, and search patterns should
become “slanted.” For instance, in Cu-filled skies,
glider pilots need to search upwind of the cloud to find
a thermal. How far upwind depends on the strength of
the wind, typical thermal strength on that day, and distance
below cloud base (the lower the glider, the further
upwind the gliders needs to be). Add to this the
fact that wind speed does not always increase at a constant
rate with height, and/or the possibility that wind
direction also can change dramatically with height, and
the task can be challenging.
Wind speed and direction at cloud base can be estimated
by watching the cloud shadows on the ground.
With all the variables, it is sometimes difficult to estimate
exactly where a thermal should be. Pay attention
to where thermals appear to be located in relation to
clouds on a given day, and use this as the search criteria
for other clouds on that day. If approaching Cu from
the downwind side, expect heavy sink near the cloud.
Head for the darkest, best defined part of the cloud
base, then continue directly into the wind. Depending
on the distance below cloud base, just about the time of
passing upwind of the cloud, fly right into the lift forming
the cloud. If approaching the cloud from a crosswind
direction (for instance, heading north with
westerly winds), try to estimate the thermal location
from others encountered that day. If only reduced sink
is found, there may be lift nearby, so a short leg upwind
or downwind may locate the thermal.
Of course, thermals drift with the wind on blue days as
well, and similar techniques are required to locate thermals
using airborne or ground-based markers. For
instance, if heading toward a circling glider but at a
adjacent flat terrain because the sun’s radiation strikes
the slope at more nearly right angles. [Figure 10-3]
Also, cooler air usually pools in low-lying areas
overnight; therefore, it takes longer to heat up during
the morning. Finally, slopes often tend to be drier than
surrounding lowlands, and hence tend to heat better.
Given the choice, it usually pays to look to the hills for
thermals first.
Whether soaring over flat or hilly terrain, some experts
suggest taking a mental stroll through the landscape to
look for thermals. Imagine strolling along the ground
where warmer areas would be found. For instance,
walking from shade into an open field the air suddenly
warms. A town surrounded by green fields will likely
heat more than the surrounding farmland. Likewise, a
yellowish harvested field will feel warmer than an adjacent
wet field with lush green vegetation. Wet areas
tend to use the sun’s radiation to evaporate the moisture
rather than heat the ground. Thus, a field with a
rocky outcrop might produce better thermals. Rocky
outcrops along a snowy slope will heat much more efficiently
than surrounding snowfields. Though this technique
works better when at lower altitudes, it can also
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Glider Flying Handbook(136)
