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and a warm, sunny day that is stable and produces no
thermals. To the earthbound general public, it matters
little–either is a nice day. Glider pilots, however, need
a better understanding of these conditions and must
often rely on their own forecasting skills.
THERMAL SHAPE AND STRUCTURE
Two primary conceptual models exist for the structure
of thermals, the bubble model and the column or plume
model. Which model best represents thermals encountered
by glider pilots is a topic of ongoing debate
among atmospheric scientists. In reality, thermals fitting
both conceptual models likely exist. Ablend of the
models, such as individual strong bubbles rising within
one plume, may be what occurs in many situations. It
must be kept in mind, these models attempt to simplify
a complex and often turbulent phenomenon, so that
many exceptions and variations are to be expected
while actually flying in thermals. Many books, articles,
and Internet resources are available for further reading
on this subject.
The bubble model describes an individual thermal
resembling a vortex ring, with rising air in the middle
and descending air on the sides. The air in the middle
of the vortex ring rises faster than the entire thermal
bubble. The model fits occasional reports from glider
pilots. At times, one glider may find no lift, when only
200 feet below another glider that climbs away. At
other times, one glider may be at the top of the bubble
climbing only slowly, while a lower glider climbs rapidly
in the stronger part of the bubble below. [Figure 9-5]
More often, a glider flying below another glider circling
in a thermal is able to contact the same thermal and
climb, even if the gliders are displaced vertically by a
1,000 feet or more. This suggests the column or plume
model of thermals is more common. [Figure 9-6]
Which of the two models best describes thermals
depends on the source or reservoir of warm air near the
surface. If the heated area is rather small, one single
bubble may rise and take with it all the warmed surface
air. On the other hand, if a large area is heated and one
spot acts as the initial trigger, surrounding warm air
flows into the relative void left by the initial thermal.
The in-rushing warm air follows the same path, creating
a thermal column or plume. Since all the warmed
air near the surface is not likely to have the exact same
temperature, it is easy to envision a column with a few
or several imbedded bubbles. Individual bubbles
within a thermal plume may merge, while at other
times, two adjacent and distinct bubbles seem to exist
side by side. No two thermals are exactly alike since
the thermal sources are not the same.
Figure 9-5. The bubble or vortex ring model of a thermal.
Figure 9-6. The column or plume model of a thermal.
9-6
Whether the thermal is considered a bubble or column,
the air in the middle of the thermal rises faster than the
air near the sides of the thermal. A horizontal slice
through an idealized thermal provides a bulls-eye pattern.
Real thermals usually are not perfectly concentric;
techniques for best using thermals are discussed in the
next chapter. [Figure 9-7]
The diameter of a typical thermal cross section is on
the order of 500–1,000 feet, though the size varies considerably.
Typically, due to mixing with the surrounding
air, thermals expand as they rise. Thus, the thermal
column may actually resemble a cone, with the narrowest
part near the ground. Thermal plumes also tilt in a
steady wind and can become quite distorted in the presence
of vertical shear. If vertical shear is strong enough,
thermals can become very turbulent or become completely
broken apart. A schematic of a thermal lifecycle
in wind shear is shown in Figure 9-8.
ATMOSPHERIC STABILITY
Stability in the atmosphere tends to hinder vertical
motion, while instability tends to promote vertical
motion. A certain amount of instability is desirable for
glider pilots, since without it, thermals would not
develop. If the air is moist enough, and the atmospheric
instability deep enough, thunderstorms and associated
hazards can form. Thus, an understanding of atmospheric
stability and its determination from available
weather data is important for soaring flight and safety.
As a note, the following discussion is concerned with
vertical stability of the atmosphere. Other horizontal
atmospheric instabilities, for instance, in the evolution
of large-scale cyclones, are not covered here.
Generally, a stable dynamic system is one in which a
　
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