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above cumulus clouds. However, directional shear is
not necessary. Cumulus waves may also be shortlived,
and difficult to work for any length of time. An
exception is when the cumulus is anchored to some
feature, such as a ridge line or short mountain range.
Figure 9-13. Circulation across a cloud street.
3H
Inversion
H
Upper Wind
Cloud
Street
Cloud
Street
Lower Wind
Figure 9-14. Cloud street wave.
9-13
In these cases, the possible influence of the ridge or
mountain in creating the wave lift becomes uncertain.
Further discussion of atmospheric waves appears later
in this chapter. As a final note, thermal waves can also
form without clouds present.
THUNDERSTORMS
An unstable atmosphere can provide great conditions
for thermal soaring. If the atmosphere is too moist
and unstable, however, cumulonimbus (Cb) or thunderclouds
can form. Thunderstorms are local storms
produced by Cb and are accompanied by lightning,
thunder, rain, graupel or hail, strong winds, turbulence,
and even tornadoes. Not all precipitating, large
cumulo-form clouds are accompanied by lightning and
thunder, although their presence is usually an indication
that conditions are ripe for full-blown thunderstorms.
Forecasters sometimes use the term “deep
convection” to refer to convection that rises to high
levels, which usually means thunderstorms. The
tremendous amount of energy associated with Cb
stems from the release of latent heat as condensation
occurs with the growing cloud.
Thunderstorms can occur any time of year, though
they are more common during the spring and summer
seasons. They can occur anywhere in the continental
United States but are not common along the immediate
West Coast, where on average only about one per
year occurs. During the summer months, the desert
southwest, extending northeastward into the Rocky
Mountains and adjacent Great Plains, experiences an
average of 30 to 40 thunderstorms annually.
Additionally, in the southeastern United States, especially
Florida, between 30 and 50 thunderstorms
occur in an average year. Thunderstorms in the cool
seasons usually occur in conjunction with some forcing
mechanism, such as a fast moving cold front or a
strong upper-level trough. [Figure 9-15]
The lifecycle of an air-mass or ordinary thunderstorm
consists of three main stages: cumulus, mature, and
dissipating. We will use the term “ordinary” to
describe this type of thunderstorm consisting of a single
Cb, since other types of thunderstorms (described
below) can occur in a uniform large-scale air mass.
The entire lifecycle takes on the order of an hour,
though remnant cloud from the dissipated Cb can last
substantially longer.
The cumulus stage is characterized by a cumulus
growing to a towering cumulus (Tcu), or cumulus
congestus. During this stage, most of the air within the
cloud is going up. The size of the updraft increases,
while the cloud base broadens to a few miles in diameter.
Since the cloud has increased in size, the strong
updraft in the middle of the cloud is not susceptible to
entrainment of dryer air from the outside. Often, other
Figure 9-15.Thunderstorm frequency in the summertime.
9-14
smaller cumulus in the vicinity of the Tcu are suppressed
by general downward motion around the cloud.
Towards the end of the cumulus stage, downdrafts and
precipitation begin to form within the cloud. On some
days, small cumulus can be around for hours, before
Tcu form, while on other days, the air is so unstable
that almost as soon as any cumulus form, they become
Tcu. [Figure 9-16]
As the evolution of the thunderstorm continues, it
reaches its mature stage. By this time, downdrafts reach
the ground and spread out in what are known as downbursts
or microbursts. These often lead to strong and
sometimes damaging surface winds. Lightning and
thunder form along with precipitation (rain, graupel, or
hail) below cloud base, which has now increased to
several miles in diameter. It may become difficult to
discern cloud from precipitation after this stage. The
cloud top reaches to the tropopause, or nearly so, and
sometimes strong cells even extend into the stratosphere.
The cloud top forms a cirrus anvil indicating
the mature stage. The direction in which the anvil
streams provides an estimate of the direction of thunderstorm
movement. Often organized circulations form
within the Cb and the longevity of the thunderstorm
partly depends on the nature of those circulations. The
　
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