Geoscience Reference
In-Depth Information
Formation of Thunderstorms
Thunderstorms are fascinating atmospheric phenomena that in-
volve complex patterns of airflow. These patterns, including up-
drafts and downdrafts, are particularly well suited for animation
to visualize how they really move. To do so, view the animation
Formation of Thunderstorms
to observe these processes in
motion and to watch the life cycle of a thunderstorm. Be sure
to answer the questions at the end of the module to test your
understanding of this concept.
these downdrafts in Figure 8.12c. These downdrafts cause the
lower part of the atmosphere to cool, which serves to stabilize
the air such that further convection cannot occur on a large
scale. When these circumstances develop, rain can continue to
fall for a short period of time, but will consist of a shower that
gradually tapers off.
lightning. The production of lightning begins when collisions
among ice crystals and rain droplets cause a separation to de-
velop in the electrical charge within clouds (Figure 8.13a). The
result is that the tops and bottoms of clouds have positive and
negative charges, respectively, while the ground is positively
charged. In stage 2 the negative charge on the bottom of the
cloud increases to the point where it overcomes the air's resis-
tance to electrical flow. At this point, negatively charged elec-
trons then begin flowing toward Earth along a zigzag, forked
path, called a
leader,
at about 97 km/sec (60 mi/sec). As these
electrons approach the ground, a positive charge collects in the
ground. These positive charges are attracted to the negative
charges of the downward-flowing leader and thus move upward
through a channel (called a
streamer
) that begins at any con-
ducting object, such as a tree, a house, and even people. Both
leaders and streamers are invisible to the naked eye.
The electrical circuit becomes complete in stage 3
(Figure 8.13c), when the upward-flowing charges meet those
moving downward. This meeting typically occurs at an alti-
tude of about 30 m (
∙
100 ft) and is the place where the actual
lightning bolt you see begins. Less than a millisecond later,
millions of volts of electricity reach the ground. The visible
bolt, however, is the return stroke from this initial ground im-
pact (Figures 8.13d and 8.13e). This stroke moves upward at a
Severe Thunderstorms
In many cases, thunderstorms are short-lived events that are not
particularly intense, consisting of brief downpours of rain, short
gusts of moderately strong winds, some isolated lightning, and
a few low rumbles of thunder. At other times, however, thunder-
storms develop into very severe storms that include extremely
heavy rain along with strong winds, intense lightning, numer-
ous loud cracks of thunder, and hail. The most intense thun-
derstorms contain tornadoes with incredibly strong winds that
can cause great damage. Although these kinds of events are
relatively rare, it is nonetheless important to understand their
development because you may encounter such a storm in the
future. At the very least, you will appreciate them more the next
time one occurs where you live.
Severe midlatitude storms form during the mature stage
of thunderstorm development. Along with very strong winds,
one of the characteristics of a severe thunderstorm is abundant
Visual ConCept CheCk
8.2
This Doppler radar image shows a line of thunder-
storms along a cold front extending across Kentucky
and Tennessee. Which one of the following choices best
explains the geography of air along the front?
a)
The warmest air temperatures were northwest of
the front.
b)
The cold front caused mT air to rapidly uplift along
the front.
c)
The air southeast of the front consisted of cP air.
d)
The most humid air was located northwest of the front.
