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Chapter 11
Lightning, Thunder,
and Tornadoes
About 80 percent of all lightning is cloud-to-cloud lightning,
or sheet lightning, which occurs when the voltage gradient
within a cloud, or between clouds, overcomes the electrical
resistance of the air. The result is a large and powerful
spark that partially equalizes the charge separation.
Cloud-to-ground lightning occurs when negative charges
accumulate in the lower portions of the cloud.
Positive charges are attracted to a relatively small area in
the ground directly beneath the cloud establishing a large
voltage difference between the ground and the cloud base.
The positive charge at the surface is a local phenomenon;
it arises because the negative charge at the
base of the cloud repels electrons on the ground below.
Farther away, the surface maintains its normal
negative charge relative to the atmosphere.
All lightning requires the initial
separation of positive and
negative charges into different
regions of a cloud. Most often
the positive charges accumulate
in the upper reaches of the
cloud, negative charges in lower
portions. Small pockets of
positive charges may also gather
near the cloud base.
The actual lightning event
is preceded by the rapid
and staggered advance
of a shaft of negatively
charged air,called a
stepped leader.
When the leader approaches
the ground, a spark surges
upward from the ground
toward the leader (top).
When the leader and the
spark connect, they create
a pathway for the flow of
electrons that initiates
the first in a sequence of
brightly illuminated strokes,
or return strokes (bottom).
Another leader (the dart leader) forms within about
a tenth of a second, and a subsequent stroke emerges
from it. This sequence of dart leaders and strokes may
repeat itself four or five times. Because the individual
strokes occur in such rapid succession, they appear
to be a single stroke that flickers and dances about.
We call the combination of strokes a lightning flash,
the net effect of which is to transfer electrons
from the cloud to the ground.
Lightning
A bizarre type of electrification called ball lightning
appears as a round, glowing mass of electrified air,
up to the size of a basketball, that seems to roll through
the air or along a surface for 15 seconds or so
before either dissipating or exploding.
St. Elmo’s fire is a rare and peculiar type of electrical event.
Ionization in the air, often just before the formation of
cloud-to-ground lightning, can cause tall objects such as
church steeples to glow as they emit a continuous barrage
of sparks producing a blue-green tint to the air,
accompanied by a hissing sound.
Sprites are very large but short-lived electrical bursts that
rise from cloud tops as lightning occurs below that look
somewhat like a giant red jellyfish, extending up to 95 km
above the clouds, with blue or green tentacles
dangling from the reddish blob.
Blue jets are upward-moving electrical ejections from
the tops of the most active regions of thunderstorms.
They shoot upward at about 100 km/sec and
attain heights of up to 50 km above the surface.
A voltage difference of about 400,000 volts exists between
Earth’s surface and the ionosphere setting up the
fair weather electric field or the mean electric field.
Electricity flows in response to the voltage gradient
with cloud-to-ground lightning discharges
transfering electrons to the surface, thus maintaining
the voltage difference and the resulting electric field.
The tremendous increase in temperature during a
lightning stroke causes the air to expand explosively
and produce the familiar sound of thunder.
The decrease in the density of air with height causes
sound waves from lightning strokes over 20 km away
be to be bent upward. As a result,
the lightning seems to occur without thunder
and is sometimes called heat lightning.
Air mass thunderstorms are the most common and least
destructive usually lasting for less than an hour.
The cumulus stage begins when unstable air begins to
rise and cool adiabatically to form fair weather cumulus clouds.
The mature stage begins when precipitation starts to fall dragging air
toward the surface as downdrafts form in the areas of intense precipitation.
As the cloud yields heavy precipitation, downdrafts occupy an increasing
portion of the cloud base, the supply of additional water vapor is cut off,
and the storm enters its dissipative stage.
Severe thunderstorms have wind speeds exceeding 93 km/hr,
have hailstones larger than 1.9 cm in diameter,
or spawn tornadoes.
They typically appear in groups with individual storms
clustered together in mesoscale convective systems (MCSs).
MCSs that occur as linear bands are called squall lines.
Mesoscale convective complexes (MCCs)
appear as oval or roughly circular organized systems containing
several thunderstorms and are self-propagating in that
their individual cells often create downdrafts,
leading to the formation of new, powerful cells nearby.
The precipitation from each thunderstorm cell creates its own downdraft,
which is enhanced by the cooling of the air as the rain evaporates
and consumes latent heat. Upon hitting the ground,
the downdrafts spread outward and converge with
the warmer surrounding air to form an outflow boundary.
The movement of thunderstorm cells in an MCC. Initially (at time t = 0)
all the cells are moving toward the northeast. The cells in row A are
the oldest, those in E the most recently formed. Later (t = 1),
the cells in row A have dissipated, but a new row, F, has formed
along the southern margin of the complex. At t = 2, row B has
dissipated while a new row, G, has formed.
Squall line thunderstorms consist of a large number of individual storm cells
arranged in a linear band, about 500 km in length. They tend to form parallel
to and about 300 to 500 km ahead of cold fronts. Wind velocities in the
direction of storm movement typically increase with height. The strong winds
aloft push the updrafts ahead of the downdrafts and allow the rising air to
feed additional moisture into the storm. As the downdrafts reach the ground,
they surge forward as a wedge of cold, dense air, called a gust front.
A supercell storm consists of a single, extremely powerful cell.
Despite their single-cell structure, supercell storms are remarkably
complex, with the updraft and downdraft bending and wrapping
around each other due to strong wind shear. The downdrafts
serve to amplify the adjacent updrafts.
Doppler radar can reveal a feature of a supercell called a hook,
which looks like a small appendage attached to the main body of the
storm whose appearance usually means tornado formation is imminent.
When displayed on a radar screen a large portion of the storm
seems to be missing. This zone, known as a vault,
is where the inflow of warm surface air enters the supercell.
Potential instability arises when a layer of dry air
rests above one that is warm and humid.
If the air is potentially unstable, lifting of an
entire layer of air can cause its temperature lapse rate
to increase, thus making it statically unstable.
Strong downdrafts may also create downbursts,
potentially deadly gusts of wind that can reach
speeds in excess of 270 km/hr. When strong downdrafts
reach the surface, they can spread outward in all
directions to form intense horizontal winds capable
of causing severe damage at the surface.
Downbursts with diameters of less than 4 km are
called microbursts, and can produce a particularly
dangerous problem when they occur near airports.
The mean distribution of thunderstorms across the U.S.
Tornadoes are zones of extremely rapid, rotating winds
beneath the base of cumulonimbus clouds.
Though the majority of tornadoes rotate cyclonically
a few spin in the opposite direction.
Strong tornadic winds result from extraordinarily large
differences in atmospheric pressure over short distances.
The first observable step in
tornado formation is the slow,
horizontal rotation (a) of a large
segment of the cloud which
begins deep within the cloud
interior. The resulting large
vortices are called
mesocyclones. Under the
right conditions, strong updrafts
cause the horizontal vortex of
air to be tilted upward (b).
The narrowing column of rotating air stretches downward,
and a portion of the cloud base protrudes downward to form
a wall cloud. Wall clouds form where cool, humid air from
zones of precipitation is drawn into the updraft feeding the
main cloud. The cool, humid air condenses at a lower height
than does the air feeding into the rest of the cloud. Wall clouds
most often occur on the southern or southwestern portions of
supercells, near areas of large hail and heavy rainfall.
Funnel clouds form when a narrow, rapidly rotating vortex
emerges from the base of the wall cloud. A funnel cloud has
all the characteristics and intensity of a true tornado;
the only difference between the two is that
a funnel cloud has yet to touch the ground.
No other country in the world has nearly as many as
the U.S. The continent covers a wide range of latitudes;
its southeastern portion borders the warm Gulf of Mexico,
while the northernmost portion extends into the Arctic.
Much of the eastern portion of the continent is flat and
no major mountain range extends in an east–west direction.
These features allow for a collision of maritime tropical air
with continental polar air along the polar front.
Tornadoes around the globe.
The distribution of tornadoes during the year by state.
Although most tornadoes rotate around a single,
central core, some of the most violent ones have
relatively small zones of intense rotations
(about 10 m in diameter) called suction vortices.
It is these small vortices that probably cause the
familiar phenomenon of one home being destroyed
while the one next door remains unscathed.
The Fujita scale provides a widely used system for ranking tornado
intensity. Documented tornadoes fall into seven levels of intensity, with
each assigned a particular F-value ranging from 0 to 5.
In the U.S., the majority (69 percent) fall into the weak category,
which includes F0 and F1 tornadoes.
A severe thunderstorm watch means that the situation
is conducive to the formation of such activity.
If a severe thunderstorm has already developed,
a severe thunderstorm warning is issued.
Likewise, tornado warnings alert the public
to the observation of an actual tornado or the
detection of tornado precursors on Doppler radar.
Waterspouts occur over warm water bodies and are
typically smaller than tornadoes, having diameters
between about 5 and 100 m. Though they are
generally weaker than tornadoes, they can have
wind speeds of up to 150 km/hr. Some waterspouts
originate when land-based tornadoes move offshore.
The majority are formed over the water itself.
The next chapter examines
tropical storms and hurricanes.