Lightning, Thunder,

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Chapter 11 Lightning, Thunder, and Tornadoes
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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.
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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.
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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.
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The actual lightning event is preceded by the
rapid and staggered advance of a shaft of
negatively charged air,called a stepped leader.
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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).
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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.
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Lightning
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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.
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St. Elmos 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.
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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.
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A voltage difference of about 400,000 volts
exists between Earths 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.
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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.
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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.
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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.
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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.
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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.
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MCC

• Mesoscale Convective Complex
• Composed of multiple single-cell storms
• Covers a large area 40,000 square miles
• Lasts for more than 6 hours
• Cloud tops are cold (lt -40 F)
• Tend to form under ridges if divergence aloft
• New cell development on a preferred flank
• Supplies the bulk of growing season rains to the
  farm belts of the US and Canada

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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.
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Squall Line

• Storms arranged in a line or band
• Lasts for 6-12 hours
• Can expand across several states
• Wind shear tilts the updraft and separates
  updraft and downdraft
• Gust front cold outflow forces warm air to rise
  at leading edge of the line
• Shelf cloud Sloping, low-level cloud formed on
  gust front
• Often form along cold fronts

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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.
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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.
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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.
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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.
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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.
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The mean distribution of thunderstorms across the
U.S.
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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.
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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).
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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.
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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.
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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 eastwest
direction. These features allow for a collision
of maritime tropical air with continental polar
air along the polar front.
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Tornadoes around the globe.
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The distribution of tornadoes during the year by
state.
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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.
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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.
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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.
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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.
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The next chapter examines tropical storms and
hurricanes.