Chapter 10: Thunderstorms II

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Chapter 10 Thunderstorms II
Met 10
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Damaging winds from thunderstorms
A downburst is a downdraft that spreads out
horizontally from the base of a thunderstorm. A
downburst with winds extending only 4 km or less
is termed a microburst. Microbursts are capable
of blowing down trees. Even with their small
size, microbursts can have winds as strong as 146
knots (168 mph)!! Microbursts are responsible
for several airline crashes.
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Flying into a microburst. At position (a), the
pilot encounters a headwind at position (b), a
strong downdraft and at position (c), a tailwind
that reduces lift and causes the aircraft to lose
altitude.
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Dust clouds rising in response to a microburst
near Denver, Colorado.
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Tornadoes
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Tornadoes
A tornado is a rapidly rotating column of air
that blows around a small area of intense low
pressure with a circulation that reaches the
ground. A tornados circulation is present on
the ground either as a funnel-shaped could or as
a swirling cloud of dust and debris. Sometimes
called twisters or cyclones, tornadoes can assume
a variety of shapes and forms that range from
twisting rope-like funnels, cylindrical-shaped
funnels. A funnel cloud, is a tornado that has
not reached the ground.
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Tornadoes
A majority of North American tornadoes rotate
counterclockwise about their central core of low
pressure. The diameter of most tornadoes is
between 100 and 600 m (300-2000 ft.), although
some are a few meters wide and others have
diameters exceeding 1 mile! Winds in tornadoes
are very destructive. Most have winds that are
125 knots or less, but the most powerful have
winds up to 220 knots!
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Tornado Winds
Table 10-2, p. 288
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Total destruction caused by an F5 tornado,
Oklahoma on May 3, 1999.
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Fig. 10-32, p. 288
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The total wind speed of a tornado is greater on
one side than on the other. When facing an
on-rushing tornado, the strongest winds will be
on your left side.
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A powerful multi-vortex tornado with three
suction vortices.
Fig. 10-31, p. 287
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Tornado Occurrence
Average number of tornadoes during each month in
the United States
Fig. 10-29, p. 285
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Tornado incidence by state
number of tornadoes reported by each state during
a 25-year period.
The lower figure is the average annual number of
tornadoes per 10,000 square miles.
Fig. 10-28, p. 285
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Table 10-1, p. 287
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1. Spinning vortex tubes created by wind shear.
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2. The strong updraft in the thunderstorm carries
the vortex tube into the thunderstorm
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Features associated with a tornado-breeding
supercell thunderstorm
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A tornado-spawning supercell thunderstorm. A hook
echo in its rainfall pattern on a Doppler radar
screen. The colors red and orange represent the
heaviest precipitation.
Fig. 10-36, p. 290
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A classic tornadic supercell thunderstorm showing
updrafts and downdrafts
Fig. 10-37, p. 291
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Fig. 10-38, p. 291
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Fig. 10-39, p. 292
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Nonsupercell Tornadoes

• Along the boundary of converging winds, the air
  rises and condenses into a cumulus congestus
  cloud.
• At the surface the converging winds along the
  boundary create a region of counterclockwise
  spin.
• (b) As the cloud moves over the area of rotation,
  the updraft draws the spinning air up into the
  cloud, producing a nonsupercell tornado, or
  landspout.

Fig. 10-40, p. 293
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Doppler radar display of winds associated with
the supercell storm that moved through parts of
Oklahoma City during the afternoon of May 3,
1999. The close packing of the horizontal winds
blowing toward the radar (green and blue shades),
and those blowing away from the radar (yellow and
red shades), indicate strong cyclonic rotation
and the presence of a tornado.
Fig. 10-41, p. 294
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Graduate students from the University of Oklahoma
use a portable Doppler radar to probe a tornado
near Hodges, Oklahoma.
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Figure 3, p. 295
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Lightning and Thunder
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Lightning and Thunder
Lightning is simply a discharge of electricity, a
giant spark, which usually occurs in mature
thunderstorms. Lightning may take place within a
cloud, from one cloud to another, from a cloud to
surrounding air, or from cloud to the ground.
Majority of lightning strikes occur within the
cloud, while only 20 strike the ground. A
lightning stroke can heat the air to 30,000C
!!! This extreme heating causes the air to
expand explosively, thus initiating a shock wave
that becomes a booming sound wave called
thunder that travels outward in all directions
from the flash.
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The lightning stroke can travel in a number of
directions
Within a cloud, from one cloud to another cloud,
from a cloud to the air, or from a cloud to the
ground. Notice that the cloud-to-ground lightning
can travel out away from the cloud, then turn
downward, striking the ground many miles from the
thunderstorm.
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Electrification of Clouds
Clouds become electrified during the formation of
precipitation when regions of separate charge
exist within tiny cloud droplets and larger
precipitation particles. When falling
precipitation collides with smaller particles,
the larger precipitation particles become
negatively charged and the smaller particles,
become positively charged. Updrafts within the
cloud then sweep the smaller positively charged
particles into the upper reaches of the cloud
while larger negative charged particles settle
toward the lower and middle parts of the cloud.
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The generalized charge distribution in a mature
thunderstorm.
Fig. 10-21, p. 279
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The Lightning Stroke
Because unlike charges attract one another, the
negative charge at the bottom of the cloud causes
a region of the ground beneath it to become
positively charged. As the thunderstorm moves
along, this region of positive charge follows the
cloud like a shadow. The positive charge is
most dense on protruding objects, such as trees,
poles, and buildings. The difference in charges
causes and electrical potential between the cloud
and ground.
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The development of a lightning stroke

• When the negative charge near the bottom of the
  cloud becomes large enough, a flow of
  electronsthe stepped leaderrushes toward the
  earth.
• As electrons approach the ground, a region of
  positive charge moves up into the air through any
  conducting object, such as trees, buildings, and
  even humans.
• (c) When the downward flow of electrons meets the
  upward surge of positive charge, a strong
  electric currenta bright return stroke carries
  positive charge upward into the cloud.

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A cloud-to-ground lightning flash hitting a
65-foot sycamore tree. It should be apparent
why one should not seek shelter under a tree
during a thunderstorm.
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Lightning BrushLightning protection
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Cloud-to-ground lightning strikes in the vicinity
of Chicago, Illinois. Detected by the National
Lightning Detection Network.
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• The four marks on the road surface represent
  areas where lightning, after striking a car,
  entered the roadway through the tires.
• Lightning flattened three of the cars tires and
  slightly damaged the radio antenna.
• The driver and a six-year-old passenger were
  taken to a nearby hospital, treated for shock,
  and released.

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The lightning rod extends above the building,
increasing the likelihood that lightning will
strike the rod rather than some other part of the
structure. After lightning strikes the metal
rod, it follows an insulated conducting wire
harmlessly into the ground.
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Summary of todays lecture
Winds associated with thunderstorms-
microbursts Tornados damage, winds,
formation Lightning causes Thunder causes