Thunder and Lightning

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Thunder and Lightning

• EAS 211
• Spring 2005
• 04/07/05

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Lightning

• As the hydrometeors grow and interact.
• Some become charged (possibly through collisions)
• It is thought that the smaller particles tend to
  acquire positive charge, while the larger
  particles acquire more negative charge.
• These particles tend to separate under the
  influence of gravity until the upper portion of
  the cloud acquires a net positive charge and
  lower portion of the cloud becomes negatively
  charged.
• This separation of charge produces enormous
  electrical resistance within the cloud and b/w
  the cloud and ground
• This can amount to millions of volts and
  eventually the electrical resistance in the air
  breaks down and a flash begins.

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Lightning, then is an electrical discharge b/w
positive and negative regions of a thunderstorm.

• A lightning flash
• Composed of a series of strokes with an average
  of about four
• The length and duration of each lightning stroke
  vary, but typically average about 30
  microseconds. (average peak power per stroke is
  about 1012 watts.)

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Cloud-to-ground

• Most damaging and dangerous form of lightning
• Although not the most common type, it is the one
  which is the best understood.
• Most flashes originate near the lower-negative
  charge center and deliver negative charge to
  Earth. (negative stroke)
• Appreciable minority of flashes carry positive
  charges to Earth (positive stroke)
• Dissipating stage of a thunderstorms life. More
  common in the winter.

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Intra-Cloud

• Most common type
• Occurs b/w oppositely charged centers with the
  same cloud.
• Usually the process takes place within the cloud
  and looks from the outside of the cloud like a
  diffuse brightening which flickers.
• The flash may exit the boundary of the cloud and
  a bright channel, similar to a cloud-to ground
  flash, can be visible.
• The ratio of cg to cc lightning can vary from
  storm to storm.
• Storms w/ greatest vertical development may
  produce only cc.
• Some suggest that the variations are latitude
  dependent, with a greater percentage of cg
  strikes occurring at higher latitudes.
• Others suggest the cloud-top height is a more
  important variable than latitude.

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Why a charge stays within a cloud or comes to the
ground?

• Not really understood.
• Perhaps a flash propagates toward the Earth when
  the electric field gradient in the lower regions
  of the cloud is stronger in the downward
  direction.
• Depending upon cloud height above the ground and
  changes in electric field strength b/w cloud and
  Earth, the discharges stays w/in the cloud or
  makes direct contact with the Earth.
• If the field strength is highest in the lower
  regions of the cloud a downward flash may occur
  from cloud to Earth.

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Inter-Cloud

• As the name implies, occurs b/w charge centers in
  two different clouds w/ the discharge bridging a
  gap of clear air b/w them.

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OTHER TYPES OF LIGHTNING

• Some identify subcategories, and others may arise
  from optical illusions, appearances, or myths.
• Some popular terms include
• Ball lightning
• Heat lightning
• Bead lightning
• Sheet lightning
• Silent lightning
• Black lightning
• Ribbon lightning
• Colored lightning
• Tubular lightning
• Meandering lightning
• Cloud-to-air lightning
• Stratospheric
• Red sprites
• Blue jets
• And elves

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Lightning Discharge Process

• Charge separation occurs w/in the cloud
• Negative cloud base induces a positive charge at
  ground level.

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• With the initial breakdown of the air in a region
  of strong electric fields, a streamer may begin
  to propagate downward toward the Earth.
• It moves in discrete steps of about 50 m each and
  is called a step-leader.

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• As it grows, it creates an ionized path
  depositing charge along the channel, and as the
  step-leader nears the Earth, a large potential
  difference is generated b/w the end of the leader
  and the Earth.

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• The return stroke completes
• Return stroke releases tremendous energy, bright
  light and thunder.
• And the negative charge from the base of the
  cloud is carried downward to the surface.

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• Dart leaderanother step-leader re-ionizes the
  conductive path

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• Same process is repeated.
• After several strokes the negative charge is
  drained from the cloud.

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• Occasionally, where a thunderstorm grows over a
  tall Earth grounded object, such as a radio
  antenna, an upward leader may propagate from the
  object toward the cloud. This ground-to-cloud
  flash generally transfers a net positive charge
  to Earth and is characterized by upward pointing
  branches.

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Time Lapse Photography of Several Lightning
Strikes
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Lightning-Induced Fire in the Bitterroot National
Forest on 6 August 2000
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Lightning Strikes and Associated Fires in the NW
US on 24 August 2000
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Induction Charging Between small and large ice
particles. Small particles obtain a positive
charge while the larger particles obtain a
negative charge.
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Charge Accumulation near a Thunderstorm
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Cloud-to-ground Lightning between the anvil and
the ground Positive Polarity Stroke
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(No Transcript)
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Distribution of Lightning Strikes per square km
from 1989-1998
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Thunder

• Sound is generated along the length of the
  lightning channel as the atmosphere is heated by
  the electric discharge to the order of 20,000
  degrees Celsius (3 times the temperature of the
  surface of sun).
• This compresses the surrounding clear air
  producing a shock wave, which then decays to an
  acoustic wave as it propagates away from the
  lightning channel.
• Although the flash and resulting thunder occur at
  essentially the same time, light travels at
  186,000 miles in a second, almost a million times
  the speed of sound.
• Sound travels at the relatively snail pace of 1/5
  of a mile in the same time.
• Thus the flash, if not obscured by clouds, is
  seen before the thunder is heard.
• By counting the seconds between the flash and the
  thunder and dividing by 5, an estimate of the
  distance to the strike (in miles) can be made.

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Thunder as a shock wave due to intense heating by
lightning