Lightning

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Lightning
ch 11 pp.307-314
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Lightnings affect on structures

• Most strikes will result in total destruction of
  the structure due to fire
• This house the exception
• Several factors
• Concrete structure
• Blessing and cause
• Rebar in walls attracted
• Concrete prevented fire
• Icining insulation
• Suppressed the fire

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Lightnings affect on aircraft
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Lightnings affect on aircraft

• Lightning can
• Melt and deform aircrafts skin
• explosive vaporization of conductors,
• sparks in the fuel system which can cause
• explosion and damage in externally mounted
  composite material
• cause interference to the electronic equipment

Blackhawk helicopter charge density
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Lightnings affect on aircraft
Prototype of Glasair III LP
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Langley F106B research aircraft Where paint is
missing.lightning attachment points
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Lightning attachment hole
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On one particular Mission, the aircraft Was
struck 72 times

• NASA found in 1980s
• That lightning strikes
• Occurred between 5oC and
• -65oC
• Most above -40oC

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St. Elmos Fire
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Lightnings affect on trees

• Height is the major attractor
• Most trees will not explode like this one but
  will become weak and susceptible to disease and
  pests
• General rule is to stay at least two tree lengths
  away from any trees

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Lightnings affect on Humans and Animals

• Most common cause of death is cardiac arrest
• In humans 70 of victims survive because CPR was
  given in time
• Animals have a nearly 100 mortality rate because
  CPR is unable to be given to them
• Most common visual injury is burns
• Do not affect the body as bad as electrical burns
  can
• Often do not have an entrance or exit point
• Most lasting injury is to the nervous system

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Who is at the highest risk of being struck

• There is an equal risk of being struck whether
  you are at work, home or outdoors doing
  activities.
• But when you are outdoors the chance of being
  struck is higher due to the lack of protection.
• The activities which tend to have the highest
  strikes and fatalities are
• Golfing
• rated at have the highest number of strikes
• Water sports
• have the highest number of fatalities
• Camping/picnicking
• has a high survivability strike rate
• Hiking
• Half the people struck wound up as a fatality

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Indoor Safety Facts

• Three main ways lightning enters a structure
• A direct strike
• Through wires or pipes that extend outside the
  structure
• Through the ground
• Lightning Safety tips for Inside the Home
• Avoid contact with corded phones
• Leading cause of indoor lightning injuries
• Avoid contact with electrical equipment or cords
• Avoid contact with plumbing
• Do not take a bath, shower, or wash dishes
• Stay away from windows and doors, and stay off
  porches

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Key Facts to Know

• Outdoors is the most dangerous place to be during
  a lightning storm
• All thunderstorms produce lightning and are
  dangerous
• Lightning often strikes as far as 10 miles away
  from rainfall
• If you hear thunder you are in danger
• Look for dark cloud bases and increasing winds
• Blue skies and lightning Lightning can travel up
  to 10 miles sideways
• At least 10 of lightning occurs without visible
  clouds overhead in the sky

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Lightning Can Strike Well Away From the Base of
the Storm..
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Outdoor Safety Rules

• If you are outdoors when a thunderstorm threatens
  go as quickly as you can to a safe shelter
• What constitutes as a safe shelter
• Safe buildings are buildings that are fully
  enclosed with a roof, walls, and floor.
• They are considered safe because they have wiring
  and plumbing
• Safe vehicles are those with hard tops.
• Make sure all doors are closed and windows rolled
  up and do not touch any metal surfaces
• Large Boats with Cabins
• Best policy is if you are on the water and a
  storm is approaching is to get out of the water

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Outdoor Safety Rules

• The question begs to be asked What do you do if
  there is no safe shelter nearby to go to.
• Move away from tall trees
• Keep away from metal objects and fences
• If you are in a group spread out with at least 15
  ft between you so that there is someone to assist
  the struck victim.
• If you feel your hair rising or your skin feel
  prickly you are about to be struck so assume the
  crouching position shown
• This position where the heels are touching
  encourages the lighting to return to ground
  without going through your whole body
• Protect your ears from the resulting thunder by
  covering them with your hands

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When thunder roars go indoors
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Lightning Types

• Cloud to ground
• Cloud to cloud or Intracloud

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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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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.
Seen escorting bomber planes in World War II just
off their wingtips. Pilots referred to it as
foo fighters as they initially were thought to
be enemy planes.
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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.