Rain and Lightning

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Rain and Lightning

• September 26, 2007

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Frequency of Lightning

• Factors effecting where lightning will strike
• Size of cumulonimbus cloud base
• Height of cloud base
• Intensity of precipitation
• Presence of tall objects
• Conductivity of soil

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Cloud Charge

• Rapid updrafts and downdrafts generate a
  separation of electric charge within clouds
• Positive charge near top, negative charge near
  bottom creating electric cell(s)
• A single thundershower can have several cells
  within itself and several possibilities for
  discharge
• Differences in charge can be horizontal or
  vertical within a cloud

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Cloud to Ground

• Flow of electrons moves from cloud base to
  surface
• Negative electrons at cloud base cause build up
  of positive electrons at surface
• A streamer of electrons carrying a negative
  charge begins to probe outward and downward from
  the cloud base stepped leader
• Electrons are seeking path of least resistance to
  the ground

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Stepped Leader
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Cloud to Ground

• When the stepped ladder is 330 to 660 ft from
  point to be stricken, a flashover of positive
  charge takes place
• The flashover meets the downward moving leader a
  short distance from the cloud base to the ground
• Massive discharge follows beginning at surface
  and moving towards cloud
• Electrons travel down to ground, but discharge
  takes place in upward direction
• This creates the return stroke, which is
  responsible for the visible flash

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Cloud to Ground

• The return stroke is only a few centimeters wide,
  but is as hot as the sun
• These often ignite fires
• Because an ionized, conducting path has been
  established by the first stroke, a second leader
  moves towards the surface much faster dart
  leader
• A return stroke follows the dart leader

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Cloud to Ground

• This can be repeated several times
• The greater the number of return strokes, the
  brighter the flash and the louder the thunder
• A multiple stroke presents a much greater fire
  hazard

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Lightning-ignited fires

• Dry lightning (lightning not accompanied by
  rainfall) often causes fires
• Accepted contributor to fires in the West
• Nay-sayers in the East claim its too wet for
  lightning-ignited fires

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Intracloud and Intercloud Lightning

• Zones of positive and negative charge arise at
  different altitudes within clouds
• This creates a multitude of possible discharge
  paths
• This type of lightning does not damage objects on
  the ground, except indirectly through an
  electromagnetic pulse
• Accounts for 80 of lightning

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Ground to Cloud Lightning

• When the surface develops a negative charge and
  the cloud base a positive charge
• Electrons flow upwards instead of downwards
• Upside down lightning
• Less destructive than Cloud to Ground lightning

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Superbolt

• The most intense form of lightning discharge
• When positive charge builds up at the top of the
  cloud and a flashover occurs from the surface to
  the cloud top
• This carries more current than ordinary lightning
  strikes
• Can strike the ground a considerable distance
  from the cloud
• Very loud thunder

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Odd forms of lightning

• Ball lightning enters building through utility
  wires
• Are spheres a few centimeters in diameter that
  roll through the air as balls of fire
• Last for a few seconds and then disappear in
  smoke
• St. Elmos fire slow discharge of electric
  charge off pointed, metal objects
• Gives lightning rods, masts, antennas halos
• Common on airplane wings

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Ball Lightning
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St. Elmos Fire

• Served as an omen of heavenly intervention for
  sailors

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Thunder

• When return stroke occurs, the air is heated
  instantaneously to several thousand degrees
  Celsius
• The rapid heating causes to air to expand
  generating a shock wave that we hear
• Thunder is usually audible for 5 miles
• Length depends on local topography, wind speed,
  and direction
• Lightning is seen before thunder is heard
• To determine the distance to a lightning flash
  count the number of seconds it takes for thunder
  to arrive, then divide by 5 to get distance in
  miles

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Electromagnetic Pulse

• In a lightning stroke, electrons accelerate as
  they jump between poles of opposite charge
• When electrons accelerate EM fields are produced
• These create bursts of energy called
  electromagnetic pulses
• These run as currents through utility wires,
  telephone lines, antennas, fences, and metal

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Electromagnetic Pulse

• This is why you should never use electrical
  appliances, telephones, or anything plugged into
  a socket during a lightning storm
• EM pulses can be produced by intercloud and
  intracloud lightning that reach the ground

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Lightning Rods

• Lightning rod- metal conductor, usually steel,
  erected vertically atop a structure. The rod is
  electrically connected to the ground by heavy
  wire
• The wire should not run inside the structure or
  near flammable material
• The wire should connect to a ground rod, driven
  into the ground away from the structure
• The height of the rods is dependent upon the
  height of the structure

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Lightning rod
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• Even if you have a lightning rod, you can still
  sustain damage from EM pulses
• Unplug appliances
• Lightning can enter through windows
• If caught outside during a storm, move to a low
  place away from tall objects and crouch down
• Get into a car, not under one
• Boats are dangerous places during lightning storms

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Lightning Isnt all Bad

• Lightning strikes generate nitrogen compounds in
  the soil, which are fertilizers
• If we didnt have lightning, the atmosphere would
  eventually explode
• Lightning may have initiated life on Earth