Lightning - What Do We Know?   or   index.html


  No one is sure how lightning gets started, but one theory is
  that incoming cosmic rays from outer space serve as the
  trigger. Here, stars shine above a thunderstorm in the Alps.

NOVA Now | Lightning (9 min)

NOVA Now | How Lightning Works

The cosmic start of lightning

  Even though lightning is a common phenomenon, the exact
  mechanism triggering a lightning discharge remains elusive.
  Scientists at the Dutch national research institute for
  mathematics CWI, the University of Groningen and the
  University of Brussels now published a realistic model
  involving large ice particles and cosmic rays.

  The big picture of lightning is clear: charge separation
  occurs inside a thundercloud and is eventually
  short-circuited by a conductive tube of ionized air.
  However, the electric field inside these clouds is usually
  an order of magnitude too low to create the conductive tube.
  This is why lightning inception is first out of the 'top ten
  questions in lightning research', according to a recent


  In short, free electrons from air showers caused by cosmic
  particles entering the atmosphere are accelerated in the
  electric field at the tip of a hydrometeor, and form
  self-propagating tubes of ionized air. These conductive
  tubes can short-circuit the built-up charge difference
  inside a thundercloud, between clouds or between a cloud and
  the earth's surface. The results, presented in a Physical
  Review Letters paper, show that this mechanism for discharge
  inception is realistic.

Cosmic rays could reveal secrets of lightning on Earth

  Scientists have used cosmic rays to peer inside
  thunderstorms, a technique that could help explain the
  mystery behind lightning's formation.

  Despite Benjamin Franklin's best efforts with a kite and a
  key, the phenomenon of lightning remains a scientific
  enigma. Now, researchers have developed a new tool that
  could help them solve some of lightning's mysteries. By
  using cosmic rays, space-traveling particles that constantly
  rain down on our atmosphere, scientists report they can peek
  inside thunderstorms and measure their electric fields,
  helping them pinpoint the conditions that cause storms'
  electrical outbursts. The advance could help researchers
  predict more precisely when and where lightning is most
  likely to strike and get people out of harm's way in time.

  Lightning is so poorly understood in part because measuring
  electric fields inside thunderstorms is challenging.
  Scientists have made measurements by sending balloons or
  small rockets into the clouds, but such probes can alter the
  electrical environment, potentially obscuring the natural
  activity they're trying to measure. But such measurements
  fail to explain lightning's origin, as they have yet to find
  fields strong enough to initiate lightning. It could be that
  the high field regions are very localized, or it could mean
  another factor is necessary to set off the light show.

  Cosmic rays could help researchers solve that puzzle. When
  cosmic rays smash into molecules in our atmosphere, the
  collisions create showers of subatomic particles, including
  electrons, positrons, and other electrically charged
  particles. As these particles travel toward the ground,
  their trajectories are bent by Earth's magnetic field,
  causing them to emit radio waves. Scientists watched those
  patterns of radio waves with the LOFAR radio telescope in
  the Netherlands and compared their observations to a
  computer model simulating the radio wave patterns produced
  by cosmic ray showers. The model was able to reproduce most
  of the showers the researchers recorded, but things got
  wonky when the weather took a turn for the worse.

Characteristics of Lightning (slow motion videography)

Wikipedea | Lightning

Downward leader formation for negative CG lightning

  In a process not well understood, a channel of ionized air,
  called a "leader", is initiated from a charged region in the
  thundercloud. Leaders are electrically conductive channels
  of partially ionized gas that travel away from a region of
  dense charge. Negative leaders propagate away from densely
  charged regions of negative charge, and positive leaders
  propagate from positively charged regions.

  The positively and negatively charged leaders proceed in
  opposite directions, positive upwards within the cloud and
  negative towards the earth. Both ionic channels proceed, in
  their respective directions, in a number of successive
  spurts. Each leader "pools" ions at the leading tips,
  shooting out one or more new leaders, momentarily pooling
  again to concentrate charged ions, then shooting out another

  Leaders often split, forming branches in a tree-like
  pattern.[19] In addition, negative leaders travel in a
  discontinuous fashion. The resulting jerky movement of these
  "stepped leader(s)" can be readily observed in slow-motion
  videos of negative leaders as they head toward ground prior
  to a negative CG lightning strike. The negative leaders
  continue to propagate and split as they head downward, often
  speeding up as they get closer to the Earth's surface.

  About 90% of ionic channel lengths between "pools" are
  approximately 45 m (148 ft) in length.[20] The establishment
  of the ionic channel takes a comparatively long amount of
  time (hundreds of milliseconds) in comparison to resulting
  discharge which occurs within a few microseconds. The
  electric current needed to establish the channel, measured
  in the tens or hundreds of amperes, is dwarfed by subsequent
  currents during the actual discharge.