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. 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. 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.