St. Elmo's fire is a popular name for so-called coronal
discharge, which frequently happens during thunderstorms. Unlike
lightning, coronal discharge is much less spectacular and less
transient in nature. It looks like a blueish flame or glow
engulfing tall sharp objects, such as powerlines, roof pinnacles,
chimneys, and lightning rods. Unless the electric field is
extremely strong, the glow is only visible at night, but can still
be heard during the day as a hissing or crackling sound.
St Elmo's fire, or coronal discharge, is a weakly luminous continuous
discharge caused by strong electric fields near sharp objects.
Cause of coronal discharge
All matter consists of positive and negative charges. The
positive charges are protons and the negative charges are the
electrons. Electrons may be free in the air, while protons are
usually bound to atoms or molecules, giving those atoms or molecules
a positive net charge (forming ions) if some electrons are missing.
Opposite charges attract eachother, while like charges repel.
At many times, the lower part of a thunderstorm is negatively
charged, while the top is positively charged. The negative
charge, which is nearest the ground, has most influence on the
charges in the ground. The negative charges are repelled a
bit, making the ground under a storm positively charged.
The different charges (at ground and in the cloud) create an
electric field in the air.
The electric field, when it becomes strong enough, is able to
pull so hard on electrons and the positive charges in atoms
that the electrons are freed from the atoms, leaving the atom
ionized (an ion). In the case of coronal discharge, a strong
electric field tears the molecules in the air apart (ionization
of the air).
Since the electric field is greatly enhanced near sharp,
conductive, protruding objects, this is the place where coronal
discharge is most likely. The air in the direct vicinity of a
sharp object will become ionized, and therefore slightly conductive.
The electrons freed from the ions are accelerated by the electric
field. These electrons may get so much energy by being accelerated
that they can ionize subsequent molecules when they collide with
them. Thus, more free electrons are created that also start
accelerating. A chain reaction develops, called an electron
avalanche. One single electron may free thousands of other
electrons, before these electrons move so far away from the region
of high electric field that the reaction stops.
There can be many electron avalanches occurring simultaneously in
a coronal discharge.
Electrons also recombine with molecules or ions, making negatively
charged ions or neutral molecules again, and are then 'lost' in
the ionization process. The recombination process produces light,
and this light becomes visible as the blueish glow of corona
Danger of coronal discharge
Coronal discharge is a gradual electric discharge into the air. The
current that feeds the discharge is so low that coronal discharge by
itself is not particularly dangerous. However, it frequently occurs
under thunderstorms when the electric field is high, and lightning
Coronal discharge also occurs briefly in the near vicinity of a cloud
to ground lightning flash. If you ever felt a jerking or pulling on
your arms, clothes and hair the instant lightning would strike very
close by, you may have had coronal discharges developing over your body
as the result of the strong electric field created by the downward
propagating lightning leader. Consider yourself lucky.
As a rule, whenever you notice or hear coronal discharge nearby, run
for cover immediately.
Photographing coronal discharge
Not many photos of natural St. Elmo's fire exist, so this subject
is very interesting to try to photograph. The glow from the
discharge is very faint however, so you should try photography
at night, and relatively close to the discharge. This may be
unsafe, so great care must be taken.
Try photographing at 800 or 1600 ISO, lowest f/ratio as possible
(largest aperture) and use bulb (B) mode, exposing as long as
possible without lightning illuminating the field. Even if you
don't see any coronal discharge but suspect it, it may be worth
doing a time exposure. The glow may be too faint to see with the
Alternative ways to photograph coronal discharge is to make or
buy a high voltage generator, such as a Walton-Cockcroft cascade,
Van de Graaff generator, Tesla coil, Whimshurst machine, or a
Marx generator. Of these, the Van de Graaff generator is easiest
and probably cheapest to build and the least unsafe. The generator
must be able to continuously provide the discharge current.