When I was a lad I learned about
only two sorts of lightning. Forked lightning came down in a zig-zag to
the earth with an almighty crash. Sheet lightning sometimes only
rumbled, but for an instant it lit up the whole sky. In fact these two
are much the same, except that when lightning strikes from cloud to
cloud the flash is diffused and lights up the whole cloud, so you don’t
see the jagged stroke. Today there are several more kinds of lightning,
including ‘mega-lightning’ (described on Five at 9 p.m. on Wednesday),
sprites, elves, and balls.
As you read this there are 2000
thunderstorms going on in the world, and lightning strikes somewhere a
hundred times every second. Thunderclouds contain violent up-draughts,
and scientists think that ice crystals rushing past water drops build up
electrical charge in the clouds. The voltage grows until it can overcome
the resistance of the air; then a small current flows jaggedly along the
path of least resistance. This ‘leader stroke’ ionizes the air along its
path, which makes the air an electrical conductor, and enables the main
discharge.
In BBC’s Science shack I
stood in a metal cage which was walloped with million-volt sparks;
scary, but much less violent than lightning. Each lightning stroke is
driven by many millions of volts, and it may carry as much power as the
whole of the British National Grid – but only for a millionth of a
second.
Sprites and elves are elusive.
They appear occasionally above particularly violent thunderstorms, and
were first spotted by astronauts and pilots flying at high altitudes,
but have since been filmed from mountain tops. SPRITES (which stands for
Stratospheric Perturbations Resulting from Intense Thunderstorm
Electrification) are small bright flashes, while elves are just blue
glows.
My favourite, however, is ball
lightning. Just occasionally, sometimes after a lightning stroke, there
appears a glowing ball of light, about the size of a football and as
bright as a 100-watt light bulb, which floats through the air like a
bubble, can bounce off surfaces, and usually goes out with a bang
variously described as like a balloon bursting or a shotgun being fired.
Such balls have been seen floating down the cabin in aircraft struck by
lightning, in submarines when the batteries were inexpertly switched, in
a Florida retirement home, and in a Scottish café, where it burned the
front of a woman’s dress.
