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					Lightning and thunder

Lightning is a huge electrical discharge (a
giant spark) either between one cloud and
another or between a cloud and the ground. I
have seen a thunderstorm where the whole
sky was lit up continuously for over an hour by
flash after flash between clouds.

Lightning is formed because of a build up of
electric charge in a cloud. In a storm there are
enormous convection currents in a cloud,
water droplets and ice particles going up and
down within it. This movement causes friction
                                                                   Lightning flash
between the particles when they collide which
charges up the cloud. Positive and negative
charges separate and an electric field is

When the charge is big enough the electric field ionises the air. The electrical resistance of
the air 'breaks down', it is no longer an insulator and the charge is discharged as a spark.
One lightning flash usually takes about 0.2 s but is made up of a few rapid discharges a few
milliseconds long, too quick for the human eye to separate them.

The voltages in a lightning flash are really huge. Remember that it takes 30 000 V to make a
spark 1 cm long in dry air so you can imagine the sort of voltages in lightning, they may
reach 100 million volts for sparks up to 3 km long! (The voltages needed to break down air
containing water vapour are a bit less than those needed for dry air.)

The current in a lightning
flash may reach 10 000 A
and the temperature 20 000
o                                                                         Ice crystals
 C,    three      times    the
temperature of the surface of
the Sun. The peak power in                                                               8 km
a lightning stroke is also
enormous and can reach
100 million million watts.         UP AND DOWN
                                   MOVEMENT OF
By the way, thunder is just
the noise of the expanding                                                               5 km
and contracting air. The air
is heated up by the lightning                                        Ice pellets
flash, expands and then
cools so contracting. This
expansion and contraction                          RAIN
makes the cracking and
rumbling sound that we call                                                              1 km

If you start counting steadily when you see the lightning flash, stop when you hear the
thunder and then divide the result by five you will get a rough idea of how far away the
lightning is in miles.

NASA research suggests that more than 2,000 thunderstorms are active throughout the
world at a given moment, producing on the order of 100 flashes per second.

The most famous early investigators of lightning and thunder were Thomas d'Alibard in
France, G.W.Richman in Russia and of course Benjamin Franklin in the United States.
Franklin is well known for his kite flying experiment and unfortunately Richman was killed by
a lightning strike.

A lightning conductor works because it ‘draws’ the charge towards it, so protecting the rest of
the building. Tall spires must be protected as they also have a build up of static charge.
Pylons that carry electricity do not need protecting because they are metal and act as their
own lightning conductors.

In a thunderstorm you should never stand up if you are in open field, you would then act like
a lightning conductor. The best thing to do is to crouch down so that if you are struck the
electric current will only go through your legs into the ground and not pass through any vital
organs such as your heart or your brain.

The correct position in a thunderstorm.
Head down – bottom up!