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 between the particles
when they collide which charges up the cloud. Positive and negative charges separate and
an electric field is formed.
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 oC, three times the
temperature of the surface of the Sun. The peak power in a lightning stroke is also enormous
and can reach 100 million million watts.
By the way, thunder is just the noise of
the expanding and contracting air. The air is heated up by the lightning flash, expands and
then cools so contracting. This expansion and contraction makes the cracking and rumbling
sound that we call thunder.
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.
