Spark image

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 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.




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

 
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© Keith Gibbs 2020