Lightning

From the dawn of history mankind has been in utter awe of it and has given many reasons for it. At one time it was thought to have supernatural origins and later, in the Greek mythology was attributed as a great weapon of the Gods.

We now know that it is an electrical discharge from cumulonimbus clouds, but even we cannot explain all the different types of lightning that we now observe, and although we can forecast it, there is still little we can do about it.

In the tropics can reach up to 60,000 feet above the surface of the Earth, which is why thunderstorms are frequent and often very violent within the tropics.

Air moving within the cloud causes huge differences in the electrical distribution. The bottom of the cloud develops a negative charge and the top of the cloud develops a positive charge. A positive electrical charge is also produced at the Earth's surface.

These electrical differences attract each other, and when the difference becomes great enough,an 'electrical discharge' is produced within a cloud, between two clouds or towards the ground. This discharge we see as lightning.

After the leader stroke reaches the ground there is a return stroke back up to the cloud which is much brighter than the initial leader, and it is this that we see and call lightning.

This discharge is just the same as rubbing your feet on the carpet then touching something. You hear a sound and feel a bit of a tingle. If you turn the light off, you will even see a tiny lightning bolt.

Of course, real lightning from thunderstorms is far more powerful. One lightning bolt can reach 30,000°C, five times hotter than the sun. Air can often smell 'burnt' after a lightning strike, as the huge amount of energy released can alter molecules in the air. This heating also causes thunder as the air rapidly expands and then contracts, causing vibrations. These vibrations, or sound waves, we hear as thunder.

As lightning is seen and thunder heard, lightning travels at the speed of light, and thunder at the speed of sound. The difference in these speeds is so great that light could travel right round the world before sound finished the 100m sprint.

This speed difference provides an easy way to see how far away the lightning struck. Count the seconds between the lightning and the thunder, each second represents 300m distance from the thunderstorm. You will be surprised at how often storms that you think are on top of you are quite a long way off.

In the late 1950s I was a meteorological observer at Christmas Island in the south Pacific whilst we were testing hydrogen bombs, and one of the very important jobs I had was lightning detection. We had to make absolutely sure that there were no thunderstorms in the test area before the bombs were detonated. We did this by using a cathode ray tube, which detected lightning strikes, and by a system of triangulation with colleagues on two neighbouring islands, we were able to pin point storms anywhere in the pacific. This lightning detection still goes on today but is now done by machines rather than humans.

There are many different types of lightning, fork and sheet being the most common. Fork lightning, described above, can be seen moving from the cloud to the ground in steps. Sheet lightning can be seen when the lightning is close to the horizon. The individual strikes can't be seen, but simply light up the distant cloud.

Ball lightning
Ball lightning is very interesting. It is very difficult to assess exactly what it is and how it is formed. It has been seen as a slow moving ball of light, or presumably heat, which then burns out or explodes.

Red Sprite and Blue Jet
Satellites looking down on to the Earth have enabled us to discover two new forms of lightning, Red Sprite and Blue Jet - both seen above large thunderstorm clouds.

Red Sprite has been described as a red burst of light above the storm clouds reaching towards the stratosphere.

Blue Jet is a similar phenomenon but is seen as a very fast blue conical shaped burst of energy above the centre of the storm.