Lightning -The Facts
By Dilys & Doug Griffiths - 02/05/2008
Do you shudder at the first distant roll of thunder? Cringe at the first flash? Alternatively, are you the “It won’t happen to me” optimist? Good thinking! Your average chances of being struck are one in 6 million and while we are at it, your average chances of winning the Lottery were quoted as one in nineteen million a few years ago.
Golfing is a dangerous sport in a thunderstorm. Some time ago the American average was - for every ten people killed, five were playing golf! They wouldn’t have joined the Departed if they had only departed for the 19th hole at the sound of the first distant roll! Sheep and cattle are hit when grazing in the open, because the animal is often the tallest object around.
On average in the UK, ten people are killed annually by lightning. Death is mostly caused because of cardiac arrest rather than burning or cell damage. People have been known to die by leaning on a metal fence, with lightning striking the fence half a mile away.
So, what is Lightning, is it so dangerous and where does it most occur ?
Lightning is an electric current between cloud and earth. An electric current is a made up of countless electrons, or negative charges, moving from one point to another. When they move from cloud to earth it is known as a lightning stroke, If they move from cloud to cloud it is known as sheet lightning. There are five or six other types of lightning, these occur more rarely, so we won’t discuss them here.
Yes, lightning can be dangerous if you don’t take precautions where necessary. Avoid open spaces and don’t use an umbrella. If you are caught walking in a wooded area choose a pine tree for shelter; it is the least likely to be struck. Your safest place is in a car (not a soft-top variety) with the doors shut, because lightning’s energy will return to earth via the tyres and you’ll be much safer inside. An all-metal car acts as a Faraday Cage. In the 19th century Michael Faraday discovered that a cage distributes electric charges evenly around its surfaces, leaving the inside of the cage free of any electricity.
Still scared? Then you’ve got a phobia. We suggest you avoid central Africa, central South America, Indonesia and, closer to home, the Grand Massif in Europe. In these places storms are most severe and range from about 120 to 180 storms each year. Most of Europe suffers less severe storms, with about 20 to 60 per year. If you are really frightened then you should emigrate to Siberia, Northern Canada, and Antarctica or, for a warmer region, the Sahara Desert.
Shall we have a stab at achieving a flash of understanding here? Lightning occurs when static electrical charges build up on clouds, usually very large clouds. They are sometimes called anvil clouds and show themselves to resemble the shape of tall cylinders with heights of up to 7 miles and diameters of 3 to 4 miles. Within these clouds, due to wind-induced friction plus radiation from the sun, some electrons separate from their atoms.
These join together into separate groups of either negative or positive electric charges. There may be two or even three groupings of negative and positive charges and a lightning flash can develop from any one grouping. A lightning flash is a movement of a group of negative or positive charges between cloud and earth. This is known as a lightning stroke or as sheet lightning if the flash occurs between clouds. Other names for less common forms of lightning are, Ball, Streak, Ribbon, Bead and Chain. These flashes have different appearances in the sky and loosely resemble their names.
The sequence for the development of a lightning stroke is, firstly, the clouds emit ‘”Leaders” which ‘feel’ their way into the air. Sometimes we can see them as short lightning strokes appearing to come and go. They make the air more conductive for one or more big strokes to flash from cloud to earth. A big stroke can carry an electric current of 1,000 – 200,000 amperes. To imagine a current of such magnitude is almost impossible, when we consider an electric kettle, for example, needing only 4 – 5 amperes for boiling water. Once the flash is over, any positive charges remaining on the cloud slowly dissipate into the atmosphere, or, sometime later they can dissipate as a very severe stroke, materializing in the high-risk areas mentioned earlier.
Sometimes it is possible for a lightning stroke to be followed by an upward flash. This occurs most often from tall masts, chimneys etc. Any edifice having a measured height of less than about 60 meters doesn’t normally give rise to upward flashes. These upward flashes produce streamers occasionally and they can be very attractive to the eye.
“Balls” of lightning or “Thunderbolts” rarely occur but it can happen, after repeated large strokes of lightning. The air forms a plasma almost devoid of free negative charges. These negative charges form themselves into a self sustaining energy ball with a diameter of 2 – 100 centimetres. This ‘ball’ can roll along fences or other metallic objects, also up or down chimneys. Thunderbolts have been known to strike large army field guns, leaving the gun barrels in a highly magnetised state. They have normally a short life but can last for as long as 15 minutes having a temperature of 500 degrees C. Needless to say, operating a field gun in a thunderstorm is not to be recommended.
Lightning conductors attract “Leaders” and can protect an area twice the height of the conductor around its base. It should be attached to all metal parts around the roof especially the corners and properly earthed.. Metal roofs or roof work can often distort or be torn by the bending forces of the enormous electric currents.
Still on the subject of vulnerable heights, power lines are protected by an earthed wire. This is the wire you can see running from the top of each pylon to the next. This line will attract the lightning and take it safely to earth. Any stroke hitting the wire can produce “side strokes”, creating voltages in excess of 300,000volts within the power lines lying beneath.. The idea that lightning never strikes the same place twice was disproved recently when the same electric pylon was hit on six separate occasions.
Finally, did you know you could enjoy making your own lightning? All you have to do is walk across a nylon carpet on a dry day. This can charge your body to about 20,000 volts; much less than lightning’s 400,000,000 volts it’s true but, take care, it is very painful when you discharge this should you touch a metal door knob or a metal filing cabinet! This is one big continuing headache for carpet manufacturers.