Lightning The most dramatic and frightening form of optical phenomena for most people is probably lightning. From deep within a mature storm cloud, electrical discharges occur that are caused by negative charges reacting with positive charges, creating lightning, and consequently thunder. Scientists estimate that there are 2000 active thunderstorms going on across the world at any one time. Most storms have are capable of producing many bolts of lightning. The top of a storm cloud is positively charged, while the middle to lower regions are negatively charged. A small area at the bottom of the storm cloud (and the ground beneath it) are positively charged, so consequently there is a huge potential for lightning to occur once the static electricity builds up inside the cloud. The type of lighting that occurs depends on the locations of the opposite electrical charges in and around the cloud. The huge electrical ‘spark’ literally jumps the gap between the opposite charges, causing a swift and intense heating of the air immediately around the lightning bolt. This causes the air to expand and then contract rapidly, creating sound waves that we call thunder. The voltage flowing through a lightning bolt can reach 200 million volts, and combined with the amperage (which can be anything from a few thousand to tens of thousands of amps) some of these beasts are killers. Lightning is actually made of air. The forks that you can see are actually nitrogen and oxygen, which has been converted into plasma. Plasma is a conductor, so simply put, in a sense lightning is like a long piece of wire stretching through the sky. There are at least six different forms of lightning, each with their own individual characteristics and colours. Fork lightning can travel cloud to ground, ground to cloud, or cloud to cloud, and it is the most spectacular of all. It is also the most destructive type of lightning. Fork lightning often damages buildings and can kill. Bolts that shoot from ground to air are less common and can strike up to 10 miles away from the cloud. Sometimes you cannot see the cloud when these bolts strike, hence the term ‘Bolt from the blue’. It is true to say that sheet lightning cannot kill, because technically it does not exist. Sheet lightning is just fork lightning that happens within a cloud, or when lightning is partly hidden by clouds. Heat lightning is like sheet lightning, but is so far away that you cannot hear the thunder. It is called this because it usually happens on a hot summer night. Sometimes heat lightning can appear to be orange. This is because of particles in the air refracting the light. Ball lightning is the rarest form, and very few people have seen or photographed it, but there are some instances of it causing damage. Nobody is known to have been injured by ball lightning though. Scientists do not know how it forms. The main difference between this type of lightning and St Elmo’s fire is that ball lightning can float around freely, where St Elmo’s Fire has to be attached to something like a ships mast or the tip of the wing of a plane. St Elmo’s fire is a green or bluish haze that hangs in the air above pointed objects on the ground. It is caused by lots of sparks and the electrical field that is generated by them. It is caused by the positive charge of the sparks interacting with the negative charge in the air around it. The proper name for this kind of lightning is corona discharge. The name St Elmos fire came about because this type of lightning was first seen by sailors on the masts of ships, and St Elmo is the patron saint of sailors. High altitude lightning has only recently been discovered. It shoots out from the top of a storm cloud and has a few different shapes and colours. It usually happens just as a lightning bolt discharges from below. This form of lightning has been given various names like Red Sprites, Green Elves, and Blue Jets, depending on the actual type and colour. The speed of sound is obviously much slower than the speed of light, and this is the reason why you can calculate your approximate distance from the storm by counting the time in seconds between the flash of lightning and the sound of thunder. Every five seconds counted is equal to approximately one mile between you and the storm. When the local topography permits, you can see lightning and the associated storm cloud from anywhere up to 200 miles away. Thunder can usually be heard from about 5 to 20 miles away, and this is why is seems that some storms are silent to the observer. The cumulonimbus cloud that you are looking at may be too far away for you to be able to hear the thunder, but close enough to see the lightning. Lightning is indeed very dangerous, so people are right to be cautious. A single bolt of lightning can carry anything up to 200 million volts, and is attracted to many types of electrical circuits and anything with pointed features such as building or pine type trees. If you were stranded in the woods during a storm, you would be a bit safer standing under a tree with large flat leaves like oak, than you would under a fir or pine tree. The safest thing of all would be to avoid standing under trees, particularly solitary ones. If there are no trees crouch low and do not lie flat on the ground. Deciduous trees and conifer trees react very differently when exposed to electricity, which plays a major part in the atmosphere. The different colours of lightning can tell us much about the composition of the air that surrounds the storm. If a lightning bolt has a red tinge to it, there is probably rain in the cloud, while if the bolt appears to be blue then the cloud has a large amount of hail in it. Yellow lightning indicates a high level of dust in the air, where as white lightning is a sign of low humidity. It has been estimated that lightning strikes the ground somewhere across the planet twenty times every second, and this means the potential for damage to buildings or harm to people is extremely high. It is said that the safest place to be during a storm is inside your car. Sadly though, people will always be fascinated by storms and tend to get caught out in the open when they occur. © Brian Taylor 2002.