WHAT IS LIGHTNING
The ancient Greeks and Romans believed that lightning was a manifestation of the power and wrath of the gods. Lightning was the weapon of Zeus, for the Greeks and Jupiter for the Romans. The American Indians believed that lightning was caused by the mythical Thunderbird. (The flash of lightning being the flash of the thunderbirds eye as it winked. In South Africa today we still have witch burnings were a person is accused of having caused the lightning to strike a home or person.
English scientist William Wall first advanced the thought that lightning might
be electricity in 1708. However it was only in 1749 that William Franklin
postulated that thunderclouds were charged and that lightning was an electrical
phenomenon. Franklin then devised his famous experiment with a kite and in
doing so proved that lightning is in fact electricity.
Since that time many advance have been made in the field of lightning research. Lightning is defined as a transient, high current discharge whose path length is measured in kilometers. There are four classifications of lightning discharge:
Cloud to cloud discharges
Cloud to air discharges
Cloud to ground discharges
For the purposes of lightning protection it is the cloud to ground discharges that are of direct concern. Due to the practical importance and potential damage cloud to ground lightning has been studied more extensively than the other forms of lightning. There have been several theories as to how the electricity is generated within a thundercloud. The first attempts were made by CTR Wilson in 1929. Today theories are still being developed; the common factor being that charge separation takes place although the exact mechanism and how the charges are distributed is still uncertain. As far as the lightning protection is concerned the important fact is that cumulonimbus clouds are formed by rapidly rising humid air and become electrified, the end result being charge separation. It is estimated that about 90% of the cloud to ground discharges are initiated by downward moving negatively charged leaders and 10% by downward moving positive leaders. This is important, as generally the peak current and charge transfer in a positive lightning strike are much larger than those encountered in a negative strike. Ground to cloud flashes may be initiated by leaders of either polarity and are comparatively rare, usually occurring from mountain peaks or tall man made structures.
THE INCIDENCE OF LIGHTNING
The traditional method of designating thunderstorm activity is by means of the isokeraunic level. (The meteorological definition being the number of days per annum during which thunder is heard i.e. thunderstorm days.). This is often given in map form, as can be seen from the world map taken from BS 6651:1999 Africa has a high incidence of lightning. In 1965 the CSIR began a project in active research into lightning flash counters (Hail-lightning project) Difficulties were encountered in discriminating between ground flashes and cloud flashes. The flash counters were improved over time to a point where it could be confirmed that 90% of the flashes recorded were due to ground flashes. In the mid 70’s it was decided to start a national program for the measurement of ground flash density in South Africa. The result was that a map of ground flash density was built up for South Africa. An updated table of ground flash densities is available in SABS 0313:1999. Globally it is estimated that some 2 000 on-going thunderstorms cause about 100 lightning strikes to the earth per second.