Thunder and lightning are caused by an atmospheric discharge of electricity. In storm clouds, positive and negative particles may become separated (polarised), the positively charged particles rise to the top of the cloud and the negatively charged particles drop to the lower part of the cloud. If a sufficiently strong electric field develops, there will be enough electric potential for lightening to form. 90% of lightning is cloud to cloud where the exchanges of lightning usually expend the electrical charge. 10% of lightning is cloud to ground. The earth below the thundercloud develops an equal but opposite charge to the cloud above. A path of negatively charged ionised air (leader) moves downward from the cloud in jumps. On the ground positive streamers, which may become a positive leader, move upwards, and when the negative and positive leaders connect, the electric field greatly increases. When it becomes strong enough to form a channel, an electric discharge occurs and this is seen as a lightning bolt. The electrical discharge super-heats the air around and creates an audible shock wave, which is heard as thunder. Leader bolts of lightning can travel 60,000 m/s and can heat the surrounding air to 30,000 degrees (C). Although a bolt of lightning produces intense light and sound, its diameter is usually only 2-5cms.
The proverbial “bolt from the blue” is rare positive lightning originating from the top of a cloud. It travels almost horizontally and strikes without warning many kilometres away from the main storm, where the skies may still be clear and sunny.
Because there is a vast difference between the speed of sound (344 m/s) and the speed of light (300,000,000 m/s) there is a delay between seeing the lightning and hearing the thunder. The time between light and sound, indicate the distance from the lightning, every 5 seconds equals about 1 mile (1.6 kms).
Three main factors pre-dispose an object to being struck by lightning – height, isolation and pointedness. About 20-30% of people struck by lightning die immediately. Intense muscle contractions may throw people considerable distances and cause fractures and blunt injury. Burns are usually superficial however most survivors sustain permanent injuries such as neurological and cardiovascular damage, eye and ear injuries, dizziness, numbness, amnesia, seizures, sleep disorders, nerve damage, fatigue; personality change and depression.
If inside during a thunderstorm, do not use telephones, water taps, electrical devices and stay away from open doors and windows. If you are outside and time to thunder is less than 30 seconds (10 kms) seek shelter such as a car or substantial building. If shelter is unavailable, get away from high ground and tall trees; stay at least 5 m away from trees and other people. Adopt a crouched position with feet together, keep hands off the ground. Do not lie down. Wait until the storm passes before moving.