- Written by Nadia O'Carroll
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 is formed 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, a electric discharge occurs and this is seen as a lightning bolt. The electrical discharge super-heats the air around it and creates a shock wave, which is heard as thunder. The delay in hearing the thunder, after seeing the lightning, is due to the difference in the Speed of light (300000 km/s) and the Speed of sound (344 m/s)
Some interesting lightning statistics – lightning strikes the Earth approximately 100 times per second, leader bolts of lightning travels 60000 m/s and heat the surrounding air to 30000 degrees (C). Lightning leaders are only about the width of a pencil.
Lightning may also originate from the positive part of a cloud, but this is a much rarer form of lightning, which behaves differently and is regarded as more dangerous. The proverbial bolt from the blue is positive lightning. It originates from the top of a cloud, travels almost horizontally until it veers into the ground, and strikes without warning many kilometres ahead of the main storm, where the sky may still be clear and sunny.
We associate lightning with destruction, fires, death and injury, but lightning also plays a part in the nitrogen cycle. Nitrogen is required by all life, but needs to be fixed into useable forms such as nitrates and nitrites. The energy of lightning breaks nitrogen molecules in the atmosphere; they combine with the oxygen in the atmosphere and then dissolve in falling rain to form nitrates, which are carried down to the earth. 5-8 % of total nitrogen is fixed in this way.
Three factors predispose an object to being struck by lightning – height, isolation and pointedness. About 8-10% of people struck by lightning die as result of cardiac arrest. Less than 1/3 of those struck have burns and these are usually superficial. Intense muscle contractions may throw people considerable distances and cause fractures and blunt injury. Eye and ear injuries, dizziness, numbness, amnesia, seizures, nerve damage, fatigue; personality change and depression are among the long-term symptoms of lightning injury.
If inside during a thunderstorm, do not use telephones, electrical devices and stay away from windows. If you are outside and time to thunder is less than 15 second (5 kms) seek shelter such as a car or substantial building. If shelter is unavailable, get away from high ground and tall trees; stay more than your height away from tree trunks; keep at least 5 m away from other people. If lightning is striking around you, remain in a crouched position with feet together and wait – once the storm cell has passed and time to thunder exceeds 15 seconds then it should be safe to move again.