Short answer: A “storm” cloud accumulates differences in electrical potential, by exchanging up and down air. The base of the cloud is negatively charged, the soil of the earth positively. The two charges therefore seek to join each other: the lightning is born.
According to Météo France, about 16 million thunderstorms are recorded each year on Earth. As you read these lines, between 1,500 and 2,000 thunderstorms hit our planet, causing, every day, between 6 and 8 million lightning bolts that can exceed intensities of nearly 200,000 amps (observe in real time the current storms on our planet on the site meteorage). This atmospheric phenomenon, which triggers thunderbolts, series of lightning strikes, heavy rains and squalls, has long fuelled many mythologies from Greeks to Chinese to Mayans and Hindus.
Let’s start from the source… thunderstorms
The storm is a short-lived phenomenon (30 min to 2 hours max.) and occurs in summer (July is the most lightning month), in the mountains and on land. Its formation is closely related to the appearance of cumulonimbus, these huge anvil-headed clouds that have a large vertical extension with a summit made up of ice crystals that can peak up to 21 km above sea level. For these large clouds to appear, an unstable atmosphere (warm air on the ground and cold air at altitude), humidity (nature of soils and vegetation) and a trigger (warm air on the ground that rises rapidly in altitude thanks to the terrain or the sun’s rays).
Why are there lightning bolts?
Unstable conditions within a cumulonimbus cause a negative charge build-up at the base of the cloud. When this load is about 3 to 4 kvolts/cm, the air heats up and the load exchange is done in the cloud or with the ground. This exchange materializes with a flash. In just a few thousandths of a second, the air warms up to ever-changing temperatures: up to 30,000 degrees Celsius.
There are four forms of lightning: those visible within a cloud, between a cloud and clear air, between two clouds and, most well known of all, between a cloud and the ground (or an aircraft). The latter is commonly referred to as“lightning”and according to Martin Allan Uman, an American lightning engineer, and Philip Krider, professor emeritus, more than half of the visible lightning in the northern hemisphere is of this nature. 1/3 of the lightning discharges observed are from the ground to the cloud, but they are usually visible during large thunderstorms.
How is a lightning bolt built?
A negatively charged cloud-to-ground flash is constructed in 3 stages: the leap tracer, the attachment process and the return arc.
You should know that love at first sight is born inside the storm cloud. The negative discharge exits the cloud and moves towards the ground in successive series of light leaps of about ten meters in length each. This is the leap tracer. This “leader”, as the Anglo-Saxons call him, rushes towards the ground at a speed of 0.15 to 1 meter per microsecond, a real Formula 1!
As a result, the leap tracer causes an intensification of the electric field as it approaches the ground, positively charged, creating several ascending discharges from high physical points (mountains, trees, building antennas, etc.). A junction point is several tens of metres high between the tracer and the ascending discharges. We are in the process of attachments.
This is when the first return arc forms and spreads to the cloud at a speed equivalent to one third of the speed of light in the air (about 1 km per microsecond). It is at the time of the return train that the temperature of the canal is at its highest (30,000oC, five times the temperature on the surface of the sun!) and that the thunder is heard.
As a result of this process, lightning can disappear or new sequences can occur, on average 3 or 4 times and up to 15 arcs back when a sufficient charge remains in the cloud, during large thunderstorms.
The curious passage may notice a perceived color of lightning, depending on the environment in which it is located and the proximity of the latter to the discharge. Generally, a white flash occurs through dry air and a pure atmosphere, a red flash in case of rain, a yellow flash in a dusty environment, and a blue flash during hail falls.
Lightning, what are the consequences for living things?
Cloud-to-ground lightning can be deadly for humans and animals, but direct lightning strikes are still very rare. The live beings struck are mostly by lateral lightning (crazy by splashing) and by the ground (the victim is near a lightning strike, the current passing through both legs).
Although the temperature and voltage of a lightning bolt can be extremely high, the contact time remains, on the other hand, very short, in the order of one thousandth to ten thousandths of a second. The body is then crossed by only a few amps, which leaves a great chance of survival for the victims of a lightning strike (between 75% and 95% of the victims survive). The cattle, on the other hand, remain the most affected during a lateral lightning strike because the current can pass through the two front legs, thus crossing the heart which is halfway. According to the National Institutes of Health, global human mortality is estimated to be between 0.2 and 1.7 deaths per million inhabitants each year.
Exceptional facts
At every event, his exploits! On25 June 2020, the World Meteorological Organization validated two records: the lightning record that covered the most distance and the longest duration of a lightning bolt. The first was measured on 31 October 2018 over a horizontal distance of 700 km in southern Brazil, which corresponds, by comparison, to the distance between London and the Swiss border. The second record, the longest lightning time, was recorded in northern Argentina for 16.73 seconds. This is all the more impressive considering that a lightning bolt lasts an average of 0.2 seconds.
Sources:
- nrcan.gc.ca (Frequently asked questions about lightning)
- nasa.gov/ (How Is Lightning Made?)
- https://fr.wikipedia.org (foudre)
- meteorage.com (real-time lightning)
- public.wmo.int (WMO certifies Megaflash lightning extremes)
