Des physiciens résolvent un mystère éclair

Ville de tempête de foudre

La foudre est un phénomène naturel qui se produit lors d’orages lorsque la décharge d’électricité dans l’atmosphère provoque un éclair de lumière intense. Il est généralement accompagné du tonnerre, qui est le son produit par l’expansion de l’air rapidement chauffé provoquée par la décharge d’électricité. La foudre est causée par l’accumulation de charges positives et négatives dans un nuage ou entre un nuage et le sol. Lorsque les différences entre ces charges deviennent trop importantes, une décharge électrique se produit, qui peut se manifester par un éclair.

Environ 8,6 millions de coups de foudre ont lieu chaque jour sur toute la planète, chacun se déplaçant à une vitesse de plus de 320 000 kilomètres à l’heure et générant une énorme quantité d’électricité.

Vous êtes-vous déjà demandé pourquoi la foudre zigzague ? Les scientifiques se sont disputés sur les raisons pour lesquelles la foudre zigzague et comment elle est liée au nuage d’orage au-dessus au cours des 50 dernières années.

Il n’y a pas eu d’explication définitive jusqu’à présent, avec un Université d’Australie du Sud (UniSA)

The answer? Singlet-delta metastable oxygen molecules.

Basically, lightning happens when electrons hit oxygen molecules with enough energy to create high-energy singlet delta oxygen molecules. After colliding with the molecules, the “detached” electrons form a highly conducting step – initially luminous – that redistribute the electric field, causing successive steps.

The conducting column connecting the step to the cloud remains dark when electrons attach to neutral oxygen molecules, followed by an immediate detachment of the electrons by singlet delta molecules.

Why is this important?

“We need to understand how lightning is initiated so we can work out how to better protect buildings, airplanes, skyscrapers, valuable churches, and people,” Dr. Lowke says.

While it is rare for humans to be hit by lightning, buildings are hit many times, especially tall and isolated ones (the Empire State Building is hit about 25 times each year).

The solution to protecting structures from lightning strikes has remained the same for hundreds of years.

A lightning rod invented by Benjamin Franklin in 1752 is basically a thick fencing wire that is attached to the top of a building and connected to the ground. It is designed to attract lightning and earth the electric charge, saving the building from being damaged.

“These Franklin rods are required for all buildings and churches today, but the uncertain factor is how many are needed on each structure,” Dr. Lowke says.

There are also hundreds of structures that are currently not protected, including shelter sheds in parks, often made from galvanized iron, and supported by wooden posts.

This could change with new Australian lightning protection standards recommending that these roofs be earthed. Dr. Lowke was a committee member of Standards Australia recommending this change.

“Improving lightning protection is so important now due to more extreme weather events from climate change. Also, while the development of environmentally-friendly composite materials in aircraft is improving fuel efficiency, these materials significantly increase the risk of damage from lightning, so we need to look at additional protection measures.

“The more we know about how lightning occurs, the better informed we will be in designing our built environment,” Dr. Lowke says.

Reference: “Toward a theory of ‘stepped-leaders’ in lightning” by John J. Lowke and Endre J. Szili, 13 December 2022, Journal of Physics D: Applied Physics.
DOI: 10.1088/1361-6463/aca103

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