August 31 2023
Boating, sailboats and motorboats - As a mass made entirely or partly of metal, floating on the water, boats are subject to storms and fear lightning. In this series of articles, we will first look at the conditions under which lightning is formed and then triggered. In a second article, we will look at the consequences and structural damage on ships that are struck by lightning, in a third article we will look at the consequences on electrical and electronic equipment, and in a final article we will look at the means of protection.
Lightning: a huge difference in electrical potential
When the weather is stormy, masses of moist air at low temperatures move through the atmosphere.
At altitude, water droplets, ice crystals and hailstones collide and exchange negatively charged electrons. The result is an overall negative charge carried by the moist cloud particles. Ions are also present in the troposphere and some are captured by the moving liquid particles. Convection movements then gather the heaviest particles (rain and hailstones) at the base of the clouds, close to the ground. All these phenomena give rise to a localised electrical potential difference between the cloud base and the surface of the ground or water.
Lightning, the transition from insulating to conductive air
In the layer of atmosphere between the ground and the cloud, the water particles ionised as they pass through the upper layers of the atmosphere generate an electrical potential that is intermediate to that of the ground. A series of precursor discharges is then produced in this layer. These discharges cross and intersect. Since air is insulating, the system becomes unstable in the presence of the electric field generated by the system.
But all insulators have a so-called dielectric limit, beyond which they become conductive and allow an electric arc to be triggered. In the sky, the precursors "align" or communicate.
The lightning strike: an arc and plasma
At ground level, smaller discharges point skywards, towards the precursors. When the precursors and the discharges from the ground meet, a very high intensity electric arc is produced in their path. The air, under the effect of the current (30,000 to 300,000 A) and a potential difference of tens of millions of volts, is transformed at very high speed (40,000 km/s) into a cylinder of incandescent plasma more than 5 times the temperature of the sun, 30,000°C.
The cylinder of plasma hitting the ground is lightning.
Thunder: a strong expansion
The plasma state is one of the states of matter that requires a strong electric field to remain. Once the arc has discharged into the ground, the cylinder of plasma cools very quickly, reforming into a gas (the atmosphere).
This extremely sudden expansion creates a shock wave that travels through the atmosphere like a sound - thunder.
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