c) Different kinds of atmospheric discharges

Flash of lightning and lightning strike are 2 diferrent phenomenons. Indeed, flash of lightning is a discharge that occurs inside a cloud or between several clouds, where as lightning strike is a discharge between the cloud and the ground or inversely.There are about 90% of flashes and 10% of lightning strikes , so only 10% of discharges touching the ground. But we are only going to explain the mechanism of lightning strike because it’s that phenomenon that pushed humans to invent lightning rods.

It would be more logic to think that lightning strike occurs from the bottom of the cloud to the ground. But there are 25% of lightning strike that occurs from the ground to the cloud.

1)descendant lightning strike

The first phase of a lightning strike is a formation of pre-discharge called a tracer. This predischarge is not very bright. It moves on like zigzag, by leaps of dozen meters, through the neutral air, with a speed of 200 km/h. This tracer searchs the trajectory where there’s the less resistance. It’s made of electrical particles extracted from the cloud by the elctrical field cloud/ground. When the tracer comes near the ground, some sparks come out of the ground and get in contact with the tracer’s point . That tracer created a small-diameter( 2 to 3 cm) ionized canal that link the cloud to the ground. Then a discharge occurs : this is the lightning strike.

2) ascendant lightning strike

That kind of lightning strike occurs when the electrical field is intense enough to let the tracer develop from the ground to the cloud. We can note that ascendant lightning strikes are quite rare in low-relief regions, to be formed, the tracer needs a great prominence like mountains for exemple.

b) Charging of clouds

During a storm, the cumulonimbus cloud is electrically charged. The top of the cloud is postivly charged whereas its bottom is negativly charged. This mechanism of charges separation in the cloud is not very well understood but there are many possible explanations. The first dated from 1892 and even nowadays none is totally convincing.

For exemple, one of these says that it is the collision between particles ( ice’s crystals ) in the cloud that created the charging. The particles would take a negative charge, where as ice’s fragments extracted by friction then would be positvly charged. Ice , lighter, is at the top of the cloud whereas water droplets are at the bottom .

Whatever the real explanation is, even if this phenomenon is complicated and misunderstood, it leads to charges seperation. The charge at the top of the cloud, mainly made of light ice’s crystals, is globally positive, where as the top of the cloud , made of heavier water droplets, is globally negative.

Because the low part is negativly charged, the ground is positivly charged by influence.

During storms, we have an electrical tripole : the top of the cloud is positive, the bottom is negative and the ground is positive. If we take a look at what happens between the bottom of the cloud and the ground, we can compare that situation to a huge condensator made by air situated between the cloud and the ground.

a)Introduction to lightning

In a normal weather, there is an electrical field at the surface of Earth. This electrical field is about 100 to 150V/m worth. Indeed, Earth atcs like a condensator with a good insulator : the low parts of the atmosphere, the tropopause, situated between 2 conductors, the ground and the inonosphere.

The ionosphere is the higher part of the atmosphere, not very dense, in which the atmosphere’s ionization by the bombardment of particles or by the radiation( UV or X-rays coming from the sun, cosmic radiation, etc …) is the most important. Indeed there are 2 phenomenons in the atmosphere: ionizations (particularly in the higher parts of the atmosphere ) and ions losses, caused by the phenomenon of ions recombinations(the less dense the atmosphere is, the less important these recombinations are). These 2 phenomenons balanced in the ionosphere and so a large number of ions are present in that zone of the atmosphere : then this zone is conductive. The ionosphere is globally neutral where as the ground is naturally neagtivly charged, wich explains the permanent presence of the magnetic field at the surface of Earth.

The atmosphere is not a perfect insulator because there is a slight outflow current between the ground and the ionosphere. This current would end up discharging the ground if there wouldn’t have been mechanisms to reload it, like storm.