We often see it on a balmy summer evening: lightning. What really happens in a cloud before such a bolt finds its way to the ground? To understand how lightning is formed, you have to look at so-called charged particles, explains physicist Sander Nijdam (Eindhoven University of Technology). “There are ice crystals and hailstones in a cloud. In principle, they are charged in a neutral manner. This means that such an ice crystal or hail is made up of as many positive as negative particles. “But these ice crystals and hailstones sometimes collide. Then a small negative charge jumps from the ice crystals to the hailstones. After such a collision you have positively charged ice crystals and negatively charged hailstones in a cloud.
In order to get lightning, something else important happens under the cloud, something that we mainly see on beautiful summer days: hot air rises from the earth. Because ice crystals are much lighter than hailstones, they are pushed up the cloud. Heavy hailstorms fall and pile up just below. So you now have a positively charged cloud at the top (ice crystals) and negatively at the bottom (hailstones). “We call this charge separation, which creates an electric field,” says Nijdam. “Nature wants to cancel this electric field – the positive and negative particles have to be mixed again. To do this, a current flows from the positive part to the negative part of the cloud. This current is lightning.
This process takes place inside the cloud, 90% of the lightning stays “inside the rooms” and is not visible unless the outer cloud ignites. We call this phenomenon lightning.
But what happens when you see a lightning bolt go to Earth? Nijdam: “The negative charge at the bottom of the cloud wants to disappear. It can go up, but also to neutral earth. This causes flashes of a mile long. Although only 10 percent of lightning occurs outside the cloud, it is very common. Worldwide, there are up to 1,800 thunderstorms at any given time and lightning strikes the earth 45 times per second.
What most people don’t know is that a bolt develops in several stages. First, there is a pre-discharge, invisible to us. Lightning then finds its way from the cloud to the shattered earth. This is happening at a speed of 150 kilometers per second. If this pre-discharge is close to the ground, a trap discharge forms towards the cloud. When the pre-discharge and the capture discharge touch each other, a kind of channel is created. It is through this channel, which is only a few centimeters in diameter, that the actual discharge takes place. So flash. Because this process is repeated several times, the lightning seems to flicker.
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