Lightning is one of the most typical and characteristic manifestations of thunderstorms. Lightning is visible in the form of electric discharges that originate from the thundercloud and hit the ground, however the phenomenon is much more complex, and since it takes place at the speed of light, it is so rapid that we are unable to understand it immediately.
Lightning is an electric discharge inside a storm system. Lightning can occur within a cloud, between adjacent clouds, or between the clouds and the ground. Generally about 80% of the electric activity of a storm cell is discharged within the cloud or between two clouds and only 20% of the discharges take place between the clouds and the ground. Notwithstanding this, it has been calculated that streaks of lightning hit the ground at a rate of over 100 per second!
Above the 0°C thermocline, water in the cloud takes the form of ice crystals which have a positive charge on the surface. Instead, under the thermocline the drops of water have a negative charge. In this way within the clouds that extend vertically beyond the 0° C isotherm, strong differences in potential form between zones with opposite electric charges. Thus electric discharges are produced between the zone with the positive charge and the negative charge zone. This occurs within the cloud, but also between two clouds when zones with different electric charges come into contact.
It is slightly more complicated to explain how lightning is discharged to the ground. The Earth’s surface generally has a negative charge, as also the base of the storm clouds. Electric discharges of the same sign tend to repel each other therefore, during a storm, on the Earth’s surface, where the negative charges have been pushed away by the negative charges of the base of the cloud, areas with a positive charge are formed.
When charges of opposite signs accumulate, due to their reciprocal attraction, a channel of ionized air forms, which propagate from the cloud to the ground, and that is known as the stepped leader. As the stepped leader reaches the ground, from the Earth’s crust a second stroke is triggered, that reaches the stepped leader before it connects to the ground: this is what we see as the main lightning stroke. Obviously this all takes place at the speed of light in only a few milliseconds, therefore it gives us the illusion that the lightning stroke travels from the cloud towards the ground. If on the ground there are concentrations of positive charges that are close to each other, a number of return strokes may leave the ground simultaneously toward the same leader, and the lightning appears in the typical shape branching downwards.

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