How does lightning occur?

Clouds become charged by particles in them rubbing against each other. Positive and negative charges build up in different parts of the clouds until eventually a spark of negative charge leaps across from one side of the cloud to the other, making "sheet" lightning. Click here to see animation.

A very big charge on the cloud can "induce" an opposite charge on a tree (or any tall object) below. An electric current then flows towards the tree, making a flash of fork (zig-zag line) or chain lightning in less than one-tenth of a second. It lasts only a very short time, but a great amount of work is done (about enough to run a 100W electric light bulb for a month). The air through which the current passes becomes very hot, but it returns to normal very quickly. If the electric current hits anything on its way to Earth, it burns it. High buildings have lightning conductors on them - strips of highly conductive metal - which take the current safely down to Earth.

Where lightning strikes the ground, solid rock may be melted by the huge power of the electrical discharge. It is this kind of lightning that sometimes kills people. However, there is not much risk of being struck by lightning unless you are at or near the highest point in the neighbourhood or have sheltered under a tall tree standing by itself. Indoor there is little risk.

What happens if I am flying in an aeroplane or in a car and lightning strikes?  Let's check it out. 

Advantage of lightning

Although lightning does a great deal of damage throughout the world it has one good effect, for the flash causes the nitrogen and the oxygen in the air to combine and to dissolve in the raindrops which fall to earth and soak into the soil, thus supplying it with nitrates. As nitrates are a valuable fertilizer - which means that they help plants and crops to grow - thunderstorms are of great benefit to plant life.