How lightning works.
A typical thunderstorm will simply last for an hour or so. During this period, there will be sufficient time for electrical charges to 'accumulate' both on the ground and at the bottom surface of storm clouds. These charges are formed through processes that involve friction among water and vapour droplets. Hence, the bottom surface of a cloud acquires the negative charge whereas the ground or the earth becomes positively charged. Although there may be positive charges on the bottom surface of a cloud and vice versa, we shall now limit our discussion to only the above mentioned so as the better understand the situation. The physics and mechanics of a lightning strike is simply governed by the law of opposite attractions between negatively and positively charged particles. Once the force of attraction between the ground and the cloud becomes large enough, the air in between will be integrated, causing it to be vulnerable to electrical breakdown. Although air is not a good conductor, but when electrical breakdown occurs the 'broken down' air is a much better conductor. The 'broken down' air is now called ionized air. Confused? Imagine the situation now that the air between the cloud layer and the ground is kinda 'wild' and ready to be broken apart! Yeah, don't forget that all this is due to the force of attraction pulling from both ends of differently charged surfaces. The diagram below demontrates well.
The ionised air rapidly grows into a large chain moving downwards. This in return creates an ionised air pathways heading downward in a smooth constant flow. The paths can be visualised as sticks being laid end-to-end and every moment adding stick to make the pathway longer. These downward-moving 'paths' of ionized air are collectively called the stepped leader. The stepped leader dimly lights up every time a new 'step' is formed, but we do not see this because the stepped leader moves downward very rapidly and because the actual 'bright' lightning we see occurs so close in time after the stepped leader. As the stepped leader approaches close to the ground, and electrical breakdown occurs causing another pathway to be formed, but this time, from the ground instead. The path is similiar to that formed by the clouds, but instead it heads upwards and is positively charged. However, these positively charged paths from the ground DO NOT move up in steps. They are moving in paths, initiated by ONE single stepped leader. By the time the stepped leader gets that close to the ground, it has many branches, so there is sort of a 'race' for which branch will reach first. Whichever downward-moving branch touches an upward-moving leader first, 'wins'! When there is a 'winner', there is now a complete path of ionized air connecting the ground and the cloud, like a big, long wire. When this connection is made, the opposing charges equalize themselves rapidly by flowing through this 'wire'. Even though the ionized air path is a better conductor than air, it is, in effect, overloaded by the intense current flowing through it, so it and all of the 'branches' light up brilliantly and heat up violently, like a big, long 'instant' heating element! This flow of current is called the first return stroke, and that is the 'lightning' that we see.
