Till now we learned that lightning strike is a discharge of static electricity accumulated in clouds but we didn't say anything about its development. The typical storm cloud responsible for lightning is "cumulonimbus" - the big, heavy cloud, ascending 15 km upward. Its base is 2 - 3 km above ground. Usually they are appearing in periods of year when the ground is warmer and the warm air is heading towards the cold atmosphere. At certain speed of that wind the drops directed to the ground are combining into smaller and bigger drops. Smaller drops are negatively charged and bigger drops are positively charged. The smaller and lighter drops are taken in higher part of cloud by the wind and then the cloud becomes negatively charged. The lower part of cloud is positively charged. That process is very complicated and the distribution of charges in cloud can be various, so it's hard to say which part of cloud will be positively or negatively charged. In that way electrified cloud could provoke the increasing of electric field near ground. If the value of electric field is between 15 and 20 kV/m it will come to breakdown. We can approximately calculate the difference between earth potential and lower part of cloud which could be a few dozen of million of volts.

The most often lightning strikes are negatively polarised. They are starting in clouds and ending on earth. That's why we will work out an example for this case. A negative charge from clouds is starting to move to the earth when intensity of electric field near the clouds exceeds the permeable strength of air and water drops (500 - 1000kV/m). The permeable strength of some medium is the largest value of electric field when it still doesn't come to breakdown. Further, the charge is advancing towards the ground in steps. The individual skips of about 50 m are happening every 40 - 100 ms. Usually after every discharge the direction is changing so the discharge looks curved. The most advanced and starting part of charge is called leader and channel it is passing through stays ionised and full of negative charge. As the leader moves down towards the ground, it attracts positive charge on the ground and on the top of high objects on it. That attraction is increasing as leader is approaching the ground. It produces more and more charge on the ground and objects on it and value of electric field is increasing very fast. When that field reaches the permeable strength of the air the positive upward leader appears and starts its way to meet the negative leader. When these leaders meet, a strong avalanche of positive charge from ground to cloud appears through the ionised channel. The charge is neutralised and it is called the main discharge. It usually lasts 70-100ms and that is when flash and sound occurs.