As a result of natural environmental phenomena, there may exist lots of positive and negative charge centers in the air. The distance between those charge centers may vary up to 500 km long and their charge distribution changes. If the electric field density in the vicinity of a negative charge center reaches a critical value ,typically 10 kV/cm, the small length of air is started to be ionized and an ionized channel is formed. This ionized channel is also called "streamer". The critical electric field value for ionization is directly proportional to the breakdown value of air. This breakdown value can be changed by humidity and temperature but ionization is most likely start in large water droplets in air.
As I said, the distance between the positive and negative charges centers in air may vary up to 500 km ,thus, the excessive negative charges may prefer to discharge through ground (another positive charge center) which is much more closer to negative charge center. After first streamer ,about a couple of ten microseconds later, the second streamer arises and it follows the same way with the first streamer but it goes a little bit further than the first streamer. This process continues a number of times ,each time the ionized channel propagates 10 to 200 metres, up to the ionized channel is reached to a critical distance to ground. This process is called "stepped leader". Each time the stepped leader propagates in air, an excessive amount of negative charges move towards ground. When the stepped leader reaches to a distance of 10 to 50 metres from earth, the field intensity of the earth (positive charge center) reaches a sufficient value to form an upwards streamer in order to make connection between stepped leader and earth. Then the excessive amount of current flows from earth to clouds along ionized path so that the negative charges neutralized. This neutralizing current is called "return stroke". The figure below explains the whole characteristics of lightning discharges.
Figure 1: Lightning Discharge Phenomenon
Reference: EE575 - Advanced High Voltage Techniques (METU)
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