There is a charge balance between earth and athmosphere in nature. Storm clouds breaks this balance and creates charges. Thunders develops to fix the balance again. Because of physical reasons storm clouds bottom sides charges with negative charge. In some circumstances there can be opposite charge with 10%-30% probability. Earth charges with positive charge as a result of negative charge on clouds. Positive charge is stronger in sharp metal objects which earthed well.
Electrical field power has to reach to 2500 kw/m to create a lightning discharge. When the electrical field power is high enough cloud-cloud and cloud-earth discharges can be seen.
When a clouds bottom sides electrical field power is enough an electron beam goes to the earth from cloud. First electron beam goes 10-50 meter range with 50 000-60 000 km/h speed. After 30-100 micro seconds pause second discharge goes the same way as the first one and goes 30-50 meters far away from the first one. Every discharge goes 30-50 meters far away and helps to get first discharge closer to earth. Electrical field power gets higher than the weathers breaking point when first discharge gets closer to earth. A discharge comes from a sharp place of earth and unites with the first discharge. This discharges ionezed channels energy from earth to sky with 50.000 km/h speed. 200 kA discharge flows to earth with 100 milion volt tension.