The lightning discharge has its origin in the electrically charged thundercloud. The exact mechanisms of the large scale charge separation within the thundercloud and the initiation of the discharge itself are still not fully understood and subject of ongoing research.

Charge distribution in a thundercloud

We have to distinguish between the microscopic charge separation of the cloud particles (water, ice) and the macroscopic charge separation within the thundercloud. It is assumed that there are two main charge centers in a thundercloud, with positive charges on ice crystals located in the upper regions and water droplets with negative charges in the lower regions of the cloud. In a first approximation the thunderstorm cloud has a charge distribution corresponding to a dipole. Usually a small layer of positive charge is observed at the cloud base region.

The classification of lightning discharges is based on the observed polarity of the charge "effectively" lowered to ground and the direction of propagation of the initial leader. Four different types of lightning discharges between cloud and earth have been identified. The four types of lightning are (1) downward negative lightning (2) upward negative lightning (3) downward positive lightning, and (4) upward positive lightning.

This form of classification was introduced by Prof. BERGER, one of the pioneers of  lightning research. The form of discharge most frequently examined is the negative cloud-to-ground lightning (downward negative lightning), as more than 90 % of all flashes meet this type of discharge. Therefore it is of fundamental importance for the design of lightning protection systems.

Each lightning stroke is composed of a downward-moving process, termed "leader", and an upward-moving process, termed "return stroke". The leader creates a conducting path between the cloud charge source region and ground and distributes electric charge from the cloud source along this path. The return stroke traverses that path moving from ground toward the cloud charge source and neutralizes the leader charge. 

The first-stroke leader in negative flashes often appears heavily branched and optically to be an intermittent process and is called "stepped leader". The tip of a subsequent-stroke leader in negative flashes appears to move continuously and is called "dart leader". The apparent difference between the two types of leaders is related to the fact that the stepped leader develops in virgin air, while the dart leader follows the "pre-conditioned" path of the preceding stroke or strokes. Sometimes a continuously moving dart leader becomes stepped, in which case it is called "dart-stepped leader".

The three phases of a cloud-to-ground lightning discharge. (A) Stepped Leader, (B) Connecting Leader, and (C) Return Stroke

When the downward leader approaches ground level a connecting leader, often initiated from the top of objects, is observed. When descending leader and upward connecting leader meet, the main discharge, the return stroke, starts. At the initiation point of the connecting leader the well-known pulse-shaped lightning current occurs with an amplitude of several 1,000 Amperes up to some 100,000 Amperes. The rise time of the current is in the microsecond range. The negative charge deposited along the leader channel is discharged  to the ground. The figure schematically shows the three main phases of a lightning discharge.

The high-current return-stroke wave rapidly heats the lightning channel to a peak temperature of about 30,000° K and creates a high channel pressure. High temperature and pressure result in channel expansion, intense optical radiation, and an outward propagating shock wave that eventually becomes the thunder (sound wave).