Duration: few 1/10 s for a flash
no. of strokes : 3.4 - 20 ~ 30 with 40,50 * 1/1000 s apart
electric current: 10,000 ~ 20,000 A - several 100,000 A
cloud-to-ground duration: few 1/1000000 s
cloud-to-ground potential difference: several hundred million V
no. of lightning discharge per s worldwide :~100
Background knowledge
Thunderclouds are tripolar, consisting of a "pancake" or layer of negative charge sandwiched between two positively charged regions. The lower one is smaller than the upper one.
At very cold temperature, ice is negatively charged. As ice begins to melt, positive charge results.
In a mature thundercloud, ice particles are in a perpetual state of growth, contraction and movement : shot upwards by convection, they are then pulled downwards by gravitational forces of precipitation ( and by downward convection ).
As they move, they collide with the cloud's mist of ice particles and supercooled water droplets, and become electrically charged.
In the upper, colder regions of the cloud, the particles become negatively charged and falling, leave behind a positively charged mist. When they reach the middle of the cloud, and pass through the region at charge-reversal temperature, collision make them positively charged, while the mist becomes negatively charged. Falling further to the bottom of the cloud, they form a region of positive charge. From there, they may either precipitate or shoot upwards once more, where the process repeats itself.
Most lightning begins with a stepped leader from the highly charged negative "pancake", but sometimes the stepped leader may start from the less highly charged positive region. In the former, negative charges, electrons, flow into the ground along the return stroke; in the latter, they flow in the cloud.
Ground-Cloud Lightning
Not far from the ground, the stepped leader contracts a travelling spark that is sent up by the object to be stuck, and produce a "return stroke", completing the path to ground.
It is the return stroke, not the weakly luminons stepped leader, that we actually see.
Each step in the leader is ~50 yards ( almost 50 m ) long and occurs
The electric current flows, astronomically high and virtually instantaneous, heats the air in the leader channel to a temperature > 30,000 C (55,000 F), 4times to 5times greater than the sun surface temperature.
The initial expansion of air produces a pressure 100 times greater than the normal atmospheric pressure and a shock wave heard as thunder.
By measuring the travel time, duration, loudness and pitch of thunder claps, peals and rumbles, length, orientation, altitude, energy of the leader channel can be estimated.
As opposed to a single clap, a rumble indicates a channel pointing away from the observer. The deeper the rumble, the more energy has been released.
The width of channel can be estimated directly by photography, though brightness overexposes the film making the recorded image broader than it should be.
According to the best photographs, the channel is 2 - 7 inches ( 5 - 18 cm ) wide; objects stuck by lightning can provide channel widths too; spiral furrows on tree trunks are between ~0.6 - 5 inches ( 1.5 - 12.7 cm ) wide; fulgurites, the fused hollow tubes with corrugated glassy walls formed when lightning strikes sand, are usually between 0.5 and 2 inches ( 1.3 -5 cm ) in length.
There are still some secrets about thunder and lightning though advanced technology.
i) flashes inside clouds ??
ii) lightning channel so infinitely tortuous ??
iii) bead lightning, channel broken up into fragments ??
iv) incandescent ball lightning ??
