The zig-zag lightning, sheet lightning, and ball lightning.
Why is lightning sometimes forked?
Because the lightning cloud is at a great distance; and the resistance of the air is so great that the electrical current is diverted into a zig-zag course.
How does the resistance of the air make the lightning zig-zag?
As the lightning condenses the air in the immediate advance of its path, it flies from side to side, in order to pass where there is the least resistance.
Why is the flash sometimes quite straight?
Because the lightning cloud is near the earth, and as the flash meets with very little resistance, it is not diverted; in other words, the flash is straight.
What is sheet lightning?
Either the reflection of distant flashes not distinctly visible or beneath the horizon, or else several flashes intermingled.