During a lightning flash, the temperature of a long channel of air rises rapidly as the bolt passes through it.This temperature increase causes the air in the channel to expand rapidly, and this expansion creates a sound wave.The channel produces sound throughout its entire length at essentially the same instant.
If the orientation of the channel is such that all of its parts are approximately the same distance from you, sounds from the different parts reach you at the same time, and you hear a short, intense thunderclap.However, if the distances between your ear and different portions of the channel vary, sounds from different portions arrive at your ears at different times. If the channel were a straight line, the resulting sound would be a steady roar, but the zigzag shape of the path produces variations in loudness.
