Strike Point- referring to the object on the ground that was the location of the termination of the lightning channel. In layman's terms, it's what the bolt hit. Stroke- referring to the flow of current through a lightning channel. The sound waves produced by the explosive heating of the air in the lightning channel during a return stroke. It originates as shock waves close to the channel, and moves radially away from the channel.
Thunder changes in pitch with varying distances from the channel.
Thunder rumbles and crackles because the lightning channel is crooked and jagged, causing the sound waves to arrive at the hearer at different times and directions. If lightning strikes closer than around 300 feet, the observer will hear one loud, startling, high-pitched bang which is not 'sound wave' thunder, but the shock wave, sometimes preceded by a faint crackling noise from a yet to be determined source. For more detailed information on thunder, click here . Voltage Gradient - The surge of voltage through the ground raidially outward from the lightning strike point. The voltage gradient can electrocute anyone standing on the ground close to a lightning strike.