People talk about thunder and lightning as if they are different, but lightning and thunder are the same thing. Lightning is the visible flash and thunder is the audible sound of a huge spark.
When static charge creates enough voltage in a thunderstorm, the result is a spark — a remarkably energetic spark. Because light travels fast (186,000 miles per second) we see it instantaneously. The shock wave set off by lightning’s hot flash travels to our ears at the more mundane speed of sound, which varies based on the density of the air but is about 1,127 feet per second.
When lightning and thunder occur simultaneously, you know it is nearby. Each second between the flash and the boom accounts for about a fifth of a mile of separation between you and the lightning. Flash! One Mississippi, two Mississippi, three Mississippi. Boom! That lightning was three fifths of a mile away.