Where there is lightning there will be thunder, and where there is thunder there will be lightning. The reason: thunder is the sound of the shockwave that lightning produces as it heats up the air to 30,000°C in less than one second. This is five times the temperature on the surface of the sun! The air surrounding the lightning channel or pathway expands explosively and creates a shockwave which produces the sound waves of thunder.

Have you ever wondered why the rumble of thunder can sound differently from one lightning strike to another? Depending on atmospheric conditions, you can hear thunder from as far away as 20 km , or as close as 8 km. How thunder sounds depends on a number of factors: the distance between you and the lightning strike, the temperature of the air, the amount of cloud and water in the air, as well as where the lightning channel is in relation to where you are situated.

Light travels at 300,000 km/second, whereas sound travels at 0.3 km/sec. This is why you can see the flash of lightning sooner than you hear the rumble of thunder that the initial lightning strike produces. As the distance to the lightning strike increases, the pitch of the thunder lowers. This is because the higher frequencies are selectively absorbed. This is just like when there is music playing in another room, you can hear the bass notes but not the high notes.

Sound waves are affected by the air temperature: they move faster in hot air and slower in cold air. They also bend or refract toward areas of lower temperatures similar to how light bends when it moves from air through water. Since the atmosphere usually cools with height, sound waves or thunder tend to deflect upwards. Typically this means if you are on the ground, more than 20 km away from a lightning strike, you will not be able to hear it. See figure A.

However there are differences between day and night. During a summer day, the air near the earth’s surface can be much hotter than the air above, so the sound waves bend toward the cooler air. See figure (A) for a graphic of this process. Conversely, at night, the air near the surface is likely cooler, so the sound waves bend towards the earth’s surface. See figure (B) for a graphic of this process.