# Propagation of sound after lightning

In an area of frequent thunderstorms, I notice a 'crack or whoosh' sound if a strike is nearby but little or no thunder. Yet I can hear for 30 seconds or so after a distant flash when the thunder first is heard, the sound starts at a high frequency and then evolves into a deep rumble that can shake the walls. It approaches the sub-sonic. What is the relationship between the distance and the energy of the strike?

• Longer frequencies travel further because higher frequencies are attenuated more quickly (we see the same with seismic sources). Also, if you are further away, you are more likely to hear "reverberations" as the "crack" is reflected off the landscape. – winwaed Jun 25 '14 at 14:23
• @winwaed you should develop your comment into an answer. – casey Jun 26 '14 at 4:19

Since the sound waves propagate not from a single point source but along the length of the lightning's path, the sound origin's varying distances from the observer can generate a rolling or rumbling effect. Perception of the sonic characteristics is further complicated by factors such as the irregular and possibly branching geometry of the lightning channel, by acoustic echoing from terrain, and by the typically multiple-stroke characteristic of the lightning strike.

In addition, thunder will sound louder when the air near the ground is colder and the air higher in the atmosphere is warmer. This is because the acoustic shock waves get "trapped" in the cold air. Such a temperature difference, called an inversion, tends to happen at night, which is why thunder often sounds louder at night.

See Wikipedia and wisegeek.org

• Thank you, I just experienced this again last night and imagined the pathway for miles. – ScottyAustin Sep 3 '14 at 4:33

Geometry of the lightning channel controls the type and duration of the sound. Imagine a lightning strike oriented directly away from you--the farthest point on it is much farther away than the nearest point, meaning that the thunder will be drawn out over a long time.

On the other hand, if the lightning strike is perfectly vertical and you're standing a good distance away, the entire lightning channel is roughly the same distance from you, meaning the thunder arrives almost all at once. This makes a sharp crack.

Rough diagrams: if the lightning is the line and the observer is the dot, then the two extreme scenarios are as follows:

______      .  : long, drawn out rumble

|
|
|           .  : sharp crack


Also, low frequencies attenuate more slowly, so distant strikes will sound lower in pitch than nearby strikes.

• Vertical strike is my experience, therefore a 'crack'. The delay in the rumble is from miles away vertical which is finally reaching the point source. True? – ScottyAustin Jul 19 '14 at 3:54