Why does thunder, that is heard about five or ten seconds after the lightning is seen, start as relatively quiet high pitched 'crackling' thunder which is, about five or ten seconds later than that, followed/replaced/drowned out by much louder lower pitched 'booming' thunder?
I am under the impression that loud low frequency sound travels much faster than quiet mid range frequency sounds (by a factor of approximately two in the case of the sound/blast of a large nuclear explosion, meaning that the blast wave of a large nuclear explosion travels at twice the 'speed of sound' i.e about 600 meters per second). So I would have expected the loud low frequency sound from the lightning to reach me earlier and not later. But the low frequency sound does seem to reach me later.
Edit: Another thing I find strange is that in many cases the later sounds (the loud booms of thunder) are remarkably distinct (or brief, being at most about 0.1 seconds in duration each, say), like a series of large bombs going off, as if coming from points or small volumes, rather than a continuous roar/rumble that gets louder and quieter at random. The latter might be what is expected if the loud later sound is coming from a randomly oriented line (with or without branches) with a length of five hundred meters to twenty kilometers.
Best Answer
I'm not an expert, but I spent some time with references 1 and 2 several years ago. This answer is based on some notes I took.
Measurements using the radio waves produced by lightning indicate that lightning bolts inside thunderclouds (the ones that we can't see directly) are often mostly horizontal, and they can be anywhere from 1/2 km in length to 20 km in length, spanning a large portion of a large thundercloud. That says that the most distant part of the lightning bolt can be several kilometers farther away than the closest part. That is at least part of the reason why thunder lasts so much longer than the flash of lightning that produced it.
Also, lightning typically has a jagged, branching shape. Experiments with smaller sparks have shown that the sound from a given segment is typically directional (louder in some directions than in others), so different segments of a jagged lightning bolt will tend to contribute different amounts to the overall sound because they're oriented differently. This is at least part of the reason for the sound's rich texture.
Based on that information, here's a guess about why the sound from moderately distant lightning often starts with a quieter higher-pitched part. I'm picturing the shape of lightning as similar to the shape of a river with many smaller tributaries contributing to it. If we're closer to one of those small tributaries than to the main part of the river, then we'll hear the sound from that small tributary first (especially if its orientation is favorable, because the sound is directional), followed by the sounds from the more substantial parts that are farther away. This at least seems to explain the quiet-to-loud trend, and maybe it also helps explain the frequency trend: smaller sparks apparently don't produce as much lower-frequency sound as larger sparks, so most of the lower frequencies will come from the larger parts, which are farther away in this scenario.
Frequency-dependent attenuation in the atmosphere and in the ground might also play a role (reference 3).
References:
Rakov and Uman (2003), Lightning: Physics and Effects (Cambridge University Press)
Holmes et al (1971), "On the power spectrum and mechanism of thunder" (https://doi.org/10.1029/JC076i009p02106)
Lamancusa (2000), "Outdoor sound propagation" (https://www.mne.psu.edu/lamancusa/me458/10_osp.pdf)