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When sound of thunder travels to the listener from its location,different parts of the same wave have to travel through air of different moisture levels,densities and molar masses.

This is what I found when i wanted to know about the reason for the rolling sound(the sound which can be heard many times and for longer times) of thunder.But as I read it i found it unclear and failed to visualize the overall process when sound travels thereby giving the rolling sound of thunder.I also failed to understand how there will be different moisture levels in the air or what it meant! I also went through Extended sound of thunder this question but it didnt serve to answer my question and made me understandd the overall process.

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Thunder is the sound caused by lightning. Depending on the nature of the lightning and distance of the listener, thunder can range from a sharp, loud crack to a long, low rumble (brontide). The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave, similar to a sonic boom, which produces the sound of thunder, often referred to as a clap, crack, or peal of thunder. The distance of the lightning can be calculated by the listener based on the time interval from when the lightning is seen to when the sound is heard.

You are talking not of the original boom, but of the rolling thunder during thunder storms.In addition to the extent of the generating lightning stroke clouds enhance the effect of rolling and can carry the sound between levels. It is evident that there are clouds at several levels when lightning strikes. These levels create corridors of different densities ( cloud density, air). These form a type of acoustic waveguide reverberating and echoing creating the roll.

There is an anecdotal story from the Greek revolution in 1821 when an ambush in the mountains was betrayed because the discussion of how it would happen was transmitted booming through the cloud layers and people were warned.

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