Sound travels fastest through warm air and conditions such as temperature inversions can help to amplify the sound of thunder (source). Conversely, during a thundersnow event the falling, and accumulated, snow can act as an acoustic suppressor (source). Originally I was thinking that the loudest thunder at ground levels would simply be at higher elevations, however the atmospheric conditions mentioned above, and others that I'm not aware of, have me wondering where the loudest thunder would actually occur.
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$\begingroup$ Is google not helpful? I found several sites just googling "loudest thunder recorded" (no quotes). $\endgroup$– user967Nov 26, 2017 at 14:39
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1$\begingroup$ I had no luck googling it. There were some youtube videos, which I admittedly didn't watch, and one q&a site where an answer of 234 db was given with no source. If you have any reliable information please post as an answer. $\endgroup$– user11318Nov 26, 2017 at 18:20