What is the loudest thunder ever recorded (dB) at ground level and where did it occur? 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.
Thunder 1 mile away-117 dB
Thunder 1/2 mile away-121 dB
Lightning strike just meters away from you-125 dB