"Ninety percent of the electrical energy of lightning is released in the form of heat, which is quickly dissipated into the atmosphere. Less than 1% of lightning's energy is converted into sound and the rest released in the form of light. A sudden increase in pressure and temperature causes surrounding air to expand violently at a rate faster than the speed of sound, similar to a sonic boom. The shock wave extends outward for the first 30 feet (10 m), after which it becomes an ordinary sound wave called thunder. The speed of sound through air at sea level is 758 mph (1,130 feet/second; 344 m/second) at 68° F (20° C). Thunder is exploding air occurring along the entire length of the lightning channel. An average thunderstorm produces thousands of mi/km of lightning channel during its lifetime."
"Thunder contains a somewhat cylindrical initial pressure shock wave along the lightning channel in excess of 10 times the normal atmospheric pressure. This shock wave decays rapidly into a sound wave within feet or meters."
-lightningsafety.com
"Regardless of whether lightning is positive or negative, thunder is produced the same way. Thunder is the acoustic shock wave resulting from the extreme heat generated by a lightning flash.
Lightning can be as hot as 54,000°F (30,000°C), a temperature that is five times hotter than the surface of the sun! When lightning occurs, it heats the air surrounding its channel to that same incredible temperature in a fraction of a second.
Like all gases, when air molecules are heated, they expand. The faster they are heated, the faster their rate of expansion. But when air is heated to 54,000°F (30,000°C) in a fraction of a second, a phenomenon known as "explosive expansion" occurs. This is where air expands so rapidly that it compresses the air in front of it, forming a shock wave similar to a sonic boom. Exploding fireworks produce a similar result."
-noaa.gov
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