Thunder and lightning are always together because they are actually two different manifestations of a single phenomenon. The noise of thunder is the sound that lightning makes when it strikes. Light travels at a speed of 299,792,458 meters/second, much faster than sound, at a speed of 340.29 meters/second at sea level. For this reason, lightning is always first with the thunder following afterward after a variable time span, depending upon how far away it is taking place from the witness who is seeing and hearing the phenomenon. The distance factor also accounts for the fact that thunder varies in pitch and tone, from high sharp cracking to low ominous rumbling. The sound is the decayed acoustic wave remnant of the sonic shock wave caused by the effects of the lightning bolt's sudden electrical discharge on the surrounding air temperature and pressure.
The lightning bolt, itself, is a plasma channel created when the gases in the air become electrically charged, or ionized. Plasma is considered the fourth state of matter, after solids, liquids, and gases. It is an extremely hot condition. The temperature within a lightning bolt can reach nearly 50,000 F, traveling on its discharge path at a speed of around 130,000 mph! This extreme and violent assault to the atmosphere causes a shock wave just like those that are formed in front of a supersonic aircraft as it breaks the sound barrier. Another example of a shock wave is the mushroom-like cloud formations formed by very large hydrogen or TNT explosions. This is an extremely fast and powerful force that easily flattens everything near to it within a certain radius.
In addition to the lightning commonly occuring in rain and hail storms, it is also frequently witnessed in the ash clouds above erupting volcanos and in the statically charged windy dust above violent forest fires.
But it is the question of what causes these electrical discharges that is the real mystery; remaining without answer for most of the two centuries since Ben Franklin's famous kite and key experiment. No answer was discovered even after Nikola Tesla was able to re-create lightning with his Tesla coil.
There are a few working hypotheses in the scientific world, on the origin of lightning, connecting it to several other environmental factors, such as humidity, air density, and the presence of ice crystals in the atmosphere, that seem to be very workable. As we continue to grow and learn more about the mystery of electricity as it interacts with our environment in the form of lightning, the closer we get to understanding many other related enigmas of our natural world.