Thunder and lightning are forever intertwined in our minds and seem inseparable, like two sides of a coin. But how do they form in the skies overhead? How does the sky go from being calm and ordinary looking to bursting with light and sound, like a 4th of July fireworks display? Let's look at each in turn.
First, we're sorry, Norse mythology fans -- thunder doesn't come from Thor. There's a much less fanciful reason for the rumble in the sky. Thunder is a byproduct of lightning, so there can be no thunder without it. Lightning is incredibly hot at its center, so hot it can even melt sand into glass. When lightning forms, it heats up the air at a very rapid rate and, after the bolt is discharged, the air cools back down very quickly. This rapid heating and subsequent cooling creates the often booming sound waves that we call thunder.
Next, of course, Curiosity readers will no doubt read the above explanation and wonder also about how lightning gets crackling. It happens thanks to the creation of an electrical circuit. Lightning strikes are created when a path of ionized air known as a "step leader" connects to an electrical field formed by an object on the ground scientists call a "positive streamer." When these two forces connect, the step leader with the positive streamer, a circuit is formed, allowing electricity to travel between the two locations. This is how a lightning bolt is formed. The connection between the leader and the streamer also results in a superheating of air in the area around the lightning strike. When the air grows hot enough, it explodes into a radial shock wave spreading out from the initial strike, giving us -- you guessed it! -- what we just learned about above: thunder.