Flash! Bang! Thunderstorms are impressive displays of the power of nature. But how, you may wonder, do lightning and thunder form?

First comes the lightning, an intense electrical discharge. While not completely understood, it is believed to form as a result of the separation of charges within a cumulonimbus cloud.  One theory of how this happens involves the collision of particles within these towering clouds, including hailstones, super-cooled liquid water droplets, and ice crystals.  When they mix and collide, according to NOAA, “electrons are sheared off the ascending particles and collect on the descending particles.” This results in a cloud with a negatively charged base and a positively charged top.

As the atmosphere is a good insulator, generally inhibiting the flow of electricity, the strength of this electrical field has to build up substantially before lightning can occur. Most discharges, about 75%, occur across the electrical field within the storm cloud itself. This is known as intra-cloud lightning.

Another electrical field can also develop below the cloud. Since the cloud base is negatively charged, it induces a positive charge on the ground below, especially in tall objects such as buildings and trees. When the charge separation becomes large enough, a negatively charged stepped leader – an invisible channel of ionized air, moves down from the base of the cloud. When it meets a channel of positive charges reaching up from the ground, known as a streamer, a visible flash of lightning can be seen.  This is called cloud to ground lightning.

Lightning can be as hot as 54,000°F, a temperature that is five times hotter than the surface of the sun. When it occurs, it heats the air around it in a fraction of a second, creating an acoustic shock wave.  This is thunder.  A nearby lightning strike will produce thunder that sounds like a sharp crack. Thunder from a distant storm will sound more like a continuous rumble.

While thunderstorms can be spectacular events to watch, they are also very dangerous.  So, as the National Weather Service recommends, “When thunder roars, go indoors.”

Credit: NWS