Why Does Lightning Happen?

Lightning is a massive electrical spark. Inside a thunderstorm, ice crystals and water droplets collide as updrafts push them upward and gravity pulls them down. These collisions transfer electrical charge, so the top of the cloud tends to become positive while the lower cloud becomes negative. When the voltage between cloud and ground becomes large enough to overcome air's resistance, ionized channels called step leaders branch downward. Positive streamers rise from the ground and tall objects. When a streamer connects with a leader, the circuit is complete. The bright flash you see is mostly the return stroke, a burst of current rushing through the completed channel. It superheats the air to around 30,000 Kelvin, causing explosive expansion that produces thunder.