Lightning

In a tall storm cloud, rising winds knock drops and tiny bits of ice together, and like a balloon rubbed on wool they trade electrons: the top of the cloud becomes positive and the bottom negative. The negative base induces a positive charge on the ground below. When the charge grows too large for the air to hold back, it jumps across in a giant spark, an electric discharge, between two clouds or between a cloud and the ground. The spark heats the air in an instant; the air bursts outwards and we hear thunder, later than we see the flash because sound is far slower than light. A lightning rod gives the charge a safe metal path down into the ground.