Lightning. Multiple cloud-to-ground lightning strikes from a thundercloud at sunset, over Tucson, Arizona. A cloud-to-ground strike starts with a streamer, a line of ionized air molecules, which develops downward from the cloud. When the streamer has reached between 5 and 30 meters from the ground, a highly-luminescent return stroke comes up to meet it, following the line of ionized molecules to the cloud. This is the visible flash, lasting only for 100 microseconds and passing a current of around 10,000 amps. The return stroke travels along the streamer at about 0.1 of the speed of light (almost 30,000 km per second).