I know during severe thunderstorms it looks as if lightning originates from the clouds and moves towards the ground, but I have recently heard that it actually originates from the ground and moves up towards the clouds. Which is true?
Posted June 9, 2008 9:00 a.m. EDT
Brandon, Both are actually correct. In a way, every lightning strike has components that go in both directions. The most common cloud-to-ground strikes originate in the clouds with an ionized channel, usually invisible to us, called a stepped leader. This leader zigs and zags its way toward the surface in a series of split-second starts and stops. When it gets close enough to the surface, a second, much shorter ionized channel, called a streamer, reaches up from the surface and joins up with the stepped leader. At that instant, the path between charge centers in the cloud and the ground is completed and the brilliant flash of lightning that we see (called a return stroke) then rushes back up toward the cloud almost instantaneously, and the main body of electrical charges is transferred between the cloud and ground. In addition, lightning is a versatile phenomenon that includes cloud to ground, ground to cloud, cloud to cloud, and intracloud strikes, and each of those can be either a positive or negative discharge, all of which mean that lighting can travel amongst almost every reasonable combination of initiation and termination point you might think of.
One other small point to note is that the term "severe thunderstorm" is usually reserved for those that produce wind gusts to 58 mph or higher, hail three quarters of an inch in diameter or larger, or tornadoes. Thunderstorms that do not produce any of those phenomena are not defined as severe, even if they involve numerous lightning strikes and/or areas of heavy rain.