Last night I was running as an increasingly alarming thunderstorm descended upon Boulder. As I clocked one of my fastest ever runs in a desperate attempt to rush home, I couldn't stop thinking about the ~300 or so reindeer who were casualties of a lightning strike in Norway last Friday. Needless to say, I've been thinking about lightning a lot lately.
So how does it work?
The thing is, scientists aren't actually sure. They know that you need powerful wind circulation in a cloud and ice particles. The goal is to drive positive charges towards the top of the cloud and negative charges will then gather along the base. Although the details of this process are largely unknown, the result is understood. The negative charges along the base of the cloud attract their opposites along the ground, causing the ground to have an excess of positive charge.
What about the actual lightning strike?
Watch this video. Lightning appears to first travel downwards. This is the negative charges forming a step ladder. Then, what our eye interprets as "lightning" shoots upwards into the cloud. Upwards?!?!? This is the more powerful return stroke; once the conductive path has been created by the downward step ladder, this return stroke follows it upwards.
However, the lightning strike itself is not the most dangerous part; the 300 reindeer in Norway learned that the ground current is far more deadly. Once the return stroke has occurred, the negative charge will dissipate along the ground.
But what about the intra-cloud lightning?
When I was running last night, I heard very little thunder. Thunder is a direct result of lighting; the lightning heats the surrounding air and causes air molecules to experience immense pressure. Immense pressure = shock wave = audible sound.
Intra-cloud lightning strikes occur between the top and bottom of the thunderheads. Last night, this was happening at a great enough distance that I heard none of the thunder that is usually associated with lightning.
In conclusion, clouds form a charge separation, lightning travels from the ground up, the ground current is the most dangerous part, and intra-cloud lightning can be quiet because it is so far away. So here's to enjoying the tail end of the summer thunderstorms!