As we've already ascertained, lightning is still a bit of a mystery to scientists. But the leading theory about how it works is that as ice particles knock around in the clouds, they either gain or lose electrons to form positive or negative charges. The bolts you see most often are those of negative lightning, which happens when those negatively charged electrons at the bottom of a cloud connect with the ground. Positive lightning, on the other hand, makes up only 5 to 10 percent of the lightning you see. It comes from the positively charged particles that gather at the top of a cloud, which usually just end up connecting with the negatively charged ones at the bottom. But when those get out of the way, like at the end of a storm or in a strong wind, positive lightning can strike the ground. There's a reason it's referred to as a "bolt from the blue" — it often comes in clear skies.
Positive lightning is what you need to watch out for. Not only does it tend to come after the storm seems to have passed, but it also produces much more energy. Whereas negative lightning produces 300 million volts and 30,000 amps, positive lightning can produce nearly a billion volts and 300,000 amps. It's rarer, but it's also more deadly. Just one more reason to be careful during a thunderstorm. If you think the storm has passed, wait a while longer just to make sure.
Correction 6/26/2019: A previous version of this article said that positive lightning is caused by positively charged electrons. Electrons always have a negative charge; positive lightning involves positively charged particles. The article has been updated to reflect this.
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