Found this article in Backpacker Magazine:
MEDICINE MAN - BUCK TILTON - ASK THE EXPERT
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Are Hammocks Safe in Lightning?
Q.} I have discovered hammock camping. I was wondering if it is safe to remain in the hammock during a thunderstorm? Should I leave the shelter of the rain fly between the trees, or set it up on the ground with trekking poles?
Submitted by: Jeremy, Cincinnati, Ohio
A.} When lightning is striking, the safety of your hammock depends on what you've tied it to—just as your safety in a tent depends on where you've pitched the tent. If you string it between two trees of an approximately uniform height with surrounding trees, lightning has nothing special to attract it. If, however, one of the trees is noticeably higher than its neighbors and/or the trees are out in the open, well, you could have a problem. But even if one of the trees to which you're is hit, the lightning will head for the ground and possibly not even travel along the rope tying your hammock to the tree. I'm not sure how attractive trekking poles will be. It will depend somewhat on what they're made of and how open the terrain around your hammock. My guess is that you might be a smidge safer if the poles are not stuck in the ground. In fact, the ground current that radiates out from a lightning strike is statistically a far greater risk than the chance of getting directly hit. Overall I think you might actually be safer in a hammock than on the ground if you've hung the hammock in a safe spot.