Dark clouds

[Ixeling]

So, I've been playing FTB Unleashed for about a week now, and I found those Dark clouds in End. The tooltip says "Creates lightning strikes", but I can't manage to make any lightning with it, I've tried to place them on the ground, 50 blocks up in the sky, of course in the Overworld, but NO LIGHTNING, so what do I have to do? I can't find anything about Dark clouds.

 

[Labhras]

Try wearing a wool sweater and finding some carpeting.