Nah, can’t have thunder without lightning. I’m no meteorologist but I’m pretty sure they are connected
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. It is possible to see lightning but not hear the thunder (heat lightning) but I think that’s generally because you are too far away to hear it. I think I read somewhere that in a flat area you can see lightning up to 100 miles away (especially at night). Thunder doesn’t travel that far. To bring it all around to the gondolas I don’t think they will suspend service for heat lightning. They will probably wait until the storm is within a certain range of miles with active strikes and then shut it down.