Normally they do not close a ride due to fog. One of my favorite memories is riding MANTIS on a low cloud day, at the top of the hill, you were above the cloud and the only things you could see were the tops of Power Tower, MAGNUM and MEAN STREAK, all standing on a "pillow" of cotton. I wish I had a camera (if it was legal :) ).
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"Carnies built this country, the carnival part of it, anyway." - Homer