Rides (most of them) can operate in rain, no problem, but it's lightning you have to watch out for! Most parks have a certain waiting period after a storm passes during which no lightning can be sighted before the coasters can be run. Think about it - would you like to be on the tallest steel coaster in the western hemisphere with lightning? :)
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Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/index.html