I think there would have to be lightning in the area ( or other dangerous conditions before they closed the experience). Depending on the time of year you're going it is a varying "problem". For much of the summer there are always the chance of afternoon thunderstorms. IF so, it's best to get to DC early and get through as much of the experiences as you can. Usually if there are storms it's after 3.00 in the afternoon, by which time you should have had your monies worth, if you pack the activities in.