Are they still open when it rains?

Yes, they are.

Sofia is right. If light rain they stay open. I have watched some movies there in the rain, but never under stormy weather.

I don't work there. What I've noticed is that if passing light rain is predicted, the drive-in will stay open. If it's strong rain, or any lightning, then they close. Check out their website when in doubt! They update their information regularly.