Hi.

The glass doors in the room lit up with a streak of brilliant blue light… The thunder rolled… Slick sheets of rain came pouring down, so hard that they were bouncing off the marble floor.. Amidst these sheets of rain, we’d see gusts of wind bringing torrential waves of rain ....

I can't visualize the "sheets." It may be that if you would try to walk through the rain, you would feel like you are breaking through curtains of rain. Is it how you would look at it?

Longman Advanced American Dictionary says a sheet of rain refers to a very large moving mass of it. A mass is geometrically not a plane (sheet). How would you look at it to see a sheet?

Hiro/ Sendai, Japan