I once hit a "dry" cell in the Caribbean in a 757 at FL350. It was night time...radar was on max manual gain tilted down -2, nothing ahead or in immediate vicinity, some stuff on either side about 80 miles abeam. Suddenly flew into a cell that caused severe turbulence for 10 seconds, over speed, and +/-500ft. Once out of it immediately ascertained everyone was ok and notified atc. Not 30 seconds later Speedbird (BA) 747 hit the same type of stuff well off to our right. There were dry cells all over the Caribbean that night. They do not show up on radar no matter how much gain (sensitivity) you have it on: not enough moisture to show on radar but plenty of punch!