Well, the setting of a dark and stormy night is a classic for horror for good reason. The night already has an air of mystery and danger about it. Add to that a storm, and it becomes a perfect backdrop for horror. The howling wind can sound like the wails of the damned. The rain pelting against windows can feel like the tapping of some unseen entity. And the flashes of lightning can be like glimpses into a nightmare world. It's as if nature itself has turned against the characters, and they are at the mercy of not only whatever horror is lurking but also the elements. This all - encompassing sense of being trapped in a hostile environment heightens the horror to a great extent.