You want a definition for atmosphere in film? Watch Don't Look Now by Nicolas Roeg. When it came out it was mostly known for the controversy it stirred with the sex scene between Julie Christie and Donald Sutherland but it has stood the test of time like only classics can. What is the film about, you ask? To be honest, I'm not 100% sure. Is it a ghost story? Might be - there is enough communication with the dead spirits throughout the film to warrant it. Is it a giallo? It sure has all the traits - the serial killer, the hood, the knife, the missing piece of the puzzle. Is there something about psychic capabilities? Most definitely - the whole plot turns around a premonition. Still, I don't know how to explain the film: I'm sure the plot had already been set in motion by the time the fatal accident of John Baxter's daughter happened (which was like the very first scene you see in the film) just as I'm sure that despite all the chaotic running through the narrow streets of Venice, the key tragedy of the film had been preordained. Ah, well... As I said in the beginning, it's all about atmosphere with this movie. If you let it engulf you for just 1 second, I promise you will never ever forget it.