I try to ask all my customers if they have a photographic memory. The look at me funny, and generally just say 'no'. I say that I don't either. I ask if they have a photographic camera, which seems to catch them off-guard, like I'm asking if they have a tail or something.
What is this photographiccamera you speak of?
Using the same, good photo points year after year, same time of year, same kinda sky, people can have good data that they will never be able to capture retro-actively.