My old setup in Chandler got ruined by a porch light like that. Markhall has the right idea about showing them a picture, it really works. I brought over a print of the Orion Nebula and explained how the light washed everything out. They ended up putting it on a motion sensor after that. Sometimes people just don't know until you show them what you're actually trying to do.

Just keep a raincoat and umbrella by the door for those damp mornings.