For looking in windows, rainy nights are the best.
You might think water on a glass pane would obscure the view, but that's just the surface. Water magnifies what is essential, what is deeper inside -- emotions, sadness. Rain reveals the exquisite feminine melancholy of a world caught in the continual flux of living and dying.
Besides, I'm much less likely to be seen snooping around a woman's house when everyone else is inside keeping dry.