I thought this was amusing: There's a trick I like to pull on my friends. Standing in a large city on an overcast night, I'll ask them to look down at the sidewalk and tell me what color the sky is. They invariably will say black or, sometimes, dark blue. Look up, I'll say. And to their astonishment the sky is neither black nor blue, but a dull, smoky red - reflecting the orange glow of all those sodium and mercury vapor streetlights. Cut from an essay about SF & the future by Michael Swanwick: http://www.michaelswanwick.com/nonfic/future.html