Thought about this one the other night driving back from 60 miles outside of St. Louis where it was 4 degrees cooler. I think its accepted as fact urban areas are warmer than adjacent "natural" or rural areas. If I understand right this is because: 1. Concrete structures holds heat. 2. Human mechanical activity creates heat. Why is this never brought up in discussions for or against global warming? It could be considered: 1. Proof human structures are raising world temperatures. 2. A reason greenhouse gasses aren't the leading cause. I'm wondering what percentage of U.S. and world wide stations are in Urban areas. Any climates where city or developed areas are cooler than the prevailing landscape? Also any other angles I should take into consideration. Personally I'm a believer in the effects of greenhouse gasses, but hey, I'll even argue with myself.