It's one of those things we remember learning at some point during grade school -- wear a black T-shirt out on the playground and it will get hotter than a white T-shirt.
Same goes for your roof and the roofs of all the buildings in your city -- dark colors absorb heat and light colors reflect it.
According to the Los Angeles Times, the recently-held Climate Change Research Conference reported that if buildings and roads in 100 of the largest cities in the US were given lighter, heat-reflective surfaces the energy savings would be huge -- we're talking $1 billion annually.
Why? A light roof will reflect heat from your house, rather than absorbing it -- meaning you run your AC less, or hopefully (depending on where you live) not at all. On a city-wide level -- light roofs and roads help cool air temperatures, which reduces smog, etc.
Starting next year all new and remodeled buildings in California will be required to install light roofs.
Get more info on the Climate Change Research Conference findings Treehugger.
image via steffe; flickr.com
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Rosenzweig 2008 and Rosenzweig et al. 2006 found that light colored surfaces are effective in reducing surface temperatures, especially in settings with a lot of existing dark colored surfaces, but where there is more available sidewalk space for planting, street trees are more effective in reducing surface and air temperatures. (large pdf: http://www.nyserda.org/Programs/Environment/EMEP/project/6681_25/6681_25_project_update.pdf)
I live in Minnesota - am I better off with a light roof or a dark one, energy-savings-wise? It's cold here for much more of the year than it's warm...