This Friday, in honor of my new facebook friend Steven Chu , I'd like to examine his "White Roofs" campaign through a Fermi problem. Steven Chu has said that we could all save energy simply by painting our roofs white. The idea is the same one behind why I don't wear a black shirt on a sunny day (and white after Labor Day). Black objects absorb light, and with it, heat, while white objects reflect it. So if you're trying to keep your house cool in the summer, it makes sense to have a roof that reflects sunlight instead of absorbing it. Here's the schpiel: But how much energy do you save, really? The Fermi Problem: The solar flux at Earth's atmosphere is about 1360 watts per square meter. About 0.3 of this is reflected back into space, while the rest is absorbed by the earth. Given the square footage of your roof, how much energy (assumed as heat) would a black roof absorb? How much energy would a white roof absorb? Now, you could give Secretary
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