Wind power is usually generated in vast, open spaces — mountaintops, prairies, or offshore in shallow waters. But the Netherlands is taking wind power to new heights, literally: the rooftops of buildings in metropolitan areas. With light, quiet, efficient designs that often blend into the surrounding architecture, these urban windmills are built to take advantage of the severe turbulence and rapid shifts in wind direction that are typical of cities. Amsterdam, the Hague, Tilburg, and Twente all plan to install urban windmills this year, at a cost of $5,000 to $12,000 each. One windmill that’s already been installed on the roof of a technical school is capable of generating about 5,000 kilowatt-hours of energy per year, or more than enough for an average Dutch family (although only about half enough for an average U.S. household).