At 1:45 in the afternoon on August 9, 2001, the temperature in Chicago was in the 90s. Eleven stories up, on the roof of City Hall, the surface temperature of the black tar??measured 169 degrees. But Mayor Daley, environmental innovator ??? yes, that Mayor Daley ??? had done something interesting. The year before, a section of the City Hall roof had been painted white.?? The surface temperature there was between 126 and 130 degrees. And much of the roof of the building, and the adjacent Cook County building, had become a garden ??? 20,000 plants in 150 varieties, chosen for their abilities to thrive without irrigation and stand up to Chicago???s notorious wind. The surface temperature of the green roof varied between 91 and 119 degrees.