What do urban building materials have in common? First of all, materials such as asphalt, steel, and brick are often very dark colors—like black, brown and grey. A dark object absorbs all wavelengths of light energy and 16 them into heat, so the object gets warm. In contrast, a white object reflects all wavelengths of light. The light is not converted into heat and the temperature of the white object does not increase noticeably. 17 , dark objects—such as building materials—absorb heat from the sun. To cool down urban heat islands, some cities are "lightening" streets. 18 is done by covering black asphalt streets, parking lots, and dark roofs with a more reflective gray coating. These changes can drop urban air temperatures dramatically, especially during the heat of summer. Planting gardens on urban rooftops can also help to cool down the city, too! 19 , a study in Los Angeles, California, calculated that changes like these would be enough to save close to $100 million per year in energy costs! Urban building materials are another reason that urban areas trap heat. Many modern building materials are impervious surfaces. This means that water can't flow through surfaces like a brick or a patch of cement like 20 would through a plant. Without a cycle of flowing and evaporating water, these surfaces have nothing to cool them down.