Add "urban greening" to your eco-vocabulary. Green Roofs for Healthy Cities is singing the praises of over 3 million square feet of leafy, sod-covered roof systems installed in 2006, an increase of 25 percent over 2005.

Green roofs are being hailed as a great way to help cool urban environments by employing the sun aborbing and air cleansing qualities of plants and trees (though even that is open to debate). In the winter they act as natural insulation.

The top ten green roof U.S. cities are as follows:

1. Chicago, IL
2. Washington, D.C.
3. Wildwood Crest, NJ
4. Dulles, VA
5. Kansas City, MO
6. Phoenix, AZ
7. Milwaukee, WI
8. New York City, NY
9. Portland, OR
10. Columbus, OH

More info on the organization is available here.