There is not any "official criteria" that makes a city "green" but there are a few key areas to look for when rating a green city. Air quality, waste management, water quality,recycling management, the amount of LEED-certified buildings, acres that are devoted to green space, the use of renewable energy, the availability of organic products and cleantransportation are a few of these areas. The Mother Nature network did some research and came up with a list of the top 10 green cities that measured up in these areas.

1. Portland, Oregon - One of the most bikeable cities in the nation, with nearly 250 miles of bike lanes, trails and paths! According to Business Insider, 8% of the entire city usesbicycles as their primary mode of transportation. The clean water supply ranks as one of the cleanest for large cities in the nation. The recycling program in Portland is soeffective that just the residential recycling alone saves nearly 250,000 tons of co2 annually. That is the equivalent of what 35,000 cars produce annually on the city streets.Portland uses 33% renewable energy. The city is also home to 175 LEED certified buildings, this is the highest number per capita of any city in the U.S. Congratulations Portland!
   
2. San Francisco, California - Even with what some may believe are "fog" restrictions, San Francisco provides nearly 26 MW of solar power and could be able to increase that amount to 50 MW by 2020. San Francisco was the first city in the U.S. to ban plastic shopping bags in 2007. In February of 2012, the Board of Supervisors voted to expand the ordinance to more neighboring cities.
   
3. Boston, Massachusetts - Boston began the CleanAir Cabs program in 2007. This program replaced inefficient fuel cabs with hybrid vehicles. Boston Cab has over 400 hybrid taxis out of its 500 vehicle fleet. The city also requires a green building standard through municipal zoning and that all large-scale projects meet the LEED's standards. There is also the Mayors Carbon Cup, showcasing its participants that have each committed at least 1,000,000 sq ft of building space to a 35% reduction in greenhouse gas emissions by the year 2020.
   
4. Oakland, California - Hybrid-electric, hydrogen fuel-cell buses run up and down the streets of Oakland, saving 130 tons of carbon-dioxide emissions annually for each diesel bus that is replaced. These buses run quiet and burn clean. The waste water treatment plant in Oakland trucks in tons of food waste and successfully creates enough bio-gas to generate enough extra power for the equivalent of 1,250 homes!
   
5. Eugene, Oregon - Eugene provides nearly 200 miles of smog-free travel in the metro area, including 42 miles of shared use bike paths, 81 miles of on street bicycle lanes and 35 miles of signed bike ways. As this bicycle network continually grows, the need for bike parking is growing along with it. The city of Eugene is currently increasing the requirements for adequate bike parking in all living, working and recreation areas.
   
6. Cambridge, Massachusetts - Annually, during the month of May, "The Best Walking City" hides 100 Golden Shoes throughout the city along the river, foot paths, and parks. People are encouraged to find these golden shoes and turn them in for a free pair of walking shoes that are donated by New Balance. This event was created to encourage people to get out and walk. "The Race to Solar Program" enables eligible, non-profit organizations with the ability to acquire a solar electric energy system for their school, church, community center, etc.
   
7. Berkeley, California - In 2009, Berkeley was awarded with a "Solar Champion" award for having on of the largest numbers of solar roofs in the state of California. The city has also added nearly 5,000 street and park trees since 2000.
   
8. Seattle, Washington - "Green Up" is Seattle City Light's voluntary green power program for bothits residential and commercial customers. This program allows customers to purchase green power for a portion of their electricity. Seattle requires that anynew construction or renovation of a city building over 5,000 square feet must achieve a LEED Gold or better certification.
   
9. Chicago, Illinois - Known as the "Urbs in Horto" or "City in a Garden" Chicago's Green Roof program is living up to its name and has established over 5.4 million square feet of city roofs supporting plant life.
   
10. Austin, Texas - WaterWise Landscape Rebate program is offering cash incentives to decrease outdoor irrigation. By converting from healthy turf grass to native plant beds and permeable hardscapes you can be eligible for rebates. There are also over 206 parks, 12 preserves, 26 greenbelts and 50 miles of trails that are devoted to green space throughout the city.

Hats off to the greenest cities in America!