– You can measure the green-friendliness of a city in many ways—like its average carbon footprint per citizen or the energy efficiency of buildings. But the event-finding app Gravy has decided to measure cities by the number of pro-environmental events each one hosts in a year, the Huffington Post reports. Is this an official barometer? Hardly, but it does reflect people's interest in the green movement. Here are the top five US cities, each with a unique Earth Day activity: Portland: Operation Clean Street San Francisco: Earth Day SF Boston: Earth Day Massive Spin Jam Austin: Annual Earth Day Festival Chicago: Chicago Earth Day 5K Click for more.