Feeling unhappy? Go find a city park — the bigger the better — and try taking a walk outdoors. So says a major new study measuring the happiness effects of city parks in the 25 largest U.S. cities. The happiness benefit of urban nature on users was akin to the mood spike people experience on holidays like Thanksgiving or New Year’s Day. The research is the largest study of its kind — using massive amounts of data from social media — to quantify the mood boosting benefits on urban nature. The happiest place on Twitter? The great outdoors.