TBILISI-Sad city dwellers should seek solace in a local park, according to a survey tracking tweets that showed a walk in leafy surroundings lifts morale among Twitter users. Trees in cities are already credited with cooling and cleaning the air, along with absorbing planet-warming gases, but a team of researchers from the University of Vermont in the United States found they also increase happiness.

"Visiting parks leads people to being happier," the survey's lead researcher, Aaron Schwartz, told Thomson Reuters Foundation by phone. The authors scanned thousands of Twitter messages posted by more than 4,600 people from 160 parks and other leafy areas, like squares and playgrounds, in San Francisco between May and August 2016 for words indicating glee and elation.

The posts were then compared with messages the same users sent out before and after the green stroll.

They found that, on average, people used happier words and expressed less negativity while visiting a park and for up to four hours afterward.

The greener and bigger the park, the happier the tweets, the researchers say in the first-of-its-kind survey published in the journal People and Nature.

The analysis was based on an online tool known as hedonometer, which ranks more than 10,000 common English terms on a scale from one to nine based on their happiness value.

"Laughter" is the happiest word on the scale, with a score of 8.5, followed by "happiness" and "love", while "trapped", "kill" and "jail" rank toward the bottom.

Overall, tweets sent from under a tree were 2.5 percent happier than the average, a jump that although numerically small, Schwartz described as "pretty big".

" (It) is about the equivalent increase that Twitter as a whole experiences on Christmas Day," he says.

The holiday was the happiest day of the year on the social media platform, according to earlier research based on the hedonometer, which collects about 50 million tweets each day and scores those written in English, he explains.

Although not everybody uses Twitter, the platform's demographic is quite wide and analysis based on it could provide valuable insights into the moods of a large group of people, the researchers say.