The relationship between nature contact and mental well-being has received increasing attention in recent years.

While a body of evidence has accumulated demonstrating a positive relationship between time in nature and mental well-being, there have been few studies comparing this relationship in different locations over long periods of time.

In this study, we estimate a sentiment benefit, the difference in happiness between in- and out-of-park tweets, for the 25 largest cities in the US by population.

People write happier words during park visits when compared with non-park user tweets collected around the same time.

While the words people write are happier in parks on average and in most cities, we find considerable variation across cities.

Tweets are happier in parks at all times of the day, week, and year, not just during the weekend or summer vacation. Across all cities, we find that the sentiment benefit is highest in parks larger than 100 acres.

Overall, our study suggests the mood benefit associated with park visitation is on par with US holidays such as Thanksgiving and New Year's Day.