We’ve been saying it for years: the highest-returning investment your city can make in its neighborhoods is planting street trees. But the importance of the shade and cooling they provide goes far beyond the financial bottom line.

NPR, in conjunction with the University of Maryland's Howard Center for Investigative Journalism, has produced a fascinating new series about the effects of urban heat on physical and mental health.

One theme that runs throughout the series is how inequitable the impact of extreme heat can be. Different neighborhoods in the same city can vary in temperature by ten degrees or more. And, perhaps not surprisingly, the hottest neighborhoods tend to be the poorest. NPR looked at the 97 most populous American cities, drawing in part on income data from the U.S. Census Bureau and thermal satellite images from NASA and the U.S. Geological Survey. The results: "In more than three-quarters of those cities, we found that where it's hotter, it also tends to be poorer."