It's long been known that cities have an effect on the weather. Most noticeable is that urban areas are warmer than their surroundings, a phenomenon called the "urban heat island." Tougher to prove, though, is whether cities have an effect on rainfall because of our scattered network of ground-based observations.

A few earlier studies in St. Louis and Chicago did suggest that cities help enhance rain a little. But now, for the first time, rainfall estimates from satellites have shown that urban areas do noticeably impact rainfall. These estimates show that, at least during summer, more rain falls in areas over and downwind of big cities such as Atlanta, Dallas, and Nashville. This is likely related to the extra heat of the city which leads to more rising air, which is essential for clouds, and also to the extra particles in the air of cities, which can serve as seeds for cloud drops.

Though it's reasonable to infer that these same effects are happening around large northeast cities such as Philadelphia or Washington, they haven't been studied yet because the satellite taking the data is actually focused on the tropics, and can't measure rainfall data this far north.