If you're like me, you grew up consulting the USDA hardiness zone map to forecast what plants could be successfully grown in your climate. These zones are defined by the average minimum temperatures experienced in a given region of the country. E.g. plants that are hardy (can survive) to 15-20F could be expected to survive in zone 8 cities such as Seattle or Dallas. An updated map is currently in the works to reflect the recently warmer climate and the extent to which major urban areas retain heat.*
You may have noticed that very different climates are included within the same zones. As an extreme, Wikipedia points out that both the Shetland Islands and southern Alabama sit on the border of zones 8 and 9. Hardiness zones don't account for numerous critical factors, including average high temperatures, rainfall patterns, humidity, probability of extremely cold temperatures and the protective effect of snow cover.
Such fine-scale differences in microclimate are influenced by both topology and regional weather patterns and account for much of the regional variation in ecosystems. The EPA currently has a project (which I'm fairly obsessed with) that is working to map and define ecoregions across the United States by geography, climate and native vegetation. It's an awesome resource for native gardeners or anyone who wants to appreciate their local wildernesses on a more holistic, "systems" level.
Sunset Books has been including more than average minimum temperature in their gardening recommendations for years. Originally specializing in the diverse gardening habitats of the Pacific and Intermountain West, they now include specific gardening recommendations across the U.S.
According to their Plant Finder, Boston is hot and humid enough in the summer (yet not too cold in the winter) for a native North American palm tree. Who would have thought?
UPDATE: here's the provisional updated hardiness map. yikes!
*I keep hearing reference to some study that predicts global warming will bring Virginia temperatures to upstate New York within a few decades. As someone who loves cold, snowy weather, I desperately hope this isn't true!