100-Year-Old Frost Maps Show How Climate Change Has Shifted the Growing Season in the U.S.

"Average Dates of Last Killing Frost in Spring," William Reed Gardner, Charles Franklin Brooks, and F.J. Marschner (1916).

 "Average Dates of First Killing Frost in Fall," William Reed Gardner, Charles Franklin Brooks, and F.J. Marschner (1916).

Via slate.com