Jon, You can download a surface for the western United States and Canada for growing season length and other climate variables: http://forest.moscowfsl.wsu.edu/climate/currentClimate.html I'm not aware of a similar surface for the entire US, but the methods used to create the surface are give in: Rehfeldt, G. E. 2006. A spline model of climate for the Western United States. Gen. Tech. Rep. RMRS-GTR-165. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 21 p.
The same approach could conceivably be applied to the entire US or other area with a dense network of weather stations. Cheers, Peter
