March 20 frost-free regions over North America. Light green is the 1950-1952 average, darker green is the additional area for the 2009-2011 average.
These visualizations show observational evidence that the growing season (climatalogical spring) is occurring earlier in the northen hemisphere. Scientists analyze recorded ground temperatures throughout each season and determine the earliest frost-free dates for each location every year. The earliest frost-free date in a growing season often does not correspond to the northern hemisphere's Spring equinox (about March 20), which is the astronomical first day of Spring.
The visualziations below show frost-free regions for March 20 and April 20. The regions colored in light green are the frost-free regions averaged from 1950 through 1952. The darker green regions that fade on are the additional areas covered by the frost-free regions averaged from 2009 through 2011. More area is frost-free in the each of the 2009-2011 averages compared to the 1950-1952 averages.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 126.96.36.199.0