The map has an explanation: About this data The 2012 USDA hardiness zones were calculated using the average lowest winter temperature for the observation peri

The map has an explanation: About this data The 2012 USDA hardiness zones were calculated using the average lowest winter temperature for the observation period of 1976-2005. The new zones are calculated using the years 1991-2020. These two observation windows overlap. Colors show the difference between the two 30-year averages for each place on the map. My area is 3.8F warmer using this method.