Growing Degree Days is an attempt to make that clock visible.
|Table from HERE|
|I wonder if the diagonal red streak starting in the southwest corner of the Lower Peninsula is an anomaly or if it is from the Chicago/Gary heat bubble.|
One quirk regarding Growing Degree Days is that it is calculated using a couple of different base temperatures. For example, the site listed above makes them available using 32F, 40F, 42F and 50F. The reason for the different base points is different organisms become biologically active at different temperatures. Pussy willow, for instance, seem to become active at any temperature above 32F. Many insects don't start clocking until the temperature is over 50F. Fruit trees and their early development of flower buds seem to start at 42F.