Peter Dodds, Lewis Mitchell, Andrew Reagan, and Christopher Danforth write:
We introduce, formalize, and explore what we believe are fundamental climatological and seasonal markers: the Summer and Winter Teletherm—the on-average hottest and coldest days of the year. We measure the Teletherms using 25 and 50 year averaging windows for 1218 stations in the contiguous United States and find strong, sometimes dramatic, shifts in Teletherm date, temperature, and extent. For climate change, Teletherm dynamics have profound implications for ecological, agricultural, and social systems, and we observe clear regional differences, as well as accelerations and reversals in the Teletherms.
Of course, the hottest and coldest days of the year are not themselves so important, but I think the idea is to have a convenient one-number summary for each season that allows us to track changes over time.
One reason this is important is because one effect of climate change is to mess up synchrony across interacting species (for example, flowers and bugs, if you’ll forgive my lack of specific biological knowledge).
Dodds et al. put a lot of work into this project. For some reason they don’t want to display simple time series plots of how the teletherms move forward and backward over the decades, but they do have this sort of cool plot showing geographic variation in time trends.
You’ll have to read their article to understand how to interpret this graph. But that’s ok, not every graph can be completely self-contained.
They also have lots more graphs and discussion here.