Here is an update of some work I did a few years ago. The research question is: is summer, and the attendant threat of fires, starting earlier?
Since one warm day does not summer make, we need to use some threshold that integrates over a period of time. As a first crack I calculated daily 7-day running mean temperatures and then set the start of summer as the first day when this value was ≥55F. Offhand, this seems to me to be correlated with increased activity in the biotic world. By this criteria, the summer started in Fairbanks this year on Friday, May 20th.
What I've plotted here is then the 5 year running mean of this date, in order to smooth out the year to year variability. As you can see, there as been a fairly dramatic trend toward an earlier start of summer since the mid 1940s, even with the short-lived spike around 2000.
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