Indian summer and autumn;
describes them beautifully;
uses them as an appropriate metaphor for stages in the decline of Technic civilization.
For both, see here.
SM Stirling's Mathilda Arminger experiences an Indian Summer:
"Indian Summer here in the Kickapoo Valley had a dishevelled beauty not quite like anything back home, full of a sadness that was like a recollection of childhood..."
-SM Stirling, The Sword Of The Lady (New York, 2009), Chapter Thirteen, p. 375.
We are told that she does not recollect her actual childhood:
"...but somehow the world itself embodying the feeling the memory brought. The security she'd felt at [her father's] effortless strength, the bitterness not just of loss, but loss of that child's innocent trust." (ibid.)
Innocence lost is the pervading theme of Anderson's Time Patrol series. See here.
Stirling goes on to describe the Indian Summer:
"The leaves were still a mantle of old birch gold and maple crimson, lit at their tops with the last light..." (ibid.)