Indian summer unfolding this autumn

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I heard the weather forecaster mention the chance of an Indian summer last week and it got me thinking about this old print my father had hanging in his office. The two juxtaposed images of a young man and his grandfather around a small bonfire: as the scene turns to night the smoke that arose from the fire reveals an image of dancing American Indians, the hay stacks in the distance become teepees and the verse that accompanies the print reads like a tale from a not so enlightened time: “Yep, sonny, this is sure enough Injun summer. Don’t know what that is, I reckon, do you?”  It goes on for several paragraphs explaining, “That’s when all the homesick Injuns come back to play.” And more on spirits and bad grammar, you get the idea.
It turns out that the image and verse by John T. McCutcheon was reprinted in the Chicago Tribune every fall, which explains why my father would have it because he was from the Chicago area. He was also a very sentimental person who loved this time of the year so the nostalgia of the print out -weighed the off color depiction and language of lore. (Remember that I have a master’s in American Indian Studies so the stereotypical imagery is far from the truth.)
Alas, the image is nostalgic for me now even if the verse proves a bit offensive; and a real Indian summer is upon us even if my version is a bit different than Mr. McCutcheon’s. Fall is a transition season that proves warm one day and cold the next. We know that the eventual outcome is winter but we hold on to the idea of just one more, warm sunny day and this year it keeps coming.
Some areas have had a hard frost, which sets the scene for a real Indian summer. Frost occurs when the surface temperature, at ground level, drops to 32 degrees or lower and dew is present. The crazy thing is that this can happen even if the ambient air temperature is as high as 40 degrees.
On still, cool, clear, nights frost is more likely because the accumulated heat in the soil radiates back into the atmosphere at a faster rate leaving more time in the morning hours, just before dawn, when the surface temperature is at or below freezing. And, we all have been taught that cool air settles and warm air rises so on still nights there are no breezes to mix the air temperatures together.
The difference between a light, hard or killing frost is really the amount of time that the surface temperatures stay at or below freezing. A short period of time just before dawn usually only allows single ice crystals to form; longer periods of time allow the crystal to connect like filigree, this is a hard or hoar frost. A killing frost is called black frost because it usually turns plant tissue black.
When the weather people say we are in for a light freeze they mean temperatures will fall between 29 and 32 degrees, cold enough to kill plant tissue of tender plants, one reason is because of more moisture in the foliage; a moderate freeze is 28 to 25 degrees, enough to blacken foliage of most plants (of course, not evergreens); and a hard freeze is 24 degrees or colder. By the time a hard freeze hits most hardy and deciduous plants have dropped foliage and gone dormant for the winter.
Indian summer is a weather event that brings back summer-like weather after a hard or killing frost. The American Meteorological Society defines it as “A time interval, in mid or late autumn, of unseasonably warm weather, generally with clear skies, sunny but hazy days, and cool nights…at least one killing frost and preferably a substantial period of normally cool weather must precede this warm spell in order for it to be considered a true Indian summer.” Mr. McCutcheon missed this part of the seasonal description.