Almost every year around this time the weather discussion turns to the topic of “Indian summer.” Whether this term is politically correct or not, and I know I have discussed this topic in the past, I think that revisiting this topic every couple of years is useful to help break down some of the misconceptions surrounding this topic.
First of all, I think the main question is, just what defines an Indian summer? The most common response that I hear or see is that Indian summer is a period of warm weather that occurs sometime in late September or October. This is what the people I overheard must be basing their assessment on. If you do a little digging around about the history or origins of Indian summer, you would discover that while this general description is actually not that bad, the problem is that there doesn’t seem to be a set agreed-upon definition. The definition of Indian summer appears to be set by the geographical region rather than by a definite set of criteria, with different areas of North America defining Indian summer a little bit differently.
For the area of the Great Plains and the Prairies of Canada, Indian summer is defined by a particular set of criteria, which should help us to determine when it is occurring. It is generally accepted that for Indian summer to occur, the following conditions (source: Keith C. Heidorn) must be met:
- There needs to have been a hard or killing frost.
- Mostly clear skies (or perhaps local fog at night).
- No precipitation.
- Light winds and generally calm nights.
- Daytime maximum temperatures greater than 18 C.
- Nighttime minimum temperatures staying above freezing.
- Conditions lasting for at least three days.
If we accept these seven conditions as being the indicators that Indian summer is occurring, then the upcoming week just might meet the criteria. The biggest issue we’ll probably have is the light winds, as the current forecast looks to be warm but fairly breezy.
The term “Indian summer” has some fairly uncertain origins. The earliest reference found in the literature dates back to Jan. 17, 1778, when J. Hector St. John de Crèvecoeur wrote in a letter:
“Sometimes the rain is followed by an interval of calm and warmth which is called the Indian summer; its characteristics are a tranquil atmosphere and general smokiness. Up to this epoch the approaches of winter are doubtful; it arrives about the middle of November, although snows and brief freezes often occur long before that date.”
The fact that St. John de Crèvecoeur stated that this was called Indian summer suggests that this term must have been in use before this time.
The meteorological conditions behind Indian summer usually involve an area of high pressure building into our region in late September or October. With strong areas of high pressure at this time of the year, the calm conditions and cool air combined with the longer nights, tend to give us our first killing frost. Once the area of high pressure pushes by to the south, the clockwise flow of air around the high helps to switch our wind to the southwesterly, ushering in warm air, something that we are expected to see this week.
In some years this area of high pressure will stall out or combine with a ridge of high pressure over the U.S. and will help to maintain warm, sunny conditions for several days. On occasion, this may last for several weeks.
Looking back over fall weather records for our region, the last time we experienced a true, long Indian summer was in 1997, when we experienced six days in a row of above-average temperatures at the end of September, following a killing frost. Probably one of the most spectacular Indian summers occurred back in 1953, when we experienced temperatures a good 10 C above average for two weeks in the middle of October, with temperature readings peaking in the mid-20s. The only flaw with this 1953 Indian summer is that there was rain on two days. The next best Indian summer that met all of the criteria occurred in 1964. After recording a temperature as cold as -8 C earlier in September, warm temperatures were experienced between Oct. 11 and 16, with temperatures peaking near 28 C by the 13th and maintaining themselves near the 20 C mark until the 16th. Some will argue that the first half of last November was a perfect example of a late Indian summer. When you look back at this period, while it was way above average, temperatures barely made it to 18 C.
As I wrap up this look at Indian summer, I think it’s appropriate to conclude with Walt Whitman’s perfect quote about it: “It is only here in large portions of Canada that wondrous second wind, the Indian summer, attains its amplitude and heavenly perfection, — the temperatures, the sunny haze; the mellow, rich, delicate, almost flavoured air: Enough to live — enough to merely be.”