As the our perfect autumn weather comes to an end -- meteorologists have projected rain this weekend, whatever that is -- many people have been left wondering: what has been different about this fall that has made it so pleasant out? And the answer may lie, not in the popular culprit of global warming, but in an age-old phenomenon called Indian summer.
Although Wisconsin has been warming since the 1950s, says Michael Notaro, an associate scientist at the Center for Climatic Research at UW-Madison, over that same period of time the autumn temperatures have actually changed quite minimally and fall has tended to be wetter -- a stark contrast to what Wisconsites have been experiencing over the past several weeks.
However an Indian summer is not just a pleasant day in October or November; the actual criteria -- though some of it appears to be in dispute among climatologists -- are much more specific than that. In Wisconsin, an Indian summer is characterized by a period of anomalously dry weather with warm temperatures above 65 degrees and minimums not below freezing, and occurs after the first frost (some scientists don't believe in the frost criteria) and before the first snowfall.
Yet, science aside, most folks just think of it as a time period of sunny and warm weather after the leaves have turned colors, says Ackerman.
An Indian summer is often most distinct in the central to eastern U.S., where temperatures fluctuate a lot in autumn due to variations in the jet stream, says Notaro. This jet stream variation typical of an Indian summer allows us Wisconsinites to bask in the limited luxury of a Pacific or Southwest air mass, before it is disrupted by the Canadians when their cold air bears down on us, ending the bout of warm weather.
Another interesting inquiry revolves around the origin of the name "Indian summer". Notaro speculates that the term originated in the 1700s, and is derived from the period when the Native Americans harvested their crops. In an essay by weather historian Bill Deedler, several other possibilities and insights into the name are given. Apparently, it was first written down by a Frenchman in 1778, but it is believed to have been used well before then. The article conjectures that the term could refer to the period in the fall when Native Americans tended to hunt game, and attack colonial settlements before winter arrived; or, the term "Indian summer" signifies a "fool's" summer.
Another author, H.E. Ware, theorizes that the name has no relation to Native Americans, rather it stems from the cargo ships that crossed the Indian Ocean and loading more goods than usual during this window of good weather; some ships even had "I.S." marked on their hulls to denote how much further submerged they could be during the calm seas of an Indian summer.
Similar weather patterns are experienced throughout Europe and carry their own names: "Old Wives' Summer" and "halcyon days" as described in Greek Mythology, writes climatologist Dick Kalnicky in an essay for the Wisconsin DNR.
Kalnicky also writes that the episode usually doesn't last more than five days in southern Wisconsin, one of the longest being 11 days in 1947. In Wisconsin, this type of warm spell awards farmers an opportunity to plant winter cover crops, harvest the season's bounty, and finish chores before the winter sets in. Even the dry weather this fall has yet to be a problem for farmers and many welcome the sunny days for the ease it provides their work, though the fall's exceptional dryness has hampered the establishment of winter wheat and dried soybeans to an extent which may affect their harvest and quality, depending on their intended use, says Shawn Conley an associate professor for the Dept. of Agronomy at UW-Madison.
But, again, for most folks this fall has provided optimal weather for all sorts of outdoor activities and regular old enjoyment of the passing days and the changing leaves. And because some scientists say it is possible to experience the phenomenon more than once in a season, and as deep into autumn as late November, you may want to cross your fingers Wisconsinites, because there could be another Indian summer on the way this year.