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Some of the most interesting weather of the year occurs in autumn when the forces of nature give Wisconsin several abrupt and sometimes unpleasant transitions from summer to winter. Several of these transitions feature sharply cooler weather leading to the first freezing temperatures and the end of the growing season. But that doesn't mean all pleasant weather has departed until next spring. In most years, the first frost is followed by one or more intervals of mild, dry, pleasant weather. This welcome temporary warmth provides both opportunity and a reason for outdoor recreation while wearing summer clothing – a last chance to do this before the long Wisconsin winter sets in.
"Indian summer" is the commonly used meteorological term for abnormally warm weather in mid- or late autumn with generally clear skies, sunny, hazy days, and cool nights. According to the Glossary of Meteorology, at least one killing frost and usually a substantial period of normally cool weather must precede the warm spell in the northeastern United States in order for it to be considered a true "Indian summer." The glossary also states that while Indian summer does not occur every year, some years may have two or three such periods.
The term "Indian summer" is most often heard in the northeastern United States, but its usage extends throughout English-speaking countries. It dates back at least 200 years, but the origin is not certain; the most probable suggestions relate it to the way the American Indians availed themselves of the extra opportunity to increase their winter stores. According to New England Native American folklore, Indian summer is sent on a southwest wind from the spirit Countantowit.
European folklore has Indian summer equivalents: "Old wives' summer" in central Europe, probably from the widespread existence of "old wives' tales" concerning this striking feature of autumn weather; "halcyon days" also in central Europe, based on a period of fine weather described in Greek mythology; and "all-hallown summer," "St. Luke's summer," and "St. Martin's summer" in England, depending on when the autumn time the weather occurs.
Indian summer is an example of a weather "singularity" – a characteristic meteorological condition that recurs on or near a specific calendar date. The concept originated in folklore: agricultural societies noted that certain animals and plants responded in set ways associated with weather that recurred on nearly the same date each year. These collective observations were eventually collected and written in guides like the Farmer's Almanac that provided calendars of favorable planting and harvesting dates.
I was fortunate to experience many wonderful Indian summers while growing up on a dairy farm in western Wisconsin. I remember jumping in piles of leaves on a hillside near a little country school I attended. I also recall walks on the family farm through the hills and valleys of birch, maple, and oak with their vibrant shades of gold, orange, and crimson.
One of the most notable Indian summers on record occurred only two and one-half months after I was born. I'm sure it left a favorable impression for as long as I can remember Indian summer has been one of my favorite times of the year.
When does Indian summer occur in Wisconsin? How often can we expect one? To answer these questions and to learn other facts about Indian summers in Wisconsin, I examined daily weather records for the last 100 years (1898-1997) for three representative locations: Medford in northwestern Wisconsin, New London in east central Wisconsin, and Madison in southern Wisconsin.
Weather textbooks do not contain precise criteria for what qualifies as an Indian summer day, so I defined what I think is appropriate. An Indian summer day in Wisconsin meets all of the following criteria:
September 28th is the median date of the first frost in Wisconsin for all 118 cooperative weather stations during the last 30 years and for Medford, New London, and Madison combined for the last 100 years. First frosts have occurred as early as August 30th in Medford (1915 and 1931), September 10th in New London (1917), and September 12th at Truax Field in Madison (1955). First frosts have occurred as late as October 19th in Medford (1938), October 24th in New London (1911), and November 12th when the weather station was on Bascom Hill in Madison (1946). However, about two-thirds of first frosts occurred in the three weeks between September 20th and October 11th.
Given this wide variation in first frost dates, Indian summer days in the last 100 years have occurred as early as August 31st and as late as November 19th. However, Indian summer is most frequent from the end of September through the first three weeks of October. My data showed October 14th to have the greatest frequency of Indian summer weather, with peak frequencies during the first few days of October as well as mid-October. During these peaks, Indian summer weather occurs 25 percent of the time or more frequently.
Indian summer, on the average, occurs earlier in northern Wisconsin than in areas further south. At Medford, the peak frequency occurred on October 2; in New London there were two peaks – on October 4 and 14; in Madison on October 14.
An average year, if one ever occurs, provides a given Wisconsin locale with eight Indian summer days and a 90 percent chance of having at least one Indian summer day. Extremes can provide more than 30 Indian summer days in a fall season (Madison had 32 in 1963 when the first frost occurred on September 14th, about three weeks earlier than normal). As you would expect, the later the date of the first frost, the smaller the number of Indian summer days on the average. Who said you could have your cake and eat it too?
The typical Indian summer day has a high temperature of 72° F and a low temperature of 45° F. Although this may seem cool when compared to typical summer highs, these temperatures are fully 6 to 10 degrees warmer than the autumn norms. Besides, when you're outdoors during the day, 72° F with low humidity can be more comfortable than 80° F with high humidity.
Occasionally autumn weather patterns get stuck and Wisconsin can sustain a prolonged Indian summer weather pattern for a spell of five days or longer. The chart below identifies the most recent of these statewide Indian summer spells. The longest lasted 11 days in 1947.
A typical Indian summer weather map shows warm high pressure at the surface in the vicinity of Wisconsin. Higher up in the atmosphere, the jet stream, which divides cold air to its north from warm air to its south, is located north of Wisconsin, preventing cold air from reaching us. This allows Wisconsin to bask in a mild dry air mass usually of Pacific Ocean or southwest United States origin. At the surface, the wind flow is usually from the west or south. Indian summer persists until a cold front passes while the jet stream drops south of Wisconsin, which allows a cold Canadian air mass to take charge, lowering temperatures to below Indian summer levels.
Wisconsin farmers take advantage of Indian summer weather to harvest crops and tackle other farm chores before the onslaught of winter. It's a good time to cut and bale the last crop of hay, finish the corn silage, and complete potato and apple harvests. Cranberry, soybean and field corn harvests usually make great progress on these days. Farmers also begin fall tillage and plowing in the warmth of Indian summer.
Those of us who "farm" on a much smaller scale have plenty to do during Indian summer.
Planting winter hardy bulbs such as daffodils and tulips, digging out and storing frost sensitive bulbs and tubers such as gladiolas, calla lilies and begonias, and raking the first of several waves of leaves are typical Indian summer gardening chores. It's also time to harvest late potatoes, carrots, Brussels sprouts, late cabbage, and the last tomatoes of the season – assuming the frost preceding Indian summer was not too severe and you protected the tender tomatoes from a freeze. Hardy mums usually provide spectacular displays that make Indian summer gardening even more enjoyable.
Don't let chores consume all your Indian summer hours. Whether by coincidence or grand design, Indian summer usually falls near the foliage peak, at the end of September to the first week of October in northern Wisconsin, and mid-October in southern Wisconsin. Hit the trail and wander The Great River Road, a Rustic Road, and just about any meandering country route to see colorful views. Or enjoy the hunting and fishing seasons of Indian summer including bowhunting for white-tailed deer, gunning for gray and fox squirrel, crow, ruffed grouse (northern and western zones), cottontail rabbit (northern zone), and angling for lake sturgeon or walleyes.
Indian summer won't wait for weekends over weekdays. With an average of only eight Indian summer days per autumn, you may need to play hooky to visit your favorite golf course for a final round, get the boat out on the lake one more time, spend a day at a state park or walk through your favorite arboretum. You owe it to yourself to get out there in a T-shirt and shorts before the first snowflakes of winter arrive in just a few weeks.
Climatologist Dick Kalnicky oversees grants and contracts to cleanup contaminated lands for DNR's Bureau of Remediation and Redevelopment.