We are not sure of the origin of this expression, but it has been used for over 200 years in reference to a weather phenomenon that occurs in the fall, usually in October for our state. It occurs when the autumn weather is characterized as sunny and warm.
Indian summer can occur only after the first frost but before the first snowfall. It occurs after the leaves have turned color and includes dry weather conditions with maximum temperatures greater than 65 degrees and minimum temperatures greater than 33 degrees.
We usually get these conditions when a high pressure system is over the eastern United States. This anticyclone pushes dry southwest winds into our area, resulting in what many consider pleasant weather conditions.
Given the above definition, Indian summer usually occurs in the first three weeks of October, and Wisconsin has, on average, eight days of Indian summer each fall. These days don’t have to be consecutive. We can have a year without an Indian summer as well as a year with more than one Indian summer.
This year, our October mean temperature was close to the average temperature for most of the state, and precipitation in southern Wisconsin was slightly above. We also have had a minimum temperature below freezing, and no snow fall. So, we are in a time when Indian summer can occur if the temperature is warm enough, like it was on Oct. 27 when the high temperature hit 75 degrees and the minimum was in the low 50s. This ties for the ninth-latest 75-degree or warmer day in a calendar year. On average, the last 75-degree day occurs on Oct. 9.