An Indian Summer is a time of year when temperatures are considerably above normal. The term dates as far back as the early 1800s, with possible connections to early American Indian tribes, and typically is associated with a stretch of warm, quiet weather occurring in the autumn months. Though the exact circumstances that constitute a true Indian Summer are debated, most agree on a few solid aspects that define one.
Many references to an Indian Summer state that it can not occur until there has been a killing frost or freeze. While this is typically and almost exclusively a weather event that occurs in the fall, some areas have reported weather conditions applicable to an Indian Summer in the winter months.
The most commonly accepted locations for an Indian Summer to occur are the Mid-Atlantic states north to New England and west across the Ohio Valley, the Great Lakes areas, and the Midwest, as well as into the Great Plains states. Essentially, anywhere that has a defined winter season can experience one.
The weather patterns that impact and cause an Indian Summer involve large areas of high pressure along the East Coast with warm temperatures from the South and Southwest being pulled up. This results in a clockwise rotation of wind around the area of high pressure. It is possible for the warmer than normal stretch of weather to last for a few days to over a week, but it is widely agreed upon that an Indian Summer involves at least three days of such weather.
Though the term "Indian Summer" is typically reserved to refer to climatic conditions in North America, other areas, such as Europe, refer to similar seasons differently. In areas where no defined winter season or freezing occurs or is likely to occur, the term has little meaning, if any at all.