An Indian summer is a meteorological phenomenon that occurs in the autumn. It refers to a period of considerably above normal temperatures, accompanied by dry and hazy conditions, usually after there has been a killing frost. Depending on latitude and elevation, the phenomenon can occur in the Northern Hemisphere between late September and mid November
The modern use of the term is when the weather is sunny and clear, and above 70 °F, after there has been a sharp frost; a period normally associated with late-October to mid-November.
- In Colonial New England and New York, Indian Summer referred only to a January Thaw, when American Indian Raiding parties could be expected in the western and northern areas: the ground had briefly lost its snow cover so tracking the Native American raiders back to their winter camps was much more difficult for the Colonials.
- In The Americans: The Colonial Experience, Daniel J. Boorstin speculates that the term originated from raids on European colonies by American Indian war parties; these raids usually ended in late autumn (due to snow covered ground), hence summer-like weather in the late fall and mid winter was an Indian Summer, a time raiding parties could be expected.
- Two other known uses of the term in the 18th century are from accounts kept by two army officers leading retaliation expeditions against Indians for winter raiding parties on settlers in Ohio and Indiana in 1790, and Pennsylvania in 1794.
- It may be so named because this was the traditional period during which early American Indians harvested their crops of squash and corn.
- In the same way that Indian giver was coined for people who take back presents they have bestowed, the phrase Indian summer may simply have been a way of saying "false summer".
- When European settlers first came across the phenomenon in America it became known as the Indian's Summer.
- The haziness of the Indian Summer weather was caused by prairie fires deliberately set by Native American tribes.
- It was the period when First Nations/Native American peoples harvested their crops.
- The phenomenon was more common in what were then North American Indian territories.
- It relates to the marine shipping trade in the Indian Ocean (this is highly dubious as it is entirely remote from the early US citations).
- It originated from raids on European settlements by Indian war parties, which usually ended in autumn.
- In a parallel with other 'Indian' terms it implied a belief in Indian falsity and untrustworthiness and that an Indian summer was an ersatz copy of the real thing.