"Then a severe frost succeeds which prepares it to receive the voluminous coat of snow which is soon to follow; though it is often preceded by a short interval of smoke and mildness, called the Indian Summer." (from The Phrase Finder)American historian Daniel Boorstin in The Americans: the Colonial Experience "speculates that the term originated from raids on European colonies by Indian war parties; these raids usually ended in autumn, hence the extension to summer-like weather in the fall as an Indian summer. Two of the three other known uses of the term in the 18th century are from accounts kept by two army officers leading retaliation expeditions against Indians for raids on settlers in Ohio and Indiana in 1790 and Pennsylvania, in 1794." But there is also the suggestion that is named for the traditional period when early Native Americans harvested squash and corn. (see Wikipedia article on Indian Summer)
In Western Europe this type of weather was called St. Martin's Summer from the feast of that saint on November 11, while Russians and East Europeans called it Old Ladies Summer. And Indian Summer is the name given to the Milwaukee ethnic festival held in September honoring Native Americans: Indian Summer Festival.
No matter what name you give it, we have been enjoying what the National Weather Service defines as, "a period of considerably above normal temperatures, accompanied by dry and hazy conditions ushered in on a south or southwesterly breeze." See the attempt by a weather historian to deal with the naming issue in the article, Just what is Indian Summer and did Indians really have anything to do with it?
If you are in the midst of Indian Summer, please enjoy it! And check out more skies on Skywatch Friday.