Indian Summer is a period of unseasonably warm, dry weather that sometimes occurs in autumn in the Northern Hemisphere. The US National Weather Service defines this as weather conditions that are sunny and clear with above normal temperatures, occurring Late-September to mid-November. It is usually described as occurring after a killing frost.
Etymology and usage
Late-19th century Boston lexicographer Albert Matthews made an exhaustive search of early American literature in an attempt to discover who coined the expression. The earliest reference he found dated from 1778, but from the context it was clearly already in widespread use. William R. Deedler (historian for National Weather Service) in a 1996 essay wrote that Matthews' 1778 reference was a letter by Frenchman St. John de Crevecoeur.
Although the exact origins of the term are uncertain, it was perhaps so-called because it was first noted in regions inhabited by Native Americans ("Indians"), or because the Native Americans first described it to Europeans, or it had been based on the warm and hazy conditions in autumn when Native Americans hunted. The title of Van Wyck Brooks' New England: Indian Summer (1940) suggests inconsistency, infertility, and depleted capabilities, a period of seemingly robust strength that is only an imitation of an earlier season of actual strength.
In British English the term is used in the same way as in North America. In the UK, observers knew of the American usage from the mid-19th century onwards, and The Indian Summer of a Forsyte is the metaphorical title of the 1918 second volume of The Forsyte Saga by John Galsworthy. However, early 20th-century climatologists Gordon Manley and Hubert Lamb used it only when referring to the American phenomenon, and the expression did not gain wide currency in Britain until the 1950s. In former times such a period was associated with the autumn feast days of St. Martin and Saint Luke.