Grayson County TXGenWeb

Indian Summer

An unseasonably warm, dry and calm weather, usually following a period of colder weather of frost in the late Autumn (or in the Southern hemisphere, where the term is less common, the late Spring).

WIKIPEDIA
An Indian summer is a period of unseasonably warm, dry weather, occurring after the end of summer proper. The U.S. National Weather Service defines this as weather conditions that are sunny and clear with tempertures above 21 C (70 F), following a sharp frost (the "Squaw Winter"). It is normally associated with late-September to mid-November. There are multiple explanations for the name. The North American Indians - native Americans - depended upon periods of fine, quiet, sunny weather at this time of the year to complete their harvest to see them through the winter. The term might refer to the weather patterns in the Indian Ocean, where ships' hulls were marked "I. S." to indicate the level at which they should be loaded during that season.

Indian summer was first recorded in Letters From an American Farmer, a 1778 work by the French-American soldier turned farmer, J. H. St. John de Crevecoeur (aka Michel-Guillaume-Jean de Crevecoeur):
"Then a severe frost succeeds which prepares it to receive the voluminous coat of snow which will soon follow; thought it is often preceded by a short of interval of smoke and mildness, called the Indian Summer."

In 1834, John Greenleaf Whitter used the term when in his poem Memories he wrote of "The Indian Summer of the heart!"

source: The Path Finder
http://www.phrases.org.uk/meanings/indian-summer.html