An Indian summer is a period of unseasonably warm, dry, and pleasant weather that occurs in autumn, often after the first frost, characterized by hazy skies and light winds. This warm spell typically happens in late October or early November in temperate regions of the Northern Hemisphere, though it can occur at other times or not at all in some years. The origin of the term is debated but is believed to relate to Native American activities, such as harvesting crops, or their presence during these periods of warm weather.
Characteristics of an Indian Summer
- Warm, Dry Weather: The primary feature is an unexpected return to warm, comfortable temperatures in the fall.
- Hazy Skies: A distinctive characteristic is the presence of a hazy atmosphere.
- After a Frost: A "true" Indian summer is often understood to occur after the first killing frost has already taken place.
- Duration: The warm period can last for several days to over a week.
Causes
A high-pressure center forms, characterized by stable air and a low-level temperature inversion, which traps smoke, dust, and aerosols near the ground, contributing to the haziness.
The jet stream plays a role, with a sharp shift to the north bringing about the warm conditions.
Origins of the Name
The term is believed to have originated in New England.
Theories suggest it could be linked to Native American traditions, such as the practice of gathering winter stores during these warm spells or the time of harvesting crops. Other theories involve the perception that Native Americans caused the smoky haze through controlled fires or that the weather was considered a gift.
Alternatives
In the United Kingdom, the phenomenon is sometimes called All-hallown summer or Old Wives' summer. Other historical names in Britain have included St Luke's Little Summer and St Martin's Summer, referring to their associated feast days.