An Indian summer is a period of unseasonably warm, dry weather that sometimes occurs in autumn in temperate regions of the northern hemisphere. The term originated in New England and probably arose from the Indians practice of gathering winter stores at this time. The exact definition of an Indian summer can vary, but there are several criteria that are commonly used to describe this weather phenomenon:
- It's a period of abnormally warm weather.
- It occurs in mid to late autumn.
- It occurs after the first frost, or more specifically the first "killing" frost.
- It is generally characterized by clear skies, sunny but hazy days, and cool nights.
Some sources also mention that Indian summer may occur several times in some years and not at all in others, and it often persists for a week or longer. The weather conditions that lead to an Indian summer in the United States are a cool, shallow polar air mass stagnating and becoming a deep, warm high-pressure center, which is characterized by a strong low-level temperature inversion that produces a stable air stratification. This results in inhibited vertical air motions and the concentration of smoke and dust near the ground, which accounts for the haziness.
