Meteorologically, it is simply described as a or a period of unseasonably warm weather following a killing frost.
If you have noticed a warm, hazy period occurring in late autumn—usually after the first frost—you are likely experiencing the phenomenon traditionally known as an "Indian Summer." However, in recent years, meteorologists, dictionaries, and style guides have moved away from that term in favor of more neutral language. what is indian summer called now
(or St. Martin's Summer )