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An Indian summer is a period of unseasonably warm, dry weather that sometimes occurs in autumn in temperate regions of the northern hemisphere during September to November. In an article on the US National Weather Service's website, weather historian William R. Deedler writes that Indian Summer can be defined as "any spell of warm, quiet, hazy weather that may occur in October or November." Several references describe a true Indian Summer as not occurring until after the first frost, or more specifically the first "killing" frost.