National Weather Service United States Department of Commerce
 

The first cold spell of fall signifying the change of seasons makes many people long for Indian Summer.  But what exactly is Indian Summer?

True Indian Summer is a period of abnormally warm weather following the killing freeze of autumn. A killing freeze occurs when the overnight temperature reaches 28 degrees or colder and may or may not occur with frost.

I
ndian Summer typically occurs in mid to late autumn and can occur more than once.

In Europe, the equivalent of Indian Summer is termed Old Wives Summer or in poetry as Halcyon Days.  In England, it is known as either St. Martin's Summer or St. Luke's Summer depending upon the date of occurrence.