This is not exactly “fall” weather. (Eric Cox)

It’s the second day of fall, and temperatures in the D.C. area are pushing 90 degrees. This summer has been relentlessly hot, and it seems like it will drag into fall. We’re hearing the phrase “Indian summer” tossed around. But what does that really mean?

“Health warnings as 31C Indian summer approaches,” a recent U.K. headline blared — London was forecast to set the highest September temperature in 50 years. But Indian summer is not just hotter than normal temperatures during harvest time. It’s more complicated than that.

According to the Farmer’s Almanac, a repository of weather folklore, Indian summer is a term “[which] is used when we experience a little revival of summer after it should have finished. The sky is usually cloudless but hazy or even smoky looking, especially toward the horizon.” Mmm… okay.

The National Weather Service defines it similarly, sans haziness: “An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.”

The term Indian summer is commonly used in the U.S. and across the pond in the U.K., but its exact origins are unknown. A weather historian for the National Weather Service, William R. Deedler, thinks the first mention of it was in a 1778 letter written by Frechman St. John de Crevecoeur. In the letter, de Crevecouer describes a rainy period in New England in November that was followed by warmth, “a tranquil atmosphere” and smoky skies.

“Deedler wrote that several hypotheses exist to explain the ‘Indian’ part of the name. Some believe the mild weather provided native ‘Indians’ with their best opportunities to hunt during that season. Others believe it was the time that ships traversing the Indian Ocean attempted their deliveries. Again, there is no clear way to tell,” said Adam Lukach in an on-line essay.

Does this apply to London’s case of warmth? What was the weather like proceeding it – was there a prolonged period of cold? Is the middle of September considered mid-Autumn? The answer to all of them is no. In the six days leading up to the heat, much of the U.K. was running above average — some for many days. What happened in London was an early autumn or late summer heat wave, not an Indian summer.

When is it appropriate to refer to an Indian summer in the Washington region? If you base it on the National Weather Service definition of “an unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather,” it would be early to mid-October. That time of year meets the criteria, and is the soonest an Indian summer should be invoked around these neck of the woods.