The leaves have changed colors and the air is crisp and cool. You've swapped your shorts and tees for jeans and sweaters, but then a warm spell comes barreling in. Guess it's Indian summer ... or is it?

We tend to refer to Indian summer as anytime we get an unseasonably warm burst of weather in the fall. But there's an official definition of Indian summer, according to the Old Farmer's Almanac, and it has to meet certain criteria:

  • It can't just be any old warm spell. The atmosphere also has to be hazy or smoky with no wind. The barometer must be standing high, and nights should be chilly and clear.
  • The haze and temperature swing between day and night are caused by a moving, cool, shallow polar air mass converting into a deep, warm stagnant high-pressure system.
  • The warm days must follow a burst of cold weather or a good hard frost.

All this must happen between St. Martin's Day (Nov. 11) and Nov. 20. The Old Farmer's Almanac has a saying: "If All Saints' (Nov. 1) brings out winter, St. Martin's brings out Indian summer."

Where the term 'Indian summer' comes from

large bonfire The name may have developed because this was the time of year when Native Americans often built large fires which created the smoky hazr in the air. (Photo: Ton Blackmarine/Shutterstock)

According to the New England Historical Society the earliest use of the expression was in the late 1700s. Boston lexicographer Albert Matthews found it in a letter written in 1778 by a New York farmer, Hector St. John de Crevecoeur, and thought it was commonly used at the time.

In "History of the Valley of Virginia," Samuel Kercheval wrote that pioneers feared Indian summer. "It afforded the Indians — who during the severe Winter never made any incursions into the settlements — another opportunity of visiting the settlements with destructive warfares."

But there were other theories too, that also involved Native Americans.

Some suggested the mini-season was named because it's the time of year when Native Americans typically hunted or because they were the ones who first described it to Europeans. Others theorized that it was named because people noticed the unseasonably warm weather in places where Native Americans lived.

Politician Daniel Webster said he thought colonial settlers developed the name because they believed Native Americans created great fires, which were responsible for the smoky haziness of the air.

Others say that settlers named the season after being told Native Americans considered the pleasant weather a gift from Cautantowwit, a god who lived in the Southwest.

In modern times, the name may raise eyebrows, but as this thoughtful discussion explores, perhaps the best answer is to be informed about the history and use the term with respect.

Outside of the U.S.

These autumn heatwaves are also common in other parts of the world. They're known as Indian summers in the United Kingdom, meteorologist Philip Eden tells the BBC. But in more rural areas, the season has other names.

"In mid-October, for instance, it would have been called 'St Luke's Little Summer' as the feast day of St Luke falls on 18 October, while in mid-November it would be 'St Martin's Summer' as St Martin's feast day is 11 November," Eden writes. "Shakespeare also used the expression 'All Halloween Summer' in Henry IV part I for a period of warm sunshine as October gives way to November. A more generic but now (sadly) politically incorrect idiom is 'Old Wives' Summer'."

According to the Farmers' Almanac, many countries like England, Italy, Sweden and Portugal have outdoor festivals leading up to St. Martin's Day or also celebrate St. Luke's Summer or "All Hallown Summer" around All Saints Day on Nov. 1.

Whatever you call it, that unseasonable little burst of sunny, warm days is a welcome bit of pseudo-summer in the middle of fall, offering a break before winter rears its much-less-temperate head.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

What exactly is an Indian summer?
That warm burst of summer in the middle of fall actually only happens in a specific window.