You have probably heard the term Indian summer used before — it is not uncommon for any warm day in the fall to be described as such. However, this is not accurate. In fact, the term refers to a specific set of meteorological conditions during a specific set of days. Here is a brief guide to what creates an Indian summer and when it occurs.

What is an Indian summer?

Trees changing color on a mountain range with a haze in the air
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At its core, the term refers to an unseasonably warm fall day. However, to be a true Indian summer, there are a few specific requirements that must be met.

First, a day can only be described as such if there has already been one hard frost. This is because another characteristic of an Indian summer is the dramatic swing in temperatures during the night and the day.

The atmospheric conditions are important as well. An Indian summer day is defined by a smoke or haze that lingers in the air, which is a result of the cool polar air mass interacting with the warm high-pressure system in the air. These slow-moving air systems also mean that an Indian summer day usually has little to no wind, which can make the haze seem even thicker.

When is an Indian summer?

Sunlight coming through trees that have red and orange leaves
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Not only must these specific atmospheric conditions be met, in order for a day to be called a part of Indian summer, it must occur in a very specific time frame. That window is from St. Martin’s Day (November 11) to November 20.

Even if an autumn day is very hot, very hazy, and has no wind, if it does not fall between the 11th and the 20th of November, it is not a part of an Indian summer.

Where did Indian summer get its name?

Haze on a lake with trees reflected in the water
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While the conditions that define an Indian summer day may be agreed upon, the origin of the term is not so universally known.

One theory is that the term is a reference to Algonquin mythology. The Algonquin indigenous people believed that the periods of warm weather were sent by their god who ruled over the southwest, Cautantowwit. Another theory states that the term was coined by settlers of Colonial America and was a reference to attacks on their winter stockpiles by Native Americans during the fall.

Neither of these theories has enough evidence to be conclusive, however. What we do know is that the first recorded usage of the term is found in the 1778 letters of Jean de Crevecouer, a French farmer and American immigrant. In his letter he describes a day after a period of frost that is marked by “a short interval of smoke and mildness,” much as the term is used today.

Indian summers around the world

Lac-Superieur, Mont-Tremblant, Quebec in the fall
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The phenomenon of an unusually warm autumn has different names all around the world.

  • In Germany, Austria, Switzerland and Hungary, a warm period in the fall is known as a “altweibersommer” or an old woman’s summer.
  • In Bulgaria, it is sometimes called a gypsy summer or a poor man’s summer.
  • In Ireland, it is called "little summer of the geese" in the native Irish language of Gaelic.

So, if you would like to see if the area you spend your fall in will experience an Indian summer, pay attention to two important details. First, keep an eye out for when the first frost falls. Second, pay attention to the temperature between the days of November 11 and November 20. If you see a hard frost before these days, with high temperatures in between them, you are in an Indian summer.