In the fall, it seems that almost any warm day is referred to by most people as "Indian summer."
And, while their error is certainly not of the world-shaking variety, they are, for the most part, in error. Here is criteria for an Indian summer:
- As well as being warm, the atmosphere during Indian summer is hazy or smoky, there is no wind, the barometer is standing high, and the nights are clear and chilly.
- A moving, cool, shallow polar air mass is converting into a deep, warm, stagnant anticyclone (high pressure) system, which has the effect of causing the haze and large swing in temperature between day and night.
- The time of occurrence is important: The warm days must follow a spell of cold weather or a good hard frost.
- The conditions described above must occur between St. Martin's Day (November 11) and November 20.
Why is Indian summer called Indian summer? There are many theories. Some say it comes from the early Algonquian Native Americans, who believed that the condition was caused by a warm wind sent from the court of their southwestern god, Cautantowwit.