Wednesday, September 25, 2013

Indian summer

From The Old Farmer's Almanac

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 are 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.
For over 200 years, The Old Farmer's Almanac has
adhered to the saying, "If All Saints' (November 1)
brings out winter, St. Martin's brings out Indian summer."

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