From: Subject: Friday Weather Topic in Greater Depth Date: Thu, 8 Oct 2009 10:02:42 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w04f_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Friday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Friday, 2 October = 2009=20

INDIAN SUMMER, FIRST FROST AND GROWING SEASON

As we approach the first week of October, many residents in the = northern half=20 of the country will become keenly aware of the approach of winter, with = the=20 rapid shrinking of the daylight duration and the rapid decrease in the = daily=20 average temperatures. With the first frost, autumn marks the end of the = growing=20 season for many of the crops and plants through many areas of the = country. Many=20 of us may also experience "Indian Summer".

INDIAN SUMMER

During early to mid autumn, a frequently occurring spell of warm = weather may=20 occur across the Northeast quadrant of the country. This sequence of = days marked=20 by pleasant weather is often called "Indian Summer" in North America. = While the=20 origin of the term is somewhat uncertain, one of the accepted legends = dating=20 back to late 18th century New England suggests that Native Americans = used this=20 warm spell in autumn for harvesting and preparing their lodges for the=20 forthcoming winter.

This weather event is marked by warm, hazy and sunny days, with cool = nights=20 that usually follow the first autumn frost. In fact, purists maintain = that a=20 true Indian Summer can only occur following the first killing frost in = autumn.=20 While this argument may be true at least in New England and the = Northeast, other=20 areas of the country can claim a similar type of quiet and warm weather = spell,=20 but without the need for a freeze.

Usually, American Indian Summer occurs in October and may extend into = November, following several cold outbreaks during late September. At = this time,=20 the atmospheric circulation regime is undergoing a change from a late = summer=20 pattern to more of an early winter one. The cold outbreaks during late = September=20 occur as the source for cold Canadian air masses become established and = storm=20 activity increases. The cold air behind the cold fronts that invade the = Great=20 Lakes and Northeast often provide the needed killing frost. Frequently = during=20 October, large high-pressure systems move slowly across the country. A = large=20 high-pressure system that remains anchored over the southeastern U.S. = sets the=20 stage for Indian Summer in the Midwest and Northeast. Light winds from = the=20 desert Southwest on the west and north flanks of the high-pressure = system keep=20 conditions mild over the Mississippi Valley, Great Lakes and into the = Northeast,=20 including New England. Since the flow of air is not directly from the = Gulf of=20 Mexico, the air is not hot and muggy. The high-pressure system is = relatively=20 stable, in that both mixing and cloud formation are suppressed, leading = to hazy,=20 but relatively cloud-free days. Nighttime temperatures cool readily = because of=20 the low moisture content in the air and weak winds as well as the longer = duration of darkness.

AN END TO THE GROWING SEASON

Many crops are sensitive to the weather, in particular to extremes in = air=20 temperature. Frost sensitive plants, such as tomatoes, peppers, and = cucumbers,=20 freeze when the temperature falls to 28 degrees Fahrenheit, while apples = are=20 damaged when the temperature drops below 20 degrees. A killing freeze = occurs=20 when widespread damage to crops terminates the growing season.

In most mid latitude climates, the growing season is often used = synonymously=20 with the frost-free season, loosely defined as the length of time = between the=20 last killing frost in spring and the first killing frost in the autumn. = While=20 the exact time span would vary by plant type, the growing season is = often=20 related to the interval when the daily minimum temperature remains above = 32=20 degrees.

The National Weather Service issues several non-precipitation weather = advisories and warnings to inform the public of the potential for damage = to=20 agricultural crops and other vegetation as a consequence of cold = weather. These=20 advisories and warnings are only issued during the growing season when = the=20 greatest threat to agriculture would occur.

Return to the Friday=20 Daily Weather Summary

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological Society. =