As we approach the first week of October, many residents in the northern half of the country will become keenly aware of the approach of winter, with the rapid shrinking of the daylight duration and the rapid decrease in the daily average temperatures. With the first frost, autumn marks the end of the growing season for many of the crops and plants through many areas of the country. Many of us may also experience "Indian Summer".
During early to mid autumn, a frequently occurring spell of warm weather may occur across the Northeast quadrant of the country. This sequence of days marked by pleasant weather is often called "Indian Summer" in North America. While the origin of the term is somewhat uncertain, one of the accepted legends dating back to late 18th century New England suggests that Native Americans used this warm spell in autumn for harvesting and preparing their lodges for the forthcoming winter.
This weather event is marked by warm, hazy and sunny days, with cool nights that usually follow the first autumn frost. In fact, purists maintain that a true Indian Summer can only occur following the first killing frost in autumn. While this argument may be true at least in New England and the Northeast, other areas of the country can claim a similar type of quiet and warm weather spell, 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, the atmospheric circulation regime is undergoing a change from a late summer pattern to more of an early winter one. The cold outbreaks during late September occur as the source for cold Canadian air masses become established and storm activity increases. The cold air behind the cold fronts that invade the Great Lakes and Northeast often provide the needed killing frost. Frequently during October, large high-pressure systems move slowly across the country. A large high-pressure system that remains anchored over the southeastern U.S. sets the stage for Indian Summer in the Midwest and Northeast. Light winds from the desert Southwest on the west and north flanks of the high-pressure system keep conditions mild over the Mississippi Valley, Great Lakes and into the Northeast, including New England. Since the flow of air is not directly from the Gulf of Mexico, the air is not hot and muggy. The high-pressure system is relatively stable, in that both mixing and cloud formation are suppressed, leading to hazy, but relatively cloud-free days. Nighttime temperatures cool readily because of the low moisture content in the air and weak winds as well as the longer duration of darkness.
Many crops are sensitive to the weather, in particular to extremes in air temperature. Frost sensitive plants, such as tomatoes, peppers, and cucumbers, freeze when the temperature falls to 28 degrees Fahrenheit, while apples are damaged when the temperature drops below 20 degrees. A killing freeze occurs when widespread damage to crops terminates the growing season.
In most mid latitude climates, the growing season is often used synonymously with the frost-free season, loosely defined as the length of time between the last killing frost in spring and the first killing frost in the autumn. While the exact time span would vary by plant type, the growing season is often related to the interval when the daily minimum temperature remains above 32 degrees. The Midwestern Regional Climate Center (MRCC) is maintaining the MRCC Frost/Freeze Guidance Project as part of the Vegetation Impact Program (VIP) with website displaying a variety of freeze maps across the 48 contiguous United States. These maps show the 28-degree and 32-degree Freeze Climatologies as well as the current freeze statistics. Check the map showing the median date of occurrence of the first 32-degree Fahrenheit temperature across the 48 coterminous United States. (The median date means that half of the occurrences of a 32-degree reading over the 30-year normal occur prior to this date, while the other half occur after this date.)
Across the continental U.S. the typical lengths of the frost free regions range from about 120 days along the Canadian border to about 220 days in Oklahoma and north Texas and over 320 days in southern sections of Florida and California. Mountainous areas provide a complex pattern, with some higher elevations having lengths that are less than 100 days. By accessing the NOWData (NOAA Online Weather Data) feature on the Climate page of your local National Weather Service, you can find the "first/last dates" for various climate reporting stations around your area.
The National Weather Service issues several non-precipitation weather advisories and warnings to inform the public of the potential for damage to agricultural crops and other vegetation as a consequence of cold weather. These advisories and warnings are only issued during the growing season when the greatest threat to agriculture would occur.