WEEKLY CLIMATE NEWS

DataStreme ECS WEEK TWO: 7-11 September 2015

ITEMS OF INTEREST

CURRENT CLIMATE STATUS

CURRENT CLIMATE MONITORING

CLIMATE AND SOCIETY

Concept of the Week: End of the Growing Season

As we move through autumn, we often contemplate the end of the growing season as daylight shortens and temperatures begin to fall across many areas of the nation. The length of the growing season depends upon the plant species, as well as the climate of the locale, meaning that several ways can be used to define the growing season.

Many crops, especially vegetables and fruits, are sensitive to relatively low air temperatures. In spring, when many crops are emerging and in various stages of development they are more vulnerable to air temperatures near 32 degrees Fahrenheit. But by fall, many of these plants have become hardy. Generally speaking, a killing frost would occur when the temperature around the plant would fall to a point that would kill all but the hardiest vegetation. Sometimes, other terminology is used. When air temperatures fall between 29 and 32 degrees Fahrenheit, a so-called "light freeze" occurs. While tender plants such as tomatoes, peppers, corn and cucumbers would be killed when temperatures fall to around 29 degrees, these conditions would have little destructive effect on other hardier vegetation. A "moderate freeze" typically occurs between 25 and 28 degrees, which would have a widely destructive effect upon most vegetation. For temperatures of 24 degrees or lower, a condition called a "severe freeze", heavy damage would occur with most plants that are not dormant. Apples are damaged when the temperature drops below 20 degrees. Under these conditions, the soil would have frozen solid to a depth dependent upon the duration of the freeze, as well as the soil type and soil moisture.

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. The National Centers for Environmental Information (formerly the National Climatic Data Center) has produced climatological tables that identify those median dates (a 50 percent occurrence) during spring and fall when the temperature at a station falls to 36, 32, 28, 24 or 20 degrees Fahrenheit for the last time in spring or the first time in autumn. While the exact time span that a plant survives would vary by plant type, the growing season for climatological purposes is often related to the interval when the daily minimum temperature remains above 32 degrees. 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.

Concept of the Week: Questions

(Place your responses on the Chapter Progress Response Form provided in the Study Guide.)

  1. Many vegetable crops would succumb when autumn air temperatures fall to [(36),(33),(28)] degrees.
  2. The frost-free season in states bordering Canada would probably be less than [(60),(90),(120)] days.

Historical Events

Return to DataStreme ECS RealTime Climate Portal

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2015, The American Meteorological Society.