Clear-Day Frequencies and“Indian Summer”at Athens, Georgia and Chattanooga, Tennessee

Abstract

Indian summer has been defined as a period of clear weather with warm days and cool nights occurring during mid- or late autumn after the first frost or freeze of the season. Weekly clear-day frequencies (using two definitions of clear days: 0.0 cloud cover and 0.0?0.3 cloud cover) are analyzed in this study for periods of more than 30 years for the southeastern sites of Athens, Georgia and Chattanooga, Tennessee in an attempt to detect Indian summer. Results indicate that distinct periods of high clear-day frequencies occur throughout much of October as well as during the first two weeks of November and the week of 29 November? 5 December. Consideration of temperature and synoptic climatological conditions during these latter weeks eliminated their consideration as part of the Indian summer phenomenon. Therefore, Indian summer as defined by clear-day frequency occurs primarily during the October weeks of 4?10, 11?17 and 18?24. As was the case in a previous study for the north-central United States, Indian summer is not usually preceded by the first autumn freeze at Athens or Chattanooga. Therefore, it is recommended that the definition of Indian summer be modified to exclude reference to occurring after the first autumn freeze.