Detroit and the rest of southeastern Michigan have a humid continental climate (Köppen Dfa) which is influenced by the Great Lakes; the city and close-in suburbs are part of USDA Hardiness zone 6b, with farther-out northern and western suburbs generally falling in zone 6a. Winters are cold, with moderate snowfall and temperatures not rising above freezing on an average 44 days annually, while dropping to or below 0 °F (−18 °C) on an average 4.4 days a year; summers are warm to hot with temperatures exceeding 90 °F (32 °C) on 12 days. The warm season runs from May to September. The monthly daily mean temperature ranges from 25.6 °F (−3.6 °C) in January to 73.6 °F (23.1 °C) in July. Official temperature extremes range from 105 °F (41 °C) on July 24, 1934 down to −21 °F (−29 °C) on January 21, 1984; the record low maximum is −4 °F (−20 °C) on January 19, 1994, while, conversely the record high minimum is 80 °F (27 °C) on August 1, 2006, the most recent of five occurrences. A decade or two may pass between readings of 100 °F (38 °C) or higher, which last occurred July 17, 2012. The average window for freezing temperatures is October 20 thru April 22, allowing a growing season of 180 days.