Boston has a continental climate with some maritime influence, and using the −3 °C (27 °F) coldest month (January) isotherm, the city lies within the transition zone from a humid subtropical climate (Köppen Cfa) to a humid continental climate (Köppen Dfa), although the suburbs north and west of the city are significantly colder in winter and solidly fall under the latter categorization; the city lies at the transition between USDA plant hardiness zones 6b (most of the city) and 7a (Downtown, South Boston, and East Boston neighborhoods). Summers are typically warm to hot, rainy, and humid, while winters oscillate between periods of cold rain and snow, with cold temperatures. Spring and fall are usually mild, with varying conditions dependent on wind direction and jet stream positioning. Prevailing wind patterns that blow offshore minimize the influence of the Atlantic Ocean. The hottest month is July, with a mean temperature of 73.4 °F (23.0 °C). The coldest month is January, with a mean of 29.0 °F (−1.7 °C). Periods exceeding 90 °F (32 °C) in summer and below freezing in winter are not uncommon but rarely extended, with about 13 and 25 days per year seeing each, respectively. The most recent sub-0 °F (−18 °C) reading occurring on February 14, 2016 when the temperature dipped down to −9 °F (−23 °C), the coldest reading since 1957. In addition, several decades may pass between 100 °F (38 °C) readings, with the most recent such occurrence on July 22, 2011 when the temperature reached 103 °F (39 °C). The city's average window for freezing temperatures is November 9 through April 5.[c] Official temperature records have ranged from −18 °F (−28 °C) on February 9, 1934, up to 104 °F (40 °C) on July 4, 1911; the record cold daily maximum is 2 °F (−17 °C) on December 30, 1917, while, conversely, the record warm daily minimum is 83 °F (28 °C) on August 2, 1975.