As is typical of Iowa, Sioux City has a humid continental climate, with very warm, humid summers, cold, dry winters, and wide temperature extremes; it is part of USDA Hardiness zone 5a. The normal monthly mean temperature ranges from 20.4 °F (−6.4 °C) in January to 74.3 °F (23.5 °C) in July. On average, there are 25 days that reach 90 °F (32 °C) or higher, 52 days that do not climb above freezing, and 17 days with a low of 0 °F (−18 °C) or below annually. The average window for freezing temperatures is October 1 thru April 26, allowing a growing season of 157 days. Extreme temperatures officially range from −35 °F (−37 °C) on January 12, 1912 up to 111 °F (44 °C) on July 4 and 17, 1936 as well as July 11, 1939; the record cold daily maximum is −22 °F (−30 °C) on February 8, 1899, while, conversely, the record warm daily minimum is 86 °F (30 °C) on August 18, 1936.
Precipitation is greatest in May and June and averages 27.7 in (700 mm) annually, but has ranged from 14.33 in (364 mm) in 1976 to 41.10 in (1,044 mm) in 1903. Snowfall averages 34.8 in (88 cm) per season, and has historically ranged from 6.9 in (18 cm) in 1895–96 to 65.9 in (167 cm) in 1961–62; the average window for measurable (≥0.1 in or 0.25 cm) snowfall is November 8 thru April 7, although snow in October occurs several times per decade. On May 14, 2013, the high temperature reached 106 °F (41 °C), setting a new all-time May record high, along with a 77 °F (43 °C) rise from the morning of the 12th.