Waterloo has a humid continental climate zone , typical of the state of Iowa, and is part of USDA Plant Hardiness zone 5a. The normal monthly mean temperature ranges from 18.5 °F (−7.5 °C) in January to 73.6 °F (23.1 °C) in July. On average, there are 22 nights annually with a low at or below 0 °F (−18 °C), 58 days annually with a high at or below freezing, and 16 days with a high at or above 90 °F (32 °C). As the mean first and last occurrence of freezing temperatures is October 1 and April 29, respectively, this allows for a growing season of 154 days. Temperature records range from −34 °F (−37 °C) on March 1, 1962 and January 16, 2009 up to 112 °F (44 °C) on July 13 and 14, 1936, during the Dust Bowl. The record cold daily maximum is −16 °F (−27 °C) on February 2, 1996, while conversely the record warm daily minimum is 80 °F (27 °C) on July 31, 1917 and August 16, 1988.
Normal annual precipitation equivalent is 34.60 inches (879 mm) spread over an average 112 days, with heavier rainfall in spring and summer, but observed annual rainfall has ranged from 17.35 to 53.07 inches (441 to 1,348 mm) in 1910 and 1993, respectively. The wettest month on record is July 1999 with 12.82 inches (326 mm); on the 2nd of that month, 5.49 inches (139 mm) of rain fell, making for the heaviest rainfall in a single calendar day. The driest months are October 1952 and November 1954 with trace amounts each.
Winter snowfall is moderate, and averages 35.3 inches (90 cm) per season, spread over an average 27 days, and snow cover of 1 inch (2.5 cm) or more is seen on 67 days, mostly from December to March. Winter snowfall has ranged from 11.6 inches (29.5 cm) in 1967–68 to 68.5 inches (174.0 cm) in 1904–05. The most snow in a calendar day and month is 13.2 inches (33.5 cm) and 33.9 inches (86.1 cm) on January 3, 1971 and in December 2000, respectively.