The county’s climate is much like that of its large neighboring city, Chicago, with hot summers and cold winters. The hottest month of the year is July, when the average high temperature is approximately 86 °F (30 °C). Temperatures in July and in summer in general, can frequently exceed 95 °F (35 °C), and occasionally exceed 100 °F (39 °C), although this does not happen each year. The coldest month of the year is January, where the average high temperature is 23 °F (−5 °C). Overnight low temperatures are usually around 8 °F (−12 °C). In winter, the low temperatures fall below 0 °F (−18 °C) on many (often as many as fifteen or twenty) occasions per year. Extremely cold nights may record temperatures as low as −20 °F (−29 °C) or lower, but this is rare and does not happen each year.

The wettest month of the year is August, when thunderstorms contribute brief, heavy downpours of rain. July is the second wettest month of the year, also mainly due to thunderstorms. However, long dry spells can also occur at this time of year, sometimes lasting weeks. The two driest months of the year are January and February, where almost all of the precipitation falls as snow. In a typical year, total precipitation is 37 in (940 mm) with a winter snowfall total of 40 in (102 cm). Large snowstorms, although rare, do occur. In many years, at least one storm will deliver 12 in (30 cm) of snow in one day. Most snow-bearing systems are Alberta Clippers, while the more infrequent heavy snows are caused by Panhandle Hooks.