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Chicago sits in the Midwest of the United States on the southwestern shore of Lake Michigan, on flat plains at approximately 41.88°N, 87.63°W. It has a humid continental climate (Köppen Dfa) with hot, humid summers and cold, snowy winters and four distinct seasons, its weather shaped year-round by the vast lake beside it and by its exposure to air masses sweeping across the continent's interior.
Summer, from June to August, is warm to hot and humid, with July the warmest month — average highs around 29°C — and hot spells that can exceed 35°C, when high humidity can drive the heat index dangerously high, as in the deadly heat wave of 1995. Lake breezes cool the immediate shoreline, sometimes making it noticeably 'cooler by the lake', and summer is the wettest and sunniest season, with frequent thunderstorms.
Winter, from December to February, is cold and snowy, with January the coldest month — average highs around freezing and lows near -8°C, and brutal cold snaps, sometimes driven by an Arctic 'polar vortex', that can plunge temperatures below -25°C with ferocious wind chills. The city averages around 90 cm of snow a year, and biting winds off the lake and the open plains earn Chicago part of its 'Windy City' reputation.
Chicago receives around 910–950 mm of precipitation a year, relatively evenly distributed but with a clear summer maximum from thunderstorms in July and August and the driest months in mid-winter; a substantial share of the cold-season total falls as snow. Live rainfall, humidity, and pressure readings for the city are shown in the panels above.
Lake Michigan is the defining influence on Chicago's weather, moderating temperatures, feeding summer humidity, cooling the lakeshore on warm afternoons and occasionally generating lake-effect snow. Sitting in the open interior, the city sees dramatic swings as continental air masses collide — from Arctic outbreaks to summer heat waves — and severe thunderstorms, including the occasional derecho and tornado, are a warm-season hazard.
To follow any single measurement in Chicago more closely, use our live instruments: the online barometer for atmospheric pressure, the thermometer for temperature, the hygrometer for humidity, the anemometer for wind speed, the wind vane for wind direction, and the rain gauge for rainfall.