St Louis sits at the confluence of the Mississippi and Missouri rivers in eastern Missouri, in the American heartland at approximately 38.63°N, 90.20°W. It has a humid continental climate (Köppen Dfa/Cfa) — with hot, humid summers and cold winters — and it sits on the boundary between the Midwest and the humid South.

Summer, from June to August, is hot and oppressively humid, with July the warmest month — average highs around 32°C — and heatwaves that can exceed 38°C, made stifling by Gulf humidity trapped in the river valley. It is the wettest season, with frequent, sometimes severe thunderstorms bringing hail and occasional tornadoes.

Winter, from December to February, is cold and often grey, with January the coldest month — average highs around 5°C and lows near -5°C. Snow falls, around 45 cm a year, though thaws are frequent, and the season alternates sharply between cold Canadian air and mild, damp Gulf air, with ice storms an occasional hazard.

St Louis receives around 1,050–1,100 mm of precipitation a year, spread through every month with a clear spring and summer maximum from thunderstorms; its confluence setting makes it vulnerable to major river flooding, as in the catastrophic Great Flood of 1993. Live rainfall, humidity, and pressure readings for the city are shown in the panels above.

St Louis sits where America's two greatest rivers meet, and its gravest weather hazard is the flooding that follows: the Great Flood of 1993 saw the Mississippi crest nearly six metres above flood stage, inundating vast areas of the Midwest for months. Its summers are among the muggiest of any large American city.

To follow any single measurement in St 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.