Riverside sits inland in the Inland Empire of southern California, some 90 km east of Los Angeles in a valley ringed by mountains at approximately 33.95°N, 117.40°W. Cut off from the ocean's moderating influence, it has a semi-arid Mediterranean climate (Köppen BSk/Csa) — with hot, dry summers and mild winters — markedly hotter than the coast.

Summer, from June to September, is hot and utterly rainless, with July and August the hottest — average highs around 35–36°C, with heatwaves exceeding 40°C — far hotter than coastal Los Angeles, since the inland valley receives little of the cooling sea breeze. The dryness makes the heat bearable, and nights cool markedly.

Winter, from December to February, is mild and the wetter season, with January the coolest month — average highs around 19°C and cool nights near 5°C, with frost occasional. Pacific storms bring most of the year's modest rain, and the surrounding mountains may be snow-capped, while the valley stays green and mild.

Riverside is very dry, receiving only around 250–280 mm of rain a year — semi-arid levels — almost all of it between December and March, while the summer is entirely rainless; the coastal mountains cast a rain shadow over the Inland Empire. Live rainfall, humidity, and pressure readings for the city are shown in the panels above.

Riverside's inland valley traps air, and the Los Angeles basin's pollution drifts eastward to settle against the mountains, historically giving the Inland Empire some of the worst air quality in the United States. The autumn Santa Ana winds bring both extreme fire danger and, briefly, the region's clearest air.

To follow any single measurement in Riverside 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.