Philadelphia sits on the Delaware River in southeastern Pennsylvania, on the coastal plain between the Appalachians and the Atlantic at approximately 39.95°N, 75.17°W. It has a humid subtropical climate (Köppen Cfa) at its northern limit — with hot, humid summers and cold winters — and four sharply distinct seasons.

Summer, from June to August, is hot and humid, with July the warmest month — average highs around 31°C — and heatwaves that can exceed 35°C, made oppressive by Atlantic humidity. It is the wettest season, with frequent afternoon thunderstorms, and the remnants of Atlantic hurricanes occasionally bring torrential rain in late summer.

Winter, from December to February, is cold, with January the coldest month — average highs around 5°C and lows near -4°C. Snow falls, around 55 cm a year, though amounts vary hugely from winter to winter; nor'easters tracking up the coast can bury the city under half a metre in a single storm, while other winters pass nearly snowless.

Philadelphia receives around 1,050–1,100 mm of precipitation a year, spread fairly evenly through every month with a modest summer maximum from thunderstorms and tropical systems. Live rainfall, humidity, and pressure readings for the city are shown in the panels above.

Philadelphia's winters swing wildly from year to year, governed by whether nor'easters — intense coastal storms tracking up the eastern seaboard — track close enough to bring heavy snow or far enough offshore to miss entirely. A single nor'easter can deliver more snow than an entire quiet winter.

To follow any single measurement in Philadelphia 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.