Porto is the second-largest city in Portugal after Lisbon and one of the major urban areas of the Iberian Peninsula. Located along the Douro river estuary in Northern Portugal, Porto is one of the oldest European centres, and its historical core was proclaimed a World Heritage Site by UNESCO in 1996. The history of Porto dates back to around 300 BC with Proto-Celtic and Celtic people being the first known inhabitants. Ruins of that period have been discovered in several areas. Porto features a warm-summer mediterranean climate. Winter temperatures typically range between 5 °C (41 °F) during morning and 15 °C (59 °F) in the afternoon but rarely drop below 0 °C (32 °F) at night. The weather is often rainy for long stretches although prolonged sunny periods do occur.