Lanzhou is the capital and largest city of Gansu Province in Northwest China. The prefecture-level city, located on the banks of the Yellow River, is a key regional transportation hub, connecting areas further west by rail to the eastern half of the country. Historically, it has been a major link on the Northern Silk Road. The city is also a center for heavy industry and petrochemical industry. Lanzhou was previously ranked as one of the cities with the worst air quality in the world, due to industrial pollution and its situation in a narrow river valley. Government measures to reduce the pollution have had their effect, and in 2015 the city was awarded China's climate progress title.
Lanzhou is home to 3,616,163 inhabitants at the 2010 census and 2,177,130 in the built-up area (urban) of 1,088 square kilometres (420 sq mi).
Originally in the territory of the Western Qiang peoples, Lanzhou became part of the territory of the State of Qin in the 6th century BC.
In 81 BC, under the Han dynasty (206 BC–220 AD), it was taken from the Huns' Huandi Chanyu and made the seat of Jincheng commandery (jùn), and later of the Jincheng county (xiàn), later renamed Yunwu. The city used to be called the Golden City, and since at least the first millennium BC it was a major link on the ancient Northern Silk Road, and also an important historic Yellow River crossing site. To protect the city, the Great Wall of China was extended as far as Yumen. Parts of the Great Wall still exist within the built-up area.
After the fall of the Han dynasty, Lanzhou became the capital of a succession of tribal states. In the 4th century it was briefly the capital of the independent state of Liang. The Northern Wei dynasty (386–534) reestablished Jincheng commandery, renaming the county Zicheng. Mixed with different cultural heritages, the area at present-day Gansu province, from the 5th to the 11th century, became a center for Buddhist study. Under the Sui Dynasty (581–618) the city became the seat of Lanzhou prefecture for the first time, retaining this name under the Tang dynasty (618–907). In 763 the area was overrun by the Tibetan Empire and in 843 was conquered by the Tang. Later it fell into the hands of the Western Xia dynasty (which flourished in Qinghai from the 11th to 13th century) and was subsequently absorbed by the Song dynasty (960–1126) in 1041. The name Lanzhou was reestablished, and the county renamed Lanzhuan.
Lanzhou is situated on the upper reaches of the Yellow River where it emerges from the mountains and has been a center since early times, being at the southern end of the route leading via the Hexi Corridor across Central Asia. It commands the approaches to the ancient capital area of Chang'an (modern Xi'an) in Shaanxi province from both the west and the northwest, as well as the area of Qinghai Lake via the upper waters of the Yellow River and its tributaries.
Lanzhou is situated in the temperate zone and enjoys a semi-arid climate (Köppen BSk) with hot summers and cold and very dry winters. The monthly 24-hour average temperature ranges from −5.3 °C (22.5 °F) in January to 22.4 °C (72.3 °F) in July. The mean annual temperature is 9.75 °C (49.5 °F), while annual rainfall is 315 millimetres (12.4 in), almost all of which falls from May to October. The winters are so dry that snow is extremely rare. With monthly percent possible sunshine ranging from 50 percent in December to 59 percent in February, sunshine is generous but not abundant, as the city receives 2,424 hours of bright sunshine annually.