Climate and environment
Lanzhou sits 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 with hot summers and cold and very dry winters. The monthly 24-hour aerage 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 (49.6 °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. Sunshine is generous, with no month having less than 50% of the potential, but not particularly abundant, averaging 2,424 hours annually.

Serious air pollution in the city means that it has some of the worst air quality of any city nationally. According to the Blacksmith Institute, Lanzhou is one of the 30 most polluted cities in the world, with its TSP (total suspended particle)] rating 247% above that of the Gansu State recommendation. Air quality is so poor that at times one cannot see Lanshan, the mountain rising straight up along the south side of the city. At one point, a controversial suggestion was put forward to bulldoze a mountain adjacent to the city, in order to let fresh air into the bowl where Lanzhou is situated. It was suggested on the premise that the surrounding mountains block a free flow of air in the city. The city is located in a narrow river valley with an unfortunate curve causing it to be hemmed in with no free air flow. Lanzhou is also the home of many factories, including some involved in petroleum processing, and suffers from large dust storms kicked up from the Gobi Desert, especially in the winter and spring. In 2011, using Chinese statistics, the World Health Organization reported that Lanzhou has the worst air quality (annual mean PM10 ug/m3 of 150) among eleven western Chinese cities, and is even worse than Beijing with its reading of 121.

The reach of the Yellow River at Lanzhou carries a high load of silt, giving the river its characteristic muddy appearance; however water quality in this reach is better than the "fetid outflow that barely passes for water two hours downstream".