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. Environmental problems The city is located in a narrow and curved river valley with surrounding mountains causing it to be hemmed in blocking a free flow of air. According to the National Environmental Analysis released by Tsinghua University and The Asian Development Bank in January 2013, Lanzhou was among the ten most air-polluted cities in the world. Air quality reportedly was so poor that at times one could not see Lanshan, the mountain rising up along the south side of the city. 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 had the worst air quality among eleven western Chinese cities. Its annual mean PM10 μg/m3 of 150 is 7 times the safe level established by the World Health Organization. It was worse than Beijing with its reading of 121. Since then, authorities have taken measures to improve air quality, which have largely been successful. "Thirteen polluting enterprises with excessive capacity have been closed, more than 200 highly polluting enterprises were suspended in winter, and 78 industrial enterprises have moved to an industrial park outside the city." In 2015 it was awarded China's climate progress title. As a city once unable to be spotted from satellites, Lanzhou has taken various measures to combat air pollution in recent years, having reduced its Air Pollution Index at the fastest speed across China. According to the 2018 WHO database, of 2700 towns listed, Lanzhou has the 158th highest level of PM 2.5 pollution, with an average of 54 micrograms per cubic meter (twice that of Milan). The large particle PM10 remains high, at 132 micrograms per cubic meter, in part as a result of sand storms. 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" (2008). In recent years, several specimens of the endangered Chinese giant salamander have been found in and near the Yellow River in Lanzhou. On April 11,