▁both ▁the ▁west ▁and ▁the ▁north west , ▁as ▁well ▁as ▁the ▁area ▁of ▁Q ing hai ▁Lake ▁via ▁the ▁upper ▁waters ▁of ▁the ▁Y ellow ▁River ▁and ▁its ▁trib ut aries . ▁ ▁Cl imate ▁▁ ▁L anz hou ▁is ▁situated ▁in ▁the ▁temper ate ▁zone ▁and ▁enjo ys ▁a ▁semi - ar id ▁climate ▁( K ö ppen ▁B Sk ) ▁with ▁hot ▁sum mers ▁and ▁cold ▁and ▁very ▁dry ▁winter s . ▁In ▁the ▁urban ▁core , ▁based ▁on ▁ 1 9 7 1 – 2 0 0 0 ▁norm als , ▁the ▁month ly ▁ 2 4 - hour ▁average ▁temperature ▁ranges ▁from ▁ ▁in ▁January ▁to ▁ ▁in ▁July . ▁The ▁mean ▁annual ▁temperature ▁is ▁, ▁while ▁annual ▁ra inf all ▁is ▁, ▁almost ▁all ▁of ▁which ▁falls ▁from ▁May ▁to ▁October . ▁The ▁winter s ▁are ▁so ▁dry ▁that ▁snow fall ▁is ▁sometimes ▁restricted ▁to ▁fall ▁and ▁spring . ▁With ▁month ly ▁percent ▁possible ▁sun sh ine ▁r anging ▁from ▁ 5 0   percent ▁in ▁December ▁to ▁ 5 9   percent ▁in ▁February , ▁sun sh ine ▁is ▁gener ous ▁but ▁not ▁abund ant , ▁as ▁the ▁city ▁receives ▁ 2 , 4 2 4 ▁hours ▁of ▁bright ▁sun sh ine ▁ann ually . ▁ ▁Environment al ▁problems ▁ ▁The ▁city ▁is ▁located ▁in ▁a ▁narrow ▁and ▁cur ved ▁river ▁valley ▁with ▁surrounding ▁mountains ▁causing ▁it ▁to ▁be ▁hem med ▁in ▁blocking ▁a ▁free ▁flow ▁of ▁air . ▁According ▁to ▁the ▁National ▁Environment al ▁Analysis