Yellowknife has a continental subarctic climate (Köppen: "Dfc"). Although winter is predominantly polar, rapid heat waves emerge at the summit of summer due to the immense path south. The city averages less than of precipitation annually, as it lies in the rain shadow of mountain ranges to the west. Due to its location on Great Slave Lake, Yellowknife has a frost-free growing season that averages slightly over 100 days. In an occasional year, the first fall frost does not come until October. Most of the limited precipitation falls between June and October, with April being the driest month of the year and August having the most rainfall. Snow that falls in winter accumulates on the ground until the spring thaw.