These are Lake Athabasca, 7898 km² (3085 mi²) in extent, of which the greater part is in the province of Saskatchewan, and the other Lesser Slave Lake 1,160 km² (600 mi²) in area. As Alberta extends for approximately from north to south, it is natural that the climate should vary considerably between parallels of 49° and 60° north and also between approximately 110° and 120° west. It is also further influenced by the different altitudes found in the province. In general, Alberta's climate is dry, due to the rain shadow effect of the Rocky Mountains and Pacific Coast Ranges to the west, and continental, due to its distance from any large body of water. Most of Southern Alberta east of the Rockies and outside of the Cypress Hills, especially Palliser's Triangle, is a dry steppe, with most locations having an annual average precipitation of 280-430mm. Under the Köppen climate classification, most places in Southern Alberta are either semi-arid (Köppen climate classification ""BSk""), or humid continental (Köppen climate classification "") (the "humid" designation denotes that these climates do not meet the criteria to be semi-arid, not that they necessarily have high humidity levels). Most places in Southern Alberta that fall under the "humid continental" classification are close to the borderline between semi-arid and humid continental. This region has the hottest summer temperatures in Alberta and quite cold winters though chinook winds ameliorate the cold winter temperatures temporarily when they pass over. The natural vegetation of this region is mainly dry mixed grass prairie, grading to mixed grass prairie, and then fescue prairie as precipitation increases with higher altitude and/or latitude. This area is prone to drought and farming here is not successful without significant irrigation. Central Alberta has a dry continental climate, with most places falling under the humid continental classification (Köppen climate classification "Dfb"), though some areas in the southeast of this region, close to the border with Saskatchewan around Oyen, are semi-arid (Köppen "BSk"). Precipitation levels here are generally higher than in Southern Alberta, though the driest areas of Alberta, around Empress, are in Central Alberta. This region generally has cooler temperatures than Southern Alberta in both summer and winter, due to the decreased amount of solar radiation received, and the lower frequency of chinooks blowing through the region.