Lower elevations of the Rocky Mountains and all but the highest elevations of the Cypress Hills have a humid continental climate (Köppen climate classification Dfb). These areas have snowy but mild winters due to the warming effect of chinook winds, and cool summers due to their high elevation. As altitude increases and the subalpine zone is reached, this grades into a subarctic climate (Köppen climate classification Dfc), with much harsher winters and even cooler summers. Above the tree line, an alpine/polar climate (Köppen climate classification ET) prevails. Conditions this high are even colder and very windy. Precipitation is higher in these elevated regions than elsewhere in Alberta, and this supports forests of lodgepole pine and trembling aspen mixed with fescue grasslands in the montane areas, Engelmann spruce and subalpine fir forests in the subalpine zone, and heather, sedges and mountain avens in the alpine zone above the tree lines.