A significant factor for human comfort is the low humidity. Generally, precipitation increases with elevation, the western slopes of the Rockies receiving the most. A hazard exacerbated by climate and topography is air pollution. The Denver-Boulder urban area is among the top ten U.S. areas in poor air quality due to carbon monoxide. Colorado has five distinct life zones, which are associated with climate and topography and provide habitat for a wide variety of plant and animal life.
Trees occur naturally only along riverbanks.
It is inhabited by gray fox, skunks, and raccoons. Above the timberline in the alpine zone are found wildflowers, mosses, and lichens. Coyotes, mule deer, and small mammals such as beaver and deer mice are ubiquitous, but several once-numerous species, including grizzly bears, bison, wolverines, lynx, wolves, and otter, are now either rare or extinct. Colorado has known deposits of more than 250 economically useful minerals, including coal, oil shale, petroleum, natural gas, molybdenum, vanadium, zinc, and uranium, with about 35 minerals actively mined. Mostly urban, the population is concentrated in a corridor along the eastern slope of the Rockies, especially the front range.