The weight of the atmosphere in Earth’s gravity field compresses the air, so that air pressure is greater and the air is more dense at sea level than at high elevation. A gulp of air at the peak of Mt. Everest has about one-third the density of a gulp of air at sea level, and thus can provide only one-third the oxygen for metabolism. The average temperature also decreases with elevation, roughly at a rate of about 6.5°C per kilometer (3.5°F per thousand feet); if it’s 25°C at the base of a 4-km-high mountain, it might be only -1°C at the top. Because of this temperature change, a hike up a mountain can take you from a desert or jungle up through temperate forests, past the tree line (above which wind and cold prevent trees from growing), into the tundra (treeless plain), and ultimately onto polar-like glaciers.

Because of the harsh climate at high elevations, permanent mountain dwellers live no higher than 4.8 km (15,800 feet) above sea level. Though these people are not genetically adapted for high-altitude living, they do develop more capillaries in their fingers and toes than do their brethren at lower altitudes, leading to better blood circulation, which inhibits frostbite; they also develop a larger lung capacity. Some high-altitude dwellers of the Andes chew coca leaves, which act as a stimulant and prevent altitude sickness. Their ancestors, the Incas, may have ventured to these levels because of their religious beliefs: they worshipped the Sun and the mountains, and thus built temples on mountain peaks.

Because of their harsh conditions and daunting cliffs, mountains serve as major obstacles to migration. Not surprisingly, in the days before modern modes of communication, mountain tribes inhabiting neighboring valleys were so isolated from one another that they developed different languages. In this way, mountains have contributed to the diversification of human culture.

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