High altitude are regions on the Earth's surface (or in its atmosphere) that are high above mean sea level. The pressure and temperature of the atmosphere at high altitude is substantially different than at sea level. These differences can affect living organisms, including humans. High altitude is sometimes defined to begin at 2,400 metres (8,000 ft) above sea level.^[1]

At high altitude, atmospheric pressure is lower compared to sea level. This is due to two competing physical effects: gravity, which causes the air to be as close as possible to the ground; and the heat content of the air, which causes the molecules to bounce off each other and expand.^[2]

The lower atmospheric pressure affects animals (including humans), due to the decrease in the partial pressure of oxygen.^[3]

Because of the lower pressure, the air expands as it rises, which causes it to cool.^[4][5] Thus, high altitude air is cold, which causes a characteristic alpine climate. This climate dramatically affects the ecology at high altitude.

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[edit] See also

• Near space
• Barometric formula
• Alpine tundra
• Effects of high altitude on humans
  • Altitude sickness
  • High altitude pulmonary edema
  • High altitude cerebral edema