Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum [数据] and a point or object. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.

Aviation altitude is measured using either mean [平均] sea level (MSL) or local ground level (above ground level, or AGL) as the reference datum.

Absolute altitude is the vertical distance of the aircraft above the terrain [地带] over which it is flying. It can be measured using a radar altimeter [高度表] (or "absolute altimeter"). Also referred to as "radar height" or feet/metres above ground level (AGL).

The Earth's atmosphere is divided into several altitude regions.

Regions on the Earth's surface (or in its atmosphere) that are high above mean sea level are referred to as high altitude. High altitude is sometimes defined to begin at 2,400 meters (8,000 ft) above sea level.

Medicine [医学] recognizes that altitudes above 1,500 metres (4,900 ft) start to affect humans. As the altitude increases, atmospheric pressure decreases, which affects humans by reducing the partial pressure of oxygen. The lack of oxygen above 2,400 metres (8,000 ft) can cause serious illnesses.

In a mixture of gases, each constituent [成分] gas has a partial pressure which is the notional [概念性的] pressure of that constituent gas if it alone occupied the entire volume of the original mixture at the same temperature.

For athletes [运动员], high altitude produces two contradictory effects on performance. For explosive events (sprints up to 400 metres, long jump, triple jump) the reduction in atmospheric pressure signifies less atmospheric resistance, which generally results in improved athletic performance. For endurance events (races of 5,000 metres or more) the predominant effect is the reduction in oxygen which generally reduces the athlete's performance at high altitude.

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