Altitude, like elevation, is the distance above sea level. Areas are often considered "high-altitude" if they reach at least 2,400 meters (8,000 feet) into the atmosphere.

The most high-altitude point on Earth is Mount Everest, in the Himalayan mountain range on the border of Nepal and the Chinese region of Tibet. Mount Everest is 8,850 meters (29,035 feet) tall. The urban area of El Alto, Bolivia, is the highest-altitude city on Earth. All 1.2 million residents live about 4,150 meters (13,615 feet) above sea level.

Altitude is related to air pressure. In fact, aviators and mountaineers can measure their altitude by measuring the air pressure around them. This is called indicated altitude, and is measured by an instrument called an altimeter.

As altitude rises, air pressure drops. In other words, if the indicated altitude is high, the air pressure is low.

This happens for two reasons. The first reason is gravity. Earth's gravity pulls air as close to the surface as possible.

The second reason is density. As altitude increases, the amount of gas molecules in the air decreases—the air becomes less dense than air nearer to sea level. This is what meteorologists and mountaineers mean by "thin air." Thin air exerts less pressure than air at a lower altitude.

High-altitude locations are usually much colder than areas closer to sea level. This is due to the low air pressure. Air expands as it rises, and the fewer gas molecules—including nitrogen, oxygen, and carbon dioxide—have fewer chances to bump into each other.

The human body struggles in high altitudes. Decreased air pressure means that less oxygen is available for breathing. One normal effect of altitude is shortness of breath, since the lungs have to work harder to deliver oxygen to the bloodstream. It can take days and even weeks for a body to adjust to high altitude and low air pressure.

People who spend too much time in high-altitude locations risk more serious symptoms of altitude sickness. These may range from headaches and dizziness to much more serious consequences, such as brain or lung damage. Above about 8,000 meters (26,000 feet), the human body cannot survive at all, and starts to shut down. Mountaineers call this altitude the "death zone."

To prevent severe altitude sickness, mountaineers bring supplemental (extra) supplies of oxygen and limit their time in the "death zone."

Different regions have different air pressures, even at the same altitude. Factors such as climate and humidity impact local air pressure. Air pressure also decreases around the poles. For this reason, if Mount Everest was located in the U.S. state of Alaska or the continent of Antarctica, it could never be summited without supplemental oxygen—the pressure would make the altitude seem 914 meters (3,000 feet) higher.

Astronomical Altitude

In astronomy, altitude has a somewhat different meaning. It describes the angle between the horizon and some point in the sky. For example, if a star is directly overhead, its altitude is 90 degrees. If a star has just set or is just about to rise, it is right at the horizon and has an altitude of 0 degrees.

The North Star, Polaris, does not rise or set because Earth's axis passes directly through it. It thus has a constant altitude when viewed from anywhere in the Northern Hemisphere. This makes it incredibly useful in celestial navigation.