Altitude

The exact definition and reference datum varies according to the context (e.g., aviation, geometry, geographical survey, sport, or atmospheric pressure).

In geometry and geographical surveys, altitude helps create accurate topographic maps and understand the terrain's elevation.

For high-altitude trekking and sports, knowing and adapting to altitude is vital for performance and safety.

When flying at a flight level, the altimeter is always set to standard pressure (29.92 inHg or 1013.25 hPa).

These regions start and finish at varying heights depending on season and distance from the poles.

[7] The thermosphere and exosphere (along with the higher parts of the mesosphere) are regions of the atmosphere that are conventionally defined as space.

[10] The human body can adapt to high altitude by breathing faster, having a higher heart rate, and adjusting its blood chemistry.

[22] Additionally, there is a dose response relationship between increasing elevation and decreasing obesity prevalence in the United States.

[23] In addition, the recent hypothesis suggests that high altitude could be protective against Alzheimer's disease via action of erythropoietin, a hormone released by kidney in response to hypoxia.

[26] 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.

The same changes that help the body cope with high altitude increase performance back at sea level.

Animals have developed physiological adaptations to enhance oxygen uptake and delivery to tissues which can be used to sustain metabolism.

As oxygen is used as a source of metabolic heat production, the hypobaric hypoxia at high altitudes is problematic.

[30] These factors may decrease productivity in high altitude habitats, meaning there will be less energy available for consumption, growth, and activity.

Altitude is a vertical measurement between a reference datum and an object.
A generic Boeing 737-800 cruising at 32,000 feet. Below it are a pack of clouds. Above it is a vivid, ambient blue sky.
A Boeing 737-800 cruising in the stratosphere , where airliners typically cruise to avoid turbulence rampant in the troposphere . The blue layer is the ozone layer , fading further to the mesosphere . The ozone heats the stratosphere, making conditions stable. The stratosphere is also the altitude limit of jet aircraft and weather balloons , as the air density there is roughly 1 ⁄ 1000 of that in the troposphere. [ 1 ]
Vertical distance comparison
Low (cyan) and Medium (yellow) Earth orbit regions to scale. The black dashed line is the geosynchronous orbit. The green dashed line is the 20,230 km orbit used for GPS satellites.