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Altitude

physical science Maturity 7-9 Next article →

Altitude is a vertical measurement between a reference datum and an object. A datum is simply a starting point or a base level used for measurement.

Altitude
Altitude

While people often use altitude to mean height above sea level, the specific term can change based on the context. In geography, the term elevation is often used for height above sea level. In aviation, altitude is vital for safe navigation. Pilots must know if they are measuring from the sea or the ground. Understanding these different types of vertical distance is essential for science, flight, and sports.

In aviation, several specific types of altitude are used to ensure safety. Indicated altitude is the reading shown on an altimeter when it is set to local barometric pressure at mean sea level. Absolute altitude is the actual vertical distance of an aircraft above the terrain below it. This is often called radar height or feet above ground level (AGL). True altitude is the actual elevation above mean sea level, corrected for non-standard temperature and pressure.

vertical distances
vertical distances

There is also pressure altitude, which is the elevation above a standard datum air-pressure plane. This standard pressure is typically 1013.25 millibars or 29.92 inches of mercury (inHg).

To measure these distances, pilots use a pressure altimeter. This instrument is actually an aneroid barometer. Instead of just showing atmospheric pressure, its face indicates distance in feet or meters. When flying at high altitudes, pilots use pressure altitude to report their flight level. In the United States, this is used in Class A airspace above roughly 18,000 feet. At these heights, the altimeter is always set to the standard pressure of 29.92 inHg. This ensures all aircraft are using the same vertical reference.

The Earth's atmosphere is divided into several layers based on altitude. The troposphere is the lowest layer, extending from the surface to the tropopause. Above this is the stratosphere, where many jet aircraft, such as the Boeing 737-800, typically cruise.

Boeing 737 view 1
Boeing 737 view 1

This layer is stable, which helps planes avoid the turbulence found in the troposphere. Higher up, the layers include the mesosphere, the thermosphere, and the exosphere. The Kármán line, located at an altitude of 100 kilometers above sea level, marks the boundary between the atmosphere and space.

Atmospheric pressure decreases as altitude increases. This happens because of two competing physical effects. First, gravity pulls air molecules close to the ground. Second, the heat content of the air causes molecules to bounce and expand. This expansion lowers the density of the air. The temperature of the air also changes with height through a process called convection. As air rises, it expands and cools. This temperature decrease is known as the adiabatic lapse rate. In a dry environment, this rate is about 9.8 degrees Celsius per kilometer. When water vapor condenses into clouds, it releases heat, changing the rate to the moist adiabatic lapse rate of 5.5 degrees Celsius per kilometer.

High altitude can have significant effects on the human body. As altitude increases, the partial pressure of oxygen decreases. This lack of oxygen can lead to altitude sickness or more serious conditions like high altitude pulmonary edema. The human body can undergo acclimatization to adapt to these changes. This process involves breathing faster and increasing the heart rate. It can take days or weeks to adjust. However, above 8,000 meters, humans enter the "death zone," where acclimatization is no longer possible.

Athletes also experience the effects of altitude in different ways. For explosive events like sprints or long jumps, low atmospheric pressure means less air resistance. This can actually improve performance. However, for endurance events like long races, the reduction in oxygen makes performance more difficult. Because of this, many athletes use altitude training. They train at high altitudes to trigger bodily changes that help them perform better when they return to sea level.

vertical distances
vertical distances

This training is common in sports like cycling, swimming, and distance running.

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File:Altitude.svg
Altitude.svg
File:Boeing 737 view 1.jpg
Boeing 737 view 1.jpg
File:vertical distances.svg
vertical distances.svg
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