Altitude is the elevation of an object from a known level or datum.
Common datums are mean sea level and the surface of the WGS-84 geoid, used by GPS.
In the United States and the UK aviation altitude is usually measured in feet.
Everywhere else in the world the altitude is measured in metres.
Atmospheric pressure decreases as altitude increases.
This principle is the basis of operation of the pressure altimeter, which is an aneroid barometer calibrated to indicate altitude instead of pressure.
It is the fall in pressure that leads to a shortage of oxygen (hypoxia) in humans on ascent to high altitude.
Altimeter An altimeter is an active instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, ... >
read more
Sea level Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level, however, ... >
read more
Boiling Boiling is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to a temperature such that its vapor pressure is above ... >
read more
Military aviation Military aviation is used to attack or defend a country through the sky. There are many types of military aircraft, but the basic types of military ... >
read more
Earth's atmosphere Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly 78% nitrogen and 21% ... >
read more
Volcanic ash Volcanic ash consists of very fine rock and mineral particles less than 2 mm in diameter that are ejected from a volcanic vent. Ash is created when ... >
read more
Speed of sound The speed of sound is a term used to describe the speed of sound waves passing through an elastic medium. The speed varies with the medium employed ... >
read more
Navigation There are several traditions of navigation. In the pre-modern history of human migration and discovery of new lands by navigating the oceans, a few ... >
read more
Jet stream Jet streams are fast flowing, relatively narrow air currents found in the atmosphere at around 12 km above the surface of the Earth, just under the ... >
read more
Water rocket Water rockets are like their model rocket cousins, except that these are powered by a combination of water and air pressure instead of a chemical ... >
read more