Altitude

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 of that in the troposphere.
Vertical distance comparison

Distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object.

- Altitude

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Radar altimeter

The dipole antenna of a radar altimeter of 1947.
Early diagram of the concept of a radar altimeter, shown in a 1922 Bell telephone magazine.
Radar altimeters are used in science, with this diagram showing how a spacecraft could detect surface smoothness on the surface of Venus.
Contemporary radar altimeter equipment in 2018.
Maintenance being performed on the radar altimeter of a Northrop Grumman EA-6B Prowler.

A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to travel to ground, reflect, and return to the craft.

Height above sea level

A cuboid demonstrating the dimensions length, width, and height.

Height above mean sea level is a measure of the vertical distance (height, elevation or altitude) of a location in reference to a historic mean sea level taken as a vertical datum.

Mesosphere

Third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere.

The Space Shuttle Endeavour straddling the stratosphere and mesosphere in this photo. The troposphere, which contains clouds, appears orange in this photo.
Diagram showing the five primary layers of the Earth's atmosphere: exosphere, thermosphere, mesosphere, stratosphere, and troposphere. The layers are to scale. From Earth's surface to the top of the stratosphere (50 km) is just under 1% of Earth's radius.

In the mesosphere, temperature decreases as altitude increases.

Troposphere

First and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur.

Atmospheric Circulation: the Three Cell Model of the circulation of the planetary atmosphere of the Earth, of which the troposphere is the lowest layer.
The atmosphere of the Earth is in five layers: (i) the exosphere at 600+ km; (ii) the thermosphere at 600 km; (iii) the mesosphere at 95–120 km; (iv) the stratosphere at 50–60 km; and (v) the troposphere at 8–15 km. The scale indicates that the layers’ distances from the planetary surface to the edge of the stratosphere is ±50 km, less than 1.0% of the radius of the Earth.
Zonal Flow: a zonal flow regime indicates the dominant west-to-east flow of the atmosphere in the 500 hPa height pattern.
Meridional Flow: The meridional flow pattern of 23 October 2003 shows amplified troughs and ridges in the 500 hPa height pattern.

The temperature of the troposphere decreases at high altitude by way of the inversion layers that occur in the tropopause, which is the atmospheric boundary that demarcates the troposphere from the stratosphere.

Altitude sickness

Altitude sickness warning – Indian Army
Left: A woman at normal altitude. Right: The same woman with a swollen face while trekking at high altitude (Annapurna Base Camp, Nepal; 4130 m).
Climbers on Mount Everest often experience altitude sickness.

Altitude sickness, the mildest form being acute mountain sickness (AMS), is the harmful effect of high altitude, caused by rapid exposure to low amounts of oxygen at high elevation.

Lapse rate

Higher Czarny Staw pod Rysami lake (elevation 1,583 m) is still frozen as the lower Morskie Oko lake has already almost melted (elevation 1,395 m. Photo from Polish side of the Tatra mountains, May 2019.
Emagram diagram showing variation of dry adiabats (bold lines) and moist adiabats (dash lines) according to pressure and temperature
The latent heat of vaporization adds energy to clouds and storms.

The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude.

Height above ground level

Height measured with respect to the underlying ground surface.

The Boeing 747

Therefore, the pilot needs reliable information on the height of the plane with respect to the landing area (usually an airport).

International Standard Atmosphere

Comparison of a graph of International Standard Atmosphere temperature and pressure and approximate altitudes of various objects and successful stratospheric jumps

The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations.

Altimeter

Diagram showing the face of the "three-pointer" sensitive aircraft altimeter displaying an altitude of 10180 ft. Reference pressure of about 29.92 inHg (1013 hPa) is showing in the Kollsman window
Digital barometric pressure sensor for altitude measurement in consumer electronic applications
Digital wrist-mounted skydiving altimeter in logbook mode, displaying the last recorded jump profile.
Skydiver in free fall, making use of a hand-mounted altimeter. The analogue face is visible, showing colour-coded decision altitudes. The depicted altimeter is electronic, despite using an analogue display.
Speaking Altimeter with helmet for skydiving
An old altimeter intended for use in aircraft
A drum-type aircraft altimeter, showing the small Kollsman windows at the bottom left (hectopascals) and bottom right (inches of mercury) of the face.
Diagram showing the internal components of the sensitive aircraft altimeter.
The altimeter on this Piper PA-28 is seen on the top row of instruments, second from right

An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level.

Flight level

The Boeing 747

In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet.