An altimeter is a device that measures altitude, the distance of a point above sea level. Altimeters are important navigation instruments for aircraft and spacecraft pilots who monitor their height above the Earth’s surface. Skydivers and mountaineers also use altimeters to pinpoint their location in the sky or on the ground.
 
The most common types of altimeters are barometric. They determine altitude by measuring air pressure. As altitude increases, air pressure decreases. This is because the density of air is lower (thinner) at high altitudes. It exerts less pressure on the Earth below. 
 
An altimeter’s readings change as elevation changes. The atmospheric pressure on Denali, Alaska, is about half that of Honolulu, Hawai'i. Denali, also known as Mount McKinley, is the highest peak in North America. Honolulu is a city at sea level.
 
Altitude readings can also change due to weather, as air pressure decreases during storms. 
 
A simple barometric altimeter includes a sealed metal chamber, a spring, and a pointer that shows altitude in meters or feet. The chamber expands as air pressure decreases and contracts as it increases, bending the spring and moving the pointer. An altimeter can be mounted on an aircraft’s instrument panel or worn on a person’s wrist. 
 
Other Types of Altimeters
 
Not all altimeters depend on air pressure. The Global Positioning System (GPS), for instance, can provide altitude as part of an area’s location by triangulating signals from different satellites.
 
Radar and laser altimeters, found on some aircraft and spacecraft, work similarly to sonar measurements of the seafloor. These altimeters send a radio or laser signal toward the surface and measure the time it takes for the signal to bounce back. The time it takes for the signal to bounce back (or echo) to the aircraft is then translated to an elevation.
 
When used in satellites, radar and laser altimeters are able to combine altitude measurements to create accurate topographic maps of both land and ocean surfaces. The radar altimeter aboard the TOPEX/Poseidon satellite, for example, measured the surface topography of 95 percent of the ice-free ocean. Developed by NASA and CNES, the French space agency, TOPEX/Poseidon’s radar altimeter was accurate to within 2 centimeters (less than 1 inch)! Coupled with another satellite, Jason-1, TOPEX/Poseidon graphed the rise in global sea levels, providing evidence of the connection between global climate change and sea level rise.
altimeter
An altimeter measures altitude.
accurate
Adjective

exact.

aircraft
Noun

vehicle able to travel and operate above the ground.

air pressure
Noun

force pressed on an object by air or atmosphere.

Noun

device for measuring altitude.

Noun

the distance above sea level.

Noun

force per unit area exerted by the mass of the atmosphere as gravity pulls it to Earth.

barometric
Adjective

having to do with atmospheric pressure, or measuring that pressure.

chamber
Noun

sealed compartment.

Noun

gradual changes in all the interconnected weather elements on our planet.

contract
Verb

to shrink or get smaller.

decrease
Verb

to lower.

Noun

number of things of one kind in a given area.

determine
Verb

to decide.

device
Noun

tool or piece of machinery.

Noun

height above or below sea level.

Noun

height above or below sea level.

evidence
Noun

data that can be measured, observed, examined, and analyzed to support a conclusion.

exert
Verb

to force or pressure.

expand
Verb

to grow or get larger.

Global Positioning System (GPS)
Noun

system of satellites and receiving devices used to determine the location of something on Earth.

graph
Verb
to depict the relation between certain sets of numbers by plotting them with reference to a set of axes.
increase
Verb

to add or become larger.

instrument
Noun

tool.

instrument panel
Noun
dashboard with an array of dials, lights, and gauges that monitor the performance of a machine or device.
laser
Noun

(acronym for light amplification by stimulated emission of radiation) an instrument that emits a thin beam of light that does not fade over long distances.

Noun

position of a particular point on the surface of the Earth.

measurement
Noun

process of determining length, width, mass (weight), volume, distance or some other quality or size.

metal
Noun

category of elements that are usually solid and shiny at room temperature.

monitor
Verb

to observe and record behavior or data.

mountaineer
Noun

someone who climbs mountains.

NASA
Noun

(National Aeronautics and Space Administration) the U.S. space agency, whose mission statement is "To reach for new heights and reveal the unknown so that what we do and learn will benefit all humankind."

Noun

art and science of determining an object's position, course, and distance traveled.

radar
Noun

(RAdio Detection And Ranging) method of determining the presence and location of an object using radio waves.

radio
Noun

wireless transmission based on electromagnetic waves.

satellite
Noun

object that orbits around something else. Satellites can be natural, like moons, or made by people.

seafloor
Noun

surface layer of the bottom of the ocean.

Noun

base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

Noun

increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year.

signal
Verb

to communicate using signs.

skydive
Verb

to jump from an airplane in flight and freefall until releasing a parachute.

sonar
Noun

method of determining the presence and location of an object using sound waves (echolocation).

spacecraft
Noun

vehicle designed for travel outside Earth's atmosphere.

storm
Noun

severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

topographic map
Noun

map showing natural and human-made features of the land, and marked by contour lines showing elevation.

translate
Verb

to turn written or spoken text into a different language.

triangulation
Noun

method of determining distance or placement of a point by measuring angles to it from known points.

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.