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.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 AltimetersNot 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.
Term Part of Speech Definition Encyclopedic Entry accurate Adjective
vehicle able to travel and operate above the ground.
air pressure Noun
force pressed on an object by air or atmosphere.
device for measuring altitude.
Encyclopedic Entry: altimeter altitude Noun
the distance above sea level.
Encyclopedic Entry: altitude atmospheric pressure Noun
force per unit area exerted by the mass of the atmosphere as gravity pulls it to Earth.
Encyclopedic Entry: atmospheric pressure barometric Adjective
having to do with atmospheric pressure, or measuring that pressure.
climate change Noun
gradual changes in all the interconnected weather elements on our planet.
Encyclopedic Entry: Earth's Changing Climate contract Verb
to shrink or get smaller.
number of things of one kind in a given area.
Encyclopedic Entry: density determine Verb
tool or piece of machinery.
height above or below sea level.
Encyclopedic Entry: elevation elevation Noun
height above or below sea level.
Encyclopedic Entry: elevation evidence Noun
data that can be measured, observed, examined, and analyzed to support a conclusion.
to force or pressure.
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 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.
position of a particular point on the surface of the Earth.
Encyclopedic Entry: location measurement Noun
process of determining length, width, mass (weight), volume, distance or some other quality or size.
category of elements that are usually solid and shiny at room temperature.
to observe and record behavior or data.
someone who climbs mountains.
(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."
art and science of determining an object's position, course, and distance traveled.
Encyclopedic Entry: navigation radar Noun
(RAdio Detection And Ranging) method of determining the presence and location of an object using radio waves.
wireless transmission based on electromagnetic waves.
object that orbits around something else. Satellites can be natural, like moons, or made by people.
surface layer of the bottom of the ocean.
sea level Noun
base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.
Encyclopedic Entry: sea level sea level rise Noun
increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year.
Encyclopedic Entry: Sea Level Rise signal Verb
to communicate using signs.
to jump from an airplane in flight and freefall until releasing a parachute.
method of determining the presence and location of an object using sound waves (echolocation).
vehicle designed for travel outside Earth's atmosphere.
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.
to turn written or spoken text into a different language.
method of determining distance or placement of a point by measuring angles to it from known points.
state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.
Encyclopedic Entry: weather