• altimeter
    An altimeter measures altitude.

    Photograph by Luis Marden, National Geographic

    Barometric Altimeters
    "An ordinary aircraft altimeter is nothing more than a sensitive barometer."
    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.
  • Term Part of Speech Definition Encyclopedic Entry
    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.

    altimeter Noun

    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.

    chamber Noun

    sealed compartment.

    climate change Noun

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

    Encyclopedic Entry: climate change
    contract Verb

    to shrink or get smaller.

    decrease Verb

    to lower.

    density Noun

    number of things of one kind in a given area.

    Encyclopedic Entry: density
    determine Verb

    to decide.

    device Noun

    tool or piece of machinery.

    elevation Noun

    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.

    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.

    location Noun

    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.

    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

    (acronym for National Aeronautics and Space Administration) U.S. agency responsible for space research and systems.

    navigation Noun

    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.

    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.

    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.

    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 two points by calculating their distance from a third point whose position is known.

    weather Noun

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

    Encyclopedic Entry: weather
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