• altitude
    Mount Everest is the highest point of altitude on Earth.

    Photograph by George F. Mobley

    High-Altitude Cooking
    Water normally boils at 100 degrees Celsius (212 degrees Fahrenheit). But for each 500-foot increase in altitude, the boiling point drops about one degree. Water therefore boils much more quickly in Denver, Colorado, than it does in Honolulu, Hawaii. But, because the actual temperature of the water does not increase, it takes longer to cook food.

    Altitude, like elevation, is the distance above sea level. Areas are often considered "high-altitude" if they reach at least 2,400 meters (8,000 feet) into the atmosphere.
     
    The most high-altitude point on Earth is Mount Everest, in the Himalayan mountain range on the border of Nepal and the Chinese region of Tibet. Mount Everest is 8,850 meters (29,035 feet) tall. The urban area of El Alto, Bolivia, is the most high-altitude city on Earth. All 1.2 million residents live about 4,150 meters (13,615 feet) above sea level.
     
    Altitude is related to air pressure. In fact, aviators and mountaineers can measure their altitude by measuring the air pressure around them. This is called indicated altitude, and is measured by an instrument called an altimeter.
     
    As altitude rises, air pressure drops. In other words, if the indicated altitude is high, the air pressure is low.
     
    This happens for two reasons. The first reason is gravity. Earth's gravity pulls air as close to the surface as possible.
     
    The second reason is density. As altitude increases, the amount of gas molecules in the air decreases—the air becomes less dense than air nearer to sea level. This is what meteorologists and mountaineers mean by "thin air." Thin air exerts less pressure than air at a lower altitude.
     
    High-altitude locations are usually much colder than areas closer to sea level. This is due to the low air pressure. Air expands as it rises, and the fewer gas molecules—including nitrogen, oxygen, and carbon dioxide—have fewer chances to bump into each other.
     
    The human body reacts to high altitudes. Decreased air pressure means that less oxygen is available for breathing. One normal effect of altitude is shortness of breath, since the lungs have to work harder to deliver oxygen to the bloodstream. It can take days and even weeks for a body to adjust to high altitude and low air pressure.
     
    People who spend too much time in high-altitude locations risk more serious symptoms of altitude sickness. These may range from headaches and dizziness to much more serious consequences, such as brain or lung damage. Above about 8,000 meters (26,000 feet), the human body cannot survive at all, and starts to shut down. Mountaineers call this altitude the "death zone."
     
    To prevent severe altitude sickness, mountaineers bring supplemental (extra) supplies of oxygen and limit their time in the "death zone."
     
    Different regions have different air pressures, even at the same altitude. Factors such as climate and humidity impact local air pressure. Air pressure also decreases around the poles. For this reason, if Mount Everest was located in the U.S. state of Alaska or the continent of Antarctica, it could never be summited without supplemental oxygen—the pressure would make the altitude seem 914 meters (3,000 feet) higher.
     
    Astronomical Altitude
     
    In astronomy, altitude has a somewhat different meaning. It describes the angle between the horizon and some point in the sky. For example, if a star is directly overhead, its altitude is 90 degrees. If a star has just set or is just about to rise, it is right at the horizon and has an altitude of 0 degrees.
     
    The North Star, Polaris, does not rise or set because the Earth's axis passes directly through it. It thus has a constant altitude when viewed from anywhere in the Northern Hemisphere. This makes it incredibly useful in celestial navigation.
  • Term Part of Speech Definition Encyclopedic Entry
    absolute altitude Noun

    elevation, or the physical distance above the ground.

    adjust Verb

    to change or modify something to fit with something else.

    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
    altitude sickness Noun

    illness caused by reduced oxygen levels at high elevations.

    angle Noun

    slanting space between two lines that ultimately meet in a point.

    ascend Verb

    to go up.

    astronomy Noun

    the study of space beyond Earth's atmosphere.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    axis Noun

    an invisible line around which an object spins.

    Encyclopedic Entry: axis
    bloodstream Noun

    flow of blood through an organism's body.

    border Noun

    natural or artificial line separating two pieces of land.

    Encyclopedic Entry: border
    celestial navigation Noun

    determining an object's position using the stars and planets as guides.

    city Noun

    large settlement with a high population density.

    climate Noun

    all weather conditions for a given location over a period of time.

    Encyclopedic Entry: climate
    consequence Noun

    result or outcome of an action or situation.

    decrease Verb

    to lower.

    density Noun

    number of things of one kind in a given area.

    Encyclopedic Entry: density
    elevation Noun

    height above or below sea level.

    Encyclopedic Entry: elevation
    exert Verb

    to force or pressure.

    expand Verb

    to grow or get larger.

    gas Noun

    state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

    gradually Adverb

    slowly, or at a measured pace.

    gravity Noun

    physical force by which objects attract, or pull toward, each other.

    horizon Noun

    line where the Earth and the sky seem to meet.

    Encyclopedic Entry: horizon
    humidity Noun

    amount of water vapor in the air.

    Encyclopedic Entry: humidity
    indicate Verb

    to display or show.

    meteorologist Noun

    person who studies patterns and changes in Earth's atmosphere.

    molecule Noun

    smallest physical unit of a substance, consisting of two or more atoms linked together.

    mountaineer Noun

    someone who climbs mountains.

    mountain range Noun

    series or chain of mountains that are close together.

    Northern Hemisphere Noun

    half of the Earth between the North Pole and the Equator.

    North Star Noun

    the star Polaris, located roughly above the North Pole. Also called the Lodestar or Pole Star.

    oxygen Noun

    chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

    Polaris Noun

    star that is currently located roughly over the North Pole. Also called the North Star or Lodestar.

    pole Noun

    extreme north or south point of the Earth's axis.

    region Noun

    any area on the Earth with one or more common characteristics. Regions are the basic units of geography.

    Encyclopedic Entry: region
    sea level Noun

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

    Encyclopedic Entry: sea level
    star Noun

    large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.

    summit Verb

    to reach the highest point of a mountain.

    symptom Noun

    sign or indication of something.

    urban area Noun

    developed, densely populated area where most inhabitants have nonagricultural jobs.

    Encyclopedic Entry: urban area
    vertical Noun

    up-down direction, or at a right angle to Earth and the horizon.

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