• atmospheric pressure
    A barometer measures atmospheric pressure, which is also called barometric pressure.

    Photograph by an anonymous contributor, Wikipedia

    Why Do Your Ears Pop in Airplanes?
    As you go up in an airplane, the atmospheric pressure becomes lower than the pressure of the air inside your ears. Your ears pop because they are trying to equalize, or match, the pressure. The same thing happens when the plane is on the way down and your ears have to adjust to a higher atmospheric pressure.

    The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth.

    Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.

    An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). One atmosphere is 1,013 millibars, or 760 millimeters (29.92 inches) of mercury.

    Atmospheric pressure drops as altitude increases. The atmospheric pressure on Denali, Alaska, is about half that of Honolulu, Hawai'i. Honolulu is a city at sea level. Denali, also known as Mount McKinley, is the highest peak in North America.

    As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die.

    Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness. Decompression sickness, also called "the bends", is also a problem for scuba divers who come to the surface too quickly.

    Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying.

    Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.

  • Term Part of Speech Definition Encyclopedic Entry
    air Noun

    the layer of gases surrounding Earth.

    Encyclopedic Entry: air
    aircraft Noun

    vehicle able to travel and operate above the ground.

    air pressure Noun

    force pressed on an object by air or atmosphere.

    altitude Noun

    the distance above sea level.

    Encyclopedic Entry: altitude
    atmosphere (atm) Noun

    (atm) unit of measurement equal to air pressure at sea level, about 14.7 pounds per square inch. Also called standard atmospheric pressure.

    atmospheric pressure Noun

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

    Encyclopedic Entry: atmospheric pressure
    barometer Noun

    an instrument that measures atmospheric pressure.

    Encyclopedic Entry: barometer
    Celsius scale Noun

    scale for measuring surface temperature, used by most of the world, in which the boiling point of water is 100 degrees.

    cloud Noun

    visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

    Encyclopedic Entry: cloud
    decompression sickness Noun

    serious condition resulting from gases forming tiny bubbles in the bloodstream as a body adjusts to a major change in atmospheric pressure. Also known as DCS, divers disease, and the bends.

    exert Verb

    to force or pressure.

    Fahrenheit scale Noun

    scale for measuring surface temperature used by Belize, Liberia, Myanmar, and the United States.

    force Noun

    power or energy that activates movement.

    gravity Noun

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

    high-pressure system Noun

    weather pattern characterized by high air pressure, usually as a result of cooling. High-pressure systems are usually associated with clear weather.

    low-pressure system Noun

    weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms.

    mercury Noun

    chemical element with the symbol Hg.

    mercury barometer Noun

    tool that determines atmospheric pressure by measuring how much mercury moves in a glass tube.

    meteorologist Noun

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

    millibar Noun

    (mbar) unit of pressure equal to .001 bar of atmospheric pressure.

    oxygen Noun

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

    precipitation Noun

    all forms in which water falls to Earth from the atmosphere.

    Encyclopedic Entry: precipitation
    scuba noun, adjective

    (self-contained underwater breathing apparatus) portable device for breathing underwater.

    sea level Noun

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

    Encyclopedic Entry: sea level
    weather Noun

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

    Encyclopedic Entry: weather
    wind Noun

    movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.

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