• air
    A parachutist glides down safely.

    Photograph by Abhishek Singh, My Shot

    Blue Skies
    The sky appears blue because of the way air scatters light from the sun. Blue light scatters more easily in air than red light does, so the sky is blue.

    Lighter Than Air
    Substances that are lighter than air simply have a density that is lower than Standard Dry Air. Substances with a density lower than the surrounding material will float on top of it. Helium has a much lower density than Standard Dry Air. Objects filled with helium, from small balloons to giant airships, float when filled with this lighter than air substance.

    Air is the invisible mixture of gases that surrounds the Earth. Air contains important substances, such as oxygen and nitrogen, that most species need to survive. Human beings, of course, are one of those species. Sometimes, the word "atmosphere" is used instead of the word "air."

    Standard Dry Air is the composition of gases that make up air at sea level. It is a standard scientific unit of measurement. Standard Dry Air is made up of nitrogen, oxygen, argon, carbon dioxide, neon, helium, krypton, hydrogen, and xenon. It does not include water vapor because the amount of vapor changes based on humidity and temperature. Because air masses are constantly moving, Standard Dry Air is not accurate everywhere at once.

    Nitrogen and oxygen make up about 99 percent of Earth’s air. People and other animals need oxygen to live. Carbon dioxide, a gas that plants depend on, makes up less than .04 percent.

    Plants and animals each produce the gases that the other needs to live. Plants need carbon dioxide—people and other animals exhale carbon dioxide as a waste product. People and other animals need oxygen—plants produce oxygen during an important process called photosynthesis, which turns the sun’s energy into nutrients.

    Water vapor in the air is sometimes visible as clouds. Water enters the atmosphere through the water cycle. The water cycle also brings molecules in the air into oceans, lakes, and rivers.

    Some gases in the air come from volcanic eruptions. Volcanic eruptions eject gases from the interior of the Earth. The most common gas emitted by volcanoes is water vapor. Other gases, such as carbon monoxide and sulfur dioxide, are toxic to most organisms. A few organisms, however, thrive on these gases. At the bottom of the ocean are bacteria that do not need oxygen or sunlight to survive. In other words, they do not need air. These strange organisms create their own nutrients using hydrogen sulfide, not carbon dioxide. The hydrogen sulfide comes from cracks, or vents, in the Earth’s crust.

    The air is different as you move higher and higher into the atmosphere. The air gets "thinner" as elevation climbs because there are fewer air molecules up there. Mountain climbers often have to use canisters of oxygen as they climb above 3,800 meters (12,500 feet) because there is not enough oxygen in the atmosphere for most people to breathe. High mountains such as Mount Everest (8,848 meters, or 29,029 feet), in Nepal and China, are littered with empty oxygen canisters that climbers discard when they are used up.

    High in the stratosphere, a layer of the Earth’s atmosphere, is a special air molecule called ozone. Ozone is made up of three atoms of oxygen. The massive collection of these molecules is called the ozone layer. The ozone layer blocks harmful ultraviolet, or UV, rays so the sun’s powerful radiation does less damage to living things on Earth.

    Unfortunately, air pollution has a negative effect on the air we breathe. Air pollution happens when harmful byproducts, like exhaust from cars, enter the air. These pollutants can clog the atmosphere with smog, a combination of smoke and fog. They can also create toxic clouds of dust. Other air pollutants, such as methane and excess amounts of carbon dioxide, can upset the balance of molecules in the air, contributing to global warming.

    Non-Atmospheric Air

    Compressed air is air kept at a consistent pressure, such as air pressure at sea level. Airplanes are usually pressurized at ground level so passengers can breathe without canisters of air.

    Often, compressed air is kept at a pressure higher than normal air pressure. Scuba divers use compressed air to breathe under water. The canisters of air allow divers to inhale through a tube and exhale into the water.

    Pneumatics is the science and work of pressurized air and other gases. The uses for pneumatics are far-ranging. Air brakes in automobiles, trucks, and trains use compressed air to slow wheel rotation and stop the vehicles. Pipe organs use compressed air at different pressures to create different musical notes.

  • Term Part of Speech Definition Encyclopedic Entry
    accurate Adjective


    air Noun

    the layer of gases surrounding Earth.

    Encyclopedic Entry: air
    air brake Noun

    system that uses compressed air to slow a vehicle, often by slowing wheel rotation.

    air mass Noun

    a large volume of air that is mostly consistent, horizontally, in temperature and humidity.

    Encyclopedic Entry: air mass
    air pollution Noun

    harmful chemicals in the atmosphere.

    Encyclopedic Entry: air pollution
    airship Noun

    aircraft filled with lighter-than-air material, usually hydrogen or helium. Also called a dirigible or blimp.

    animal Noun

    organisms that have a well-defined shape and limited growth, can move voluntarily, acquire food and digest it internally, and can respond rapidly to stimuli.

    argon Noun

    chemical element (gas) with the symbol Ar.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    atom Noun

    the basic unit of an element, composed of three major parts: electrons, protons, and neutrons.

    automobile Noun

    self-propelled vehicle that can be controlled by a driver.

    bacteria Plural Noun

    (singular: bacterium) single-celled organisms found in every ecosystem on Earth.

    balloon Noun

    bag, often made of rubber, filled with air or another gas.

    byproduct Noun

    substance that is created by the production of another material.

    canister Noun

    container, usually shaped like a long tube.

    carbon dioxide Noun

    greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

    carbon monoxide Noun

    toxic gas.

    clog Verb

    to obstruct or prevent travel.

    cloud Noun

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

    Encyclopedic Entry: cloud
    compressed air Noun

    container of air kept at a consistent pressure, such as that found at sea level.

    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    density Noun

    number of things of one kind in a given area.

    Encyclopedic Entry: density
    discard Verb

    to throw away.

    dust Noun

    tiny, dry particles of material solid enough for wind to carry.

    Encyclopedic Entry: dust
    Earth Noun

    our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

    Encyclopedic Entry: Earth
    eject Verb

    to get rid of or throw out.

    elevation Noun

    height above or below sea level.

    Encyclopedic Entry: elevation
    energy Noun

    capacity to do work.

    excess Noun

    extra or surplus.

    exhale Verb

    to breathe out.

    exhaust Noun

    gases and particles expelled from an engine.

    float Verb

    to rest on the surface of a liquid.

    gas Noun

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

    global warming Noun

    increase in the average temperature of the Earth's air and oceans.

    Encyclopedic Entry: global warming
    helium Noun

    a light, colorless gas with the chemical symbol He.

    hydrogen Noun

    chemical element with the symbol H.

    hydrogen sulfide Noun

    chemical compound gas responsible for the foul odor of rotten eggs.

    inhale Verb

    to breathe in.

    interior Noun

    internal or inland.

    invisible Adjective

    unable to be seen.

    krypton Noun

    chemical element (gas) with the symbol Kr.

    lake Noun

    body of water surrounded by land.

    litter Noun

    trash or other scattered objects left in an open area or natural habitat.

    methane Noun

    chemical compound that is the basic ingredient of natural gas.

    molecule Noun

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

    musical note Noun

    tone and length of a sound.

    neon Noun

    chemical element (gas) with the symbol Ne.

    nitrogen Noun

    chemical element with the symbol N, whose gas form is 78% of the Earth's atmosphere.

    nutrient Noun

    substance an organism needs for energy, growth, and life.

    Encyclopedic Entry: nutrient
    ocean Noun

    large body of salt water that covers most of the Earth.

    Encyclopedic Entry: ocean
    organism Noun

    living or once-living thing.

    oxygen Noun

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

    ozone Noun

    form of oxygen that absorbs ultraviolet radiation.

    ozone layer Noun

    layer in the atmosphere containing the gas ozone, which absorbs most of the sun's ultraviolet radiation.

    Encyclopedic Entry: ozone layer
    photosynthesis Noun

    process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

    plant Noun

    organism that produces its own food through photosynthesis and whose cells have walls.

    pneumatics Noun

    study of the uses and properties of air and other gases.

    pollutant Noun

    chemical or other substance that harms a natural resource.

    pressurize Verb

    to adjust and maintain the atmospheric pressure in a contained area.

    radiation Noun

    energy, emitted as waves or particles, radiating outward from a source.

    scatter Verb

    to disperse or distribute without a clear pattern.

    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
    sky Noun

    upper atmosphere.

    smog Noun

    type of air pollution common in manufacturing areas or areas with high traffic.

    Encyclopedic Entry: smog
    Standard Dry Air Noun

    standard unit of measurement of gases that make up air at sea level, excluding water vapor.

    stratosphere Noun

    highest level of Earth's atmosphere, extending from 10 kilometers (6 miles) to 50 kilometers (31 miles) above the surface of the Earth.

    substance Noun

    physical material.

    sulfur dioxide Noun

    greenhouse gas that can cause acid rain.

    survive Verb

    to live.

    thrive Verb

    to develop and be successful.

    toxic Adjective


    train Noun

    connected railroad cars pulled by a single engine.

    truck Noun

    motor vehicle used for transporting cargo, often with an open space behind the driver in which to load the cargo.

    ultraviolet radiation Noun

    powerful light waves that are too short for humans to see, but can penetrate Earth's atmosphere. Ultraviolet is often shortened to UV.

    unit of measurement Noun

    standard of size or composition.

    vehicle Noun

    device used for transportation.

    visible Adjective

    able to be seen.

    volcanic eruption Noun

    activity that includes a discharge of gas, ash, or lava from a volcano.

    waste Noun

    material that has been used and thrown away.

    water cycle Noun

    movement of water between atmosphere, land, and ocean.

    Encyclopedic Entry: water cycle
    weather Noun

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

    Encyclopedic Entry: weather
    wheel Noun

    rotating circular device used in a wide variety of tools and machinery.

    xenon Noun

    chemical element with the symbol Xe.

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