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 29,035 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.

air
A parachutist glides down safely.

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.

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.

accurate
Adjective

exact.

Noun

layer of gases surrounding Earth.

air brake
Noun

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

Noun

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

Noun

harmful chemicals in the atmosphere.

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.

Noun

layers of gases surrounding a planet or other celestial body.

atom
Noun

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

Noun

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

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

colorless, odorless, and tasteless gas that is slightly less dense than air. It can be toxic to humans.

clog
Verb

to obstruct or prevent travel.

Noun

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

compressed air
Noun

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

Noun

rocky outermost layer of Earth or other planet.

Noun

number of things of one kind in a given area.

discard
Verb

to throw away.

Noun

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

Noun

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

eject
Verb

to get rid of or throw out.

Noun

height above or below sea level.

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.

Noun

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

helium
Noun

a light, colorless gas with the chemical symbol He.

hydrogen
Noun

chemical element with the symbol H, whose most common isotope consists of a single electron and a single proton.

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.

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.

Noun

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

Noun

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

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.

Noun

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

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.

Noun

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

sky
Noun

upper atmosphere.

Noun

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

Standard Dry Air
Noun

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

stratosphere
Noun

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

poisonous.

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.

Noun

movement of water between atmosphere, land, and ocean.

Noun

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

wheel
Noun

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

xenon
Noun

chemical element with the symbol Xe.