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 (homo sapiens), 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 the air at sea level. is a standard scientific unit of measure. Standard dry air is made up of nitrogen, oxygen, argon, carbon dioxide, neon, helium, krypton, hydrogen, and xenon. does not include water vapor because the amount of vapor changes with humidity and temperature. Because air masses are constantly moving, standard dry air is not accurate everywhere at the same time. Nitrogen and oxygen make up about 99 percent of the earth’s air. people and other animals need oxygen to live. carbon dioxide, a gas that plants depend on, makes up less than 0.04 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 converts the sun’s energy into nutrients. water vapor in the air is sometimes seen as clouds. Water enters the atmosphere through the water cycle. The water cycle also brings molecules from the air into the oceans, lakes, and rivers. Some gases in the air come from volcanic eruptions. Volcanic eruptions expel 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. some organisms, however, thrive on these gases. At the bottom of the ocean there are bacteria that do not need oxygen or sunlight to survive. in other words, they don’t need air. these strange organisms create their own nutrients using hydrogen sulfide, not carbon dioxide. hydrogen sulfide comes from cracks or vents in the earth’s crust.
The air is different as you go higher and higher in the atmosphere. air gets “thinner” as elevation increases because there are fewer air molecules up there. climbers often have to use oxygen canisters when climbing above 3,800 meters (12,500 feet) because there isn’t enough oxygen in the atmosphere for most people to breathe. High mountains like Mount Everest (8,848 meters or 29,035 feet) in Nepal and China are full of empty oxygen canisters that climbers discard when they run out. High in the stratosphere, a layer of Earth’s atmosphere, is a special airborne molecule called ozone. Ozone is made up of three oxygen atoms. 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 harm to living things on earth. Unfortunately, air pollution has a negative effect on the air we breathe. Air pollution occurs when harmful byproducts, such as car exhaust, 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 excessive amounts of carbon dioxide, can upset the balance of molecules in the air, contributing to global warming. non-atmospheric aircompressed air is air that is maintained at a constant pressure, such as air pressure at sea level. airplanes are usually pressurized at ground level so passengers can breathe without canisters of air. Compressed air is often held at a higher pressure than normal air pressure. divers use compressed air to breathe underwater. airboats 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 of pneumatics are very varied. Air brakes on cars, trucks, and trains use compressed air to reduce wheel spin and stop vehicles. pipe organs use compressed air at different pressures to create different musical notes.
