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Asking the question ‘what is air’ opens a wide variety of avenues for possible answers. The first thing that most people think of is what composes the air that we breath. That is simple. Our air is made up of mixture of gases. It is 78% nitrogen, 21% oxygen, and 1% various other gases such as xenon, carbon dioxide, hydrogen, argon, neon, helium, and krypton. The allergy sufferers out there would not want us to forget the water vapor, pollen, dust, and other solids that get mixed in there. This mixture can vary by altitude.
Another way to look at air composition is by its density. Air density is the mass per unit volume of the Earth’s atmosphere. It is a value that is very useful in many sciences, especially aerodynamics. Air density and air pressure decrease with increasing altitude. They also change with differences in temperature and/or humidity. At sea level and 20° C, air has a density of approximately 1.2 kg/m3.
A third way to answer ‘what is air’ is to look at the viscosity of air. The air viscosity depends mostly on the temperature. At say 15°C, the viscosity of air is 1.78×10-5 kg/(m•s). The less viscous something is, the greater its ease of movement (fluidity). The viscosity of air is important in aerodynamics. It is crucial in figuring drag coefficients for the purposes of flight and estimating fuel usage.
A final way to interpret what air is would be to look at wind. Wind is the flow of gases on a large scale. On Earth wind consists of the bulk movement of air. In outer space solar wind is the movement of gases or charged particles from the Sun through space. Planetary wind is the outgassing of light chemical elements from a planet’s atmosphere. The strongest observed winds on a planet in our solar system are on Saturn and Neptune.
The answer to ‘what is air’ is relative to your conversation. Above are four ways to look at the question, but you could even approach it as to the air on individual planets or in space itself. The question gets bigger the more you think about it.
If you’d like more info on the Air, check out an article about The Properties of Air. And here’s a link to NASA’s Eye for Clean Air – The Aura Satellite Discussion Page.
We’ve also recorded an episode of Astronomy Cast all about the Atmosphere. Listen here, Episode 151: Atmospheres.