Good question ... what IS in the air?
The simple answer is that the air ... the Earth's atmosphere ... is about 78% nitrogen, 21% oxygen, with a tiny amount of some other gases including water vapor. Then, there's dirt. I want to talk a little about the oxygen, one of the other gases (carbon dioxide to be exact), the water vapor, and the dirt.
Oxygen
The oxygen is one of the most important parts to us because we (and all the other animals) need it to breath. To me, what is most interesting about the oxygen is that in the old days ... before any animals or plants evolved but life existed on the earth ... there was probably very little oxygen in the atmosphere. To the bacteria, about the only thing that was around at that time, oxygen was a poison, and in fact, it is what those bacteria released as a waste product. They farted oxygen.
As more and more oxygen was released into the atmosphere, some of the bacteria happened to have a mechanism to trap this gas so that it would not harm them, and with more oxygen, the bacteria with this ability became more common. The molecules that trapped oxygen and made it harmless to the bacteria eventually evolved to become the molecules that animals use today to move oxygen around in their bodies (in blood) in a useful way. So this important feature of the air ... the oxygen ... is actually there because of the way in which life evolved.
Carbon dioxide
Everyone knows that carbon dioxide is a "greenhouse" gas. What does that mean, though, really? Well, energy (light) from the sun hits the surface of the earth and is transformed into heat energy, which then goes back into outer space. But on the way out of the atmosphere, the heat energy keeps hitting molecules of gas (like nitrogen) in the air, essentially passing through these molecules. But some molecules act differently. When the heat energy hits these molecules, the molecule absorbs the heat into the internal structure of the molecule and atoms, holds it for a while, then spits it out.
When heat is passing outwards towards space through nitrogen, it goes pretty quickly and is not hampered by the nigrogen, but when heat hits these other molecules, the heat is sometimes spit out back towards the earth again (and sometimes not). This means it takes the heat energy much much longer to leave the Earth's atmosphere. The gases made of this kind of molecule are the greenhouse gases, and carbon dioxide is one of the main greenhouse gases. The more carbon dioxide in the atmosphere, the warmer the atmosphere will be, on average.
Water Vapor
Water vapor turns into rain, snow, hail, fog, or frost. Then, the liquid water evaporates back into the air again. Then it turns into rain again. And so on. This is obviously part of what makes the weather.
The only reason I wanted to mention water vapor is to show you this cool video. This is a video of an undular bore, which is a big giant "wave" of cloud-stuff (water vapor) rolling across the landscape:
And now, we come to dirt.
There is a lot of "dirt" in the atmosphere, and it is very very important to me (and other scientists interested in the past). There are several kinds of "dirt" that float around in the air and eventually settle, especially in lakes or ponds or the oceans, that then become trapped in the muck at the bottom of the pond or lake or ocean, that can be examined hundreds or thousands of years later to see what kind of dirt was floating around in the past.
Since some of that dirt is pollen from plants, we can see what kinds of plants were around in the past. Some of that dirt comes from fresh water diatoms, which are like plankton in lakes, which, when they die, are sometimes blown into the air. Darwin collected fresh water plankton while he was in the middle of the Atlantic Ocean during his voyage on the Beagle. Here's a picture of some diatoms:
Sometimes the dirt is stuff exploded out of meteors or asteroids that hit the earth. When we find that kind of dirt, it tells us that there was some kind of impact at a particular time, which is important because some of these impacts caused mass extinctions or changed the climate.
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As you know, oxygen is a deadly poison. We will all die of oxygen poisioning unless something else gets us first. I read someplace, many years ago, that the threshold value of enough oxygen in the atmosphere to make it worthwhile to go to oxidative metabolism was 4%. Water vapor is less dense that air in general and high humidity decreases air density and also decreases percentage of oxygen in the air. Pilots are well aware of what is called "density altitude", a mix of real altitude, humidity and temperature. On a hot wet day, up in the mountains, you don't want to load your airplane too heavy.
My information has water vapor as less dense than air. Also, bear in mind that water vapor is "invisible", ie clear and colorless.
Please ignore above post I misread the comment. Time for my nap.
Yes, I remember this from The Physics of Baseball as the reason that you're likely to see more home runs on humid days.
It's a testimony to the power of the hydrogen bond that water has the smallest MW of all the major components of air (and I'm even throwing Argon in for good measure), and yet is the only component that is a stable liquid at ambient temperature and pressure.
And don't forget, in the nuclear navy (US) air is: 80% oxygen, 20% nitrogen, and 1% other.
Thanks, Greg - my roommate's 9-year-old son asked me that last week and I found myself stumbling for an answer. All us common folk know oxygen is a major component, but, seeing as how pure oxygen is rocket fuel, and obviously we don't go around sucking in pure rocket fuel 24/7, I could not remember the other primary components.
I was forgetting about the nitrogen. Also what you have written on the "dirt" is very interesting.
Could someone say something more about nitrogen - that is appearing more and more in debates about climate change. Apparently nitrogen is a major component of many animals'- especially cattle - waste products.
@yogi-one, above me.
The form of nitrogen you're thinking of is ammonia (NH3), which results from the decomposition of animal wastes. However, this is not a gas, and doesn't enter the atmosphere.
However, another product of animal digestion is methane (CH3) which is, in fact, a greenhouse gas. In terms of a scale, methane is a stronger greehouse gas than carbon dioxide, but it exists in much smaller amounts, so isn't as big a player. You are right in saying that the amount of livestock we keep can and does contribute to global warming through methane levels. I don't believe we can accurately predict what that affect is, yet, unfortunately.
Hi Greg, do you know of any links to data regarding present day oxygen levels around the world and any previous data recorded, the only dates I have found information on is 1950 at 21% and 1997 at 19%, with the increase of fresh water into the artic regions along with pollution and forest devastation I would say that humanity is in big trouble
I would assume the Hawaiian research station famous for its CO2 data has that as well. There are national archives of atmospheric data. Poke around in NOAH, you'll find it.
Report back.