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.OxygenThe 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 dioxideEveryone 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 VaporWater 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.