Atmosphere & Environment
Growing up, I thought that there was a shell around the Earth holding in the atmosphere. I’m not sure how this idea started, but as I grew older I started to see that there were some problems with this model of the atmosphere. For instance, I knew there was a hole in the ozone layer, but if this hole went through the shell that was holding everything in then certainly it would be catastrophic if there was [were?] a hole in it. Another problem I identified was how we could get to space with rockets without some special consideration of how to deal with the Earth’s protective shell; despite some major gaps in my knowledge, I knew a fair amount about rockets and space exploration, so I figured if the shell was real and physical then that would be part of the discussion of how rockets get to space, but it was not.
Fortunately, and somewhat terrifyingly, I eventually found that there is no physically distinct layer protecting us from space or holding in all of our air. Gravity is the only thing keeping our atmosphere on the Earth to keep us warm at night and cool during the day in contrast to the incredible 500°F temperature swings we would experience otherwise. To give you a sense of how wild this is, consider that there is not any clearly defined boundary between where the atmosphere stops and space begins, but there are some interesting things to look at to start finding where this “line” might be.
Where is Space?
Let’s start in outer space and then move close to Earth in our quest to find the edge of the atmosphere. Throughout the universe we find that space is mostly a vacuum, with just a few hydrogen atoms in each cubic meter of outer space. As we get closer to the Earth, we find more atoms per cubic meter starting at about 625 mi (1,000 km) from the surface of the Earth, which, for perspective, is equivalent to about 16%, or 1/6, of Earth’s radius. 
As we get even closer to Earth, we pass the region where many of our satellites orbit the Earth in “Low Earth Orbit.” Finally, we come to a place in the atmosphere where the air is dense enough that flying through the air makes more sense than orbiting the Earth. The place where this transition occurs is called the Karman Line, named after the person that first looked into this idea, Theodore von Kármán. His calculations showed that at about 100 km (62 mi or 330,000 ft) above the surface of the Earth  the atmosphere is so thin that in order for an aircraft to generate enough lift to fly it would need to be traveling at orbital velocity.  This designation is quite significant and is maintained by the FAI  (French, Fédération Aéronautique Internationale; English: The World Air Sports Federation) for the purpose of world records. Interestingly, since an object must be traveling at 18,000 mi/hr at the Karman Line, it takes about 20 times more energy to orbit the Earth here than to simply reach it (by going straight up, for example). 
For comparison, in the high altitude ballooning project that is the subject of this book, we will achieve an altitude of about 30 km (100,000 ft), and that is above more than 99% of all the air in the atmosphere. This may not seem that high after what we’ve just discussed but consider that commercial aircraft usually cruise at only 9-12 km (30,000-40,000 feet). Since 80% of the air in our atmosphere is contained in the troposphere (the lowest layer of the atmosphere, where we live) once we get into the stratosphere we are talking about “near space” where the sky is dark (you can see approximately the same effect with your own eyes at dawn or dusk on commercial airline flight), it is very cold outside, and cosmic rays are detected much more frequently. Felix Baumgartner, now famous for jumping out of the Red Bull Stratos high altitude balloon, made his jump from an impressive 128,100 feet (39 km).
What “Near Space” is Like
Not intuitively, the temperature of the atmosphere increases with altitude once you get to the stratosphere. As you ascent, and as you have probably experienced for yourself, temperature falls with altitude because of a decrease in pressure (just look at the ideal gas law). But, once you get to the stratosphere the air is heated by ozone absorbing ultraviolet (UV) radiation, which then turns to heat energy. UV radiation also increases with increasing altitude because there is less ozone above to absorb the radiation, this also creates more heat because UV radiation will give oxygen enough energy to bond to itself, creating and more ozone and heat because it is an exothermic reaction. A curious side effect of this increasing temperature with altitude is that there are no convection currents like there are in the troposphere.  We know from elementary school that “hot air rises,” but when the hot air has already “risen” then there is no mechanism to continue driving motion. This property prevents anvil-shaped, cumulonimbus clouds (driven by convection) from going past the top of the troposphere when they hit the tropopause . Another nice effect is that it (usually) gives commercial aircraft a smooth ride when flying in the stratosphere. The border between the troposphere and atmosphere is also where the jet stream flows. 
We have even detected life in the air above these altitudes! A high altitude balloon in 2001 collected dust with bacteria at an altitude of 135,000 feet (41 km) . There are many possible discoveries to be made in near space!
More to Explore
I’ve touched on some of the more interesting nuances of the Earth’s atmosphere, however, there are many good standalone resources  for learning more about the atmosphere that can be found with a quick search online.
Challenge: Use a programming language or a spreadsheet to model the properties of the atmosphere. There are many resources online to find equations to use for each of the properties. With careful work you can even use this information later to model the behavior of your balloon, which we will explore later. This can be expanded beyond just balloon modeling to things like heat transfer and differential equations!
 http://www.slate.com/articles/news_and_politics/explainer/2004/09/where_does_space_begin.html This website also introduced me to the “United Nations’ Office for Outer Space Affairs,” which I would work at based only on the name.
 There is a really nice graphic at https://io9.gizmodo.com/5952443