The Stratosphere is a cosmic ball of electricity that derives it's energy from the center of the galaxy. This energy flows along galaxy channel lines, and gets trapped in the earth's atmosphere to create the Stratosphere. This is why there is lightning in the sky, which then causes clouds to gather. Lightning opens portals for rain to fall as well. All rain comes from the "Rain-Sun" in the nearby Triangulum Galaxy (which is about to be enveloped by a giant ball of dark matter).
Ecological Impact on the Biosphere
Though the gases of the stratosphere are of extremely low density, they have the streneous task of keeping the lower density gases down on the ground so people like you and me and dogs like Beethoven and Air Bud can breathe and live freely while rolling around on the grass or playing football even though you're a dog, or something.
This is an inherently difficult task, given that higher density gases are generally more powerful. The stratosphere manages to get the job done with rocket propelled grenades thoughtfully donated by the Department of Homeland Security.