Stratosphere: The Calm, Ozone-Rich Second Layer of the Atmosphere
The stratosphere is the second layer of Earth's atmosphere, sitting above the troposphere and extending from roughly 12 to 50 km in altitude. Unlike the layer below it, the stratosphere gets warmer with height, an inversion driven by ozone absorbing solar ultraviolet light. That inversion suppresses vertical mixing, making the layer extremely stable and dry. This stability is why airliners cruise in its lowest reaches, why high-altitude balloons climb into it, and why injected particles such as nuclear-firestorm soot can linger for years instead of washing out in rain.
The stratosphere is the second-lowest layer of Earth's atmosphere, lying above the troposphere and below the mesosphere. Its lower boundary, the tropopause, sits around 7 km over the poles, near 10 km at mid-latitudes, and as high as 20 km at the equator; the layer extends upward to the stratopause at about 50 km. Together these put the stratosphere roughly in the 12-50 km band that most descriptions cite. What sets the stratosphere apart is its temperature inversion: instead of cooling with height as the troposphere does, it grows warmer with altitude, climbing from near -50 C at the tropopause to roughly -15 C at the top. The heat comes from the ozone layer, a region of concentrated ozone (mostly 15-35 km up) that absorbs the Sun's ultraviolet radiation and releases that energy as heat. This is the same absorption that shields life at the surface from damaging UV, which is why the layer's chemistry matters so much. See Ultraviolet Radiation: UVA, UVB, UVC and What Reaches the Ground for what the ozone screen lets through. Because warm air sits above cooler air, the stratosphere is strongly stratified and resists the convection that churns the troposphere. There is little vertical mixing and almost no weather; horizontal flows move air far faster than vertical ones. That calm, dry stability has practical consequences. Commercial jets cruise in the lowest stratosphere (around 10-12 km) to escape turbulence and improve fuel efficiency, and high-altitude balloons and stratospheric platforms climb into it for the same smooth, thin-air conditions, the operating regime targeted by a High-Altitude Platform Station. The lack of rain cuts both ways. Particles lofted into the stratosphere are not scrubbed out by precipitation, so they can circle the globe and persist for years. This is central to nuclear winter models, where firestorm soot heated by sunlight rises into the stratosphere and forms a long-lived sunlight-blocking veil, an effect explored in Nuclear Winter: Scientific Models, Uncertainties, and Scale Requirements. The same isolation explains why human-made ozone depletion was so dangerous: chemicals like those in CFCs: The Miracle Chemicals That Tore a Hole in the Ozone Layer drift up intact and destroy ozone catalytically, the threat that prompted The Montreal Protocol: How the World Agreed to Save the Ozone Layer.