The Earth is enveloped in an atmosphere that contains different layers of air that act in different ways. The main layers of the atmosphere are the troposphere, the stratosphere, the mesosphere and the ionosphere. The word "stratosphere" derives from the Latin word "stratus," which means stretched out. While most of the Earth's weather happens in the very unstable environment of the troposphere, the air remains very stable in the stratosphere. Particles and moisture that collect in the stratosphere stretch out rather than sink or rise.
The stratosphere is said to be a stable layer of the Earth's atmosphere. Cool air sinks and warm air rises. In the troposphere, where we are, the air gets cooler aloft. In the stratosphere, the air gets warmer aloft. Because cool air has a tendency to sink, the air is not continually going down then up in the stratosphere. Because the air remains relatively stationary in the stratosphere, particles that enter the stratosphere can remain there for a long time. A stable atmosphere does not often produce weather.
The stratosphere contains 90 percent of the Earth's ozone. Ozone is a gas that absorbs ultraviolet radiation. Since the 1970s, scientists have observed the levels of ozone decreasing in the stratosphere.
Because the stratosphere is a very stable layer, particles that enter it will expand rather than rise or sink. Various types of pollutants can enter the stratosphere, and when they do, they stratify, or stretch out. Volcanic ash stays in the stratosphere for years after the volcanic eruption from which it came.
Cloud formation usually requires the upward vertical motion of air in an unstable environment. Since the stratosphere is very stable, clouds usually do not form there; however, there are some minor exceptions. Cumulonimbus clouds are thunderstorm clouds that begin to form very low in the atmosphere. The tops of most thunderstorm clouds reach very high into the troposphere, but when storms are extremely strong, the tops can reach into the stratosphere. The most notable cloud that forms in the stratosphere is the polar stratospheric cloud. These clouds require very cold temperatures and form over polar regions. Polar stratospheric clouds absorb ozone, and recent to 2011 scientists have discovered an alarming increase in polar stratospheric cloud formation over the Antarctic region.