Polar stratospheric clouds form at very high altitudes in polar regions of the stratosphere. Polar stratospheric clouds are clouds that occur in the lower stratosphere at very low temperatures. The tropopause is the boundary region between the troposphere and the stratosphere. Atop the troposphere is the tropopause, which is the border between the troposphere and stratosphere. Ozone exists naturally in the stratosphere, not the troposphere. The tropopause is the boundary in the Earth's atmosphere between the troposphere and the stratosphere. Space weather is distinct from terrestrial weather of the troposphere and stratosphere. However, CFCs are well mixed globally in the troposphere and stratosphere. They are seen in the Earth's homosphere, which includes the troposphere, stratosphere, and mesosphere. Ozone absorbs large amounts of ultraviolet radiation in the stratosphere. Weather does not occur in the higher stratosphere or thermosphere. Ozone depletion is a risk of techniques involving sulfur delivery into the stratosphere. These experiments led to his discovery of the tropopause and stratosphere. The most prominent decrease in ozone has been in the lower stratosphere. This compound is also notable for catalyzing the breakup of ozone in the stratosphere.