Stratospheric Thermal Structure and Ozone

Core Idea

Temperature in the stratosphere increases with altitude due to absorption of shortwave ultraviolet radiation by ozone (O₃), creating a temperature inversion unlike the troposphere. This thermal structure controls stratospheric dynamics and limits convection. Ozone depletion over polar regions reduces UV absorption, intensifying the temperature inversion and strengthening the polar vortex.

Explainer

From your study of the tropopause, you know that the troposphere — the lowest layer of the atmosphere where weather occurs — is characterized by temperature decreasing with altitude. Air near the surface is warmed by contact with the sun-heated ground, and as you go up, temperatures drop at roughly 6.5°C per kilometer. But at the tropopause, this trend abruptly stops. Above it, in the stratosphere, temperature begins to *increase* with altitude. Understanding why requires looking at what is absorbing energy up there: ozone.

The stratosphere contains the ozone layer, concentrated between roughly 15 and 35 km altitude, with peak density near 20–25 km. Ozone molecules (O₃) are extraordinarily efficient at absorbing ultraviolet (UV) radiation from the sun, particularly the most energetic UV-B and UV-C wavelengths. When an ozone molecule absorbs a UV photon, the energy breaks the molecule apart, and the resulting fragments recombine and release heat. This absorption warms the surrounding air. Because more UV is absorbed at higher altitudes (where the incoming solar radiation has not yet been attenuated), the upper stratosphere is warmer than the lower stratosphere. The result is a temperature inversion — temperature increasing with height — that is the defining thermal feature of this layer.

This inversion has profound dynamical consequences. In the troposphere, warm air below cold air is unstable — it drives convection, clouds, and weather. In the stratosphere, the arrangement is reversed: warm air sits above cooler air, creating a stable stratification that strongly suppresses vertical mixing. Air parcels that try to rise encounter increasingly warm surroundings and are pushed back down. This is why the stratosphere is almost cloudless (except for rare polar stratospheric clouds at extreme cold), why volcanic ash injected into the stratosphere can persist for years, and why pollutants that reach this layer have exceptionally long residence times.

The connection between ozone and temperature creates a feedback when ozone is depleted. Over Antarctica each spring, chemical reactions on polar stratospheric cloud particles (involving chlorine from human-made CFCs) destroy ozone in the lower stratosphere. With less ozone to absorb UV, the lower stratosphere cools dramatically — temperature drops of 10°C or more have been observed within the ozone hole. This enhanced cooling strengthens the temperature contrast between polar and mid-latitude stratosphere, which in turn tightens and accelerates the polar vortex — the ring of westerly winds encircling the pole. A stronger polar vortex further isolates polar air, preventing mixing with warmer mid-latitude air and perpetuating the conditions for continued ozone destruction. This coupling between chemistry, radiation, and dynamics illustrates why the stratosphere, though far above the weather, profoundly influences the climate system.