Above the bustling, weather-filled troposphere lies a very different atmospheric layer: the stratosphere. The stratosphere extends from the tropopause (the top of the troposphere, at 8-18 kilometres depending on latitude) up to about 50 kilometres altitude. The defining feature of the stratosphere is its temperature profile — temperature increases with altitude rather than decreasing, the opposite of the troposphere. This is because the stratosphere contains the ozone layer, where ultraviolet radiation from the Sun is absorbed and converted to heat. The stratosphere is also extremely dry, generally stable (vertical motion is suppressed), and largely free of the day-to-day weather variability we experience at the surface. Commercial airliners typically cruise in the lower stratosphere — that smooth, often turbulence-free environment is one of the reasons the lower stratosphere is preferred for transcontinental flight.
The Discovery
The stratosphere was discovered by the French meteorologist Léon Teisserenc de Bort in 1902. Using high-altitude balloons to measure atmospheric temperature, Teisserenc de Bort found that temperature decreased steadily with altitude up to about 11 kilometres — as expected from earlier theoretical considerations — but then unexpectedly stopped decreasing and remained constant or even increased slightly. The boundary at which the change occurred (the tropopause) and the region above it (the stratosphere, from "stratus" meaning layered) were both named by Teisserenc de Bort. The discovery was initially controversial because it contradicted the simple picture of an atmosphere cooling smoothly with altitude up to its top. Subsequent measurements confirmed the existence of the temperature inversion. The mechanism — absorption of solar UV by ozone — was not fully understood until later in the 20th century.
The Temperature Profile
The stratosphere starts at the tropopause (around -55°C in the mid-latitudes, even colder in the tropics) and warms with altitude up to the stratopause at about 50 kilometres, where temperatures reach approximately 0°C. The warming is caused by the absorption of solar ultraviolet radiation by ozone (O₃). Ozone is most concentrated in a layer between about 15 and 35 kilometres altitude — the ozone layer proper. UV photons absorbed by ozone provide the energy that warms the surrounding gas. The temperature increase with altitude makes the stratosphere very stable to vertical motion — warm air aloft cannot easily descend, cold air below cannot easily rise. This is why convection and weather, both driven by vertical instability, are largely absent from the stratosphere.
The Ozone Layer
The defining chemical feature of the stratosphere is the ozone layer. Ozone is a molecule made of three oxygen atoms (O₃), formed in the stratosphere when UV radiation breaks ordinary oxygen molecules (O₂) apart and the resulting oxygen atoms combine with other O₂ molecules. This is the Chapman cycle, first proposed by the British scientist Sydney Chapman in 1930. The natural balance between production and destruction of ozone maintains a stratospheric ozone concentration of a few parts per million by volume — much higher than in the troposphere, but still very dilute. The total amount of ozone in the atmosphere is small — if all the stratospheric ozone were brought to the surface at standard pressure, it would form a layer only about 3 millimetres thick. Despite this small amount, the ozone layer absorbs almost all of the Sun's ultraviolet B (UV-B) and C (UV-C) radiation, preventing it from reaching the surface where it would damage DNA in living organisms.
The Ozone Hole and Recovery
In 1985, British Antarctic Survey scientists led by Joseph Farman published their discovery that the ozone layer over Antarctica was being severely depleted each spring. The "ozone hole" turned out to be caused by chlorofluorocarbon (CFC) compounds released into the atmosphere by human industrial activities. CFCs are inert in the lower atmosphere but, once they rise into the stratosphere, are broken apart by UV radiation to release chlorine atoms that catalyse rapid ozone destruction. The discovery led to the 1987 Montreal Protocol, an international treaty that progressively banned ozone-depleting substances. The treaty has been remarkably successful — atmospheric concentrations of most banned substances have been falling for decades, and the ozone layer is gradually recovering. The Antarctic ozone hole reached its largest extent in the late 1990s and early 2000s and has been gradually shrinking since. Full recovery is expected by approximately 2070. The Montreal Protocol is often cited as the most successful international environmental treaty ever ratified.
Stratospheric Dryness
The stratosphere is extremely dry, with water vapour concentrations of only a few parts per million by volume — about three to four orders of magnitude less than in the moist tropical troposphere. The dryness is enforced by the cold temperatures at the tropopause: as moist tropospheric air rises through the cold tropopause, the water vapour freezes out as ice crystals that fall back into the troposphere. This "cold trap" mechanism keeps the stratosphere persistently dry. Volcanic eruptions can inject substantial water vapour directly into the stratosphere, and the 2022 Hunga Tonga eruption did so on an unusual scale — but normally, the only source of stratospheric water vapour is the slow upward leakage through the tropical tropopause, and the residence time is years to decades.
Stratospheric Aerosols
The stratosphere normally contains low levels of fine aerosol particles. These can be substantially boosted by major volcanic eruptions that inject sulfur dioxide directly into the stratosphere. The SO₂ converts to sulfate aerosols, which then circulate globally and persist for 1-3 years before settling out. Large eruptions like Tambora (1815), Krakatoa (1883), and Pinatubo (1991) produced enough stratospheric sulfate to cool the global climate by several tenths of a degree for a year or more. The 1991 Pinatubo eruption produced about 0.5°C of global cooling that lasted for two years and provided a real-world test of climate models. The proposed geoengineering technique of "stratospheric aerosol injection" — deliberately introducing aerosols into the stratosphere to cool the climate — is essentially an artificial volcanic eruption and would have similar effects, along with various potential side effects.
Polar Stratospheric Clouds
The stratosphere is normally cloudless, but at very high latitudes in winter, temperatures can drop low enough to form polar stratospheric clouds (PSCs). These rare clouds, also called nacreous clouds for their iridescent mother-of-pearl appearance, form at temperatures below about -78°C. They consist of ice particles, sometimes with nitric acid or sulfuric acid components. PSCs play a critical role in the ozone hole formation — they provide surfaces on which chlorine compounds can be processed into ozone-destroying forms that activate when sunlight returns in spring. PSCs are seen mostly over Antarctica because the Antarctic stratosphere is colder than the Arctic's — the strong polar vortex around Antarctica isolates the cold air and prevents warmer mid-latitude air from mixing in. The Arctic also produces PSCs but less frequently and not for as long, so Arctic ozone depletion has been less severe than Antarctic.
Cruising Altitude
Commercial passenger jets cruise at altitudes of 9-12 kilometres — typically in the lower stratosphere over the mid-latitudes, or in the upper troposphere. The lower stratosphere offers several advantages: no weather (the smooth ride passengers experience), lower air density (less drag, better fuel economy), and avoidance of jet streams when flying westbound. The lower stratosphere is therefore where most international air travel happens. Supersonic aircraft like Concorde flew higher — Concorde cruised at about 18 kilometres altitude, well into the stratosphere. Future supersonic and hypersonic aircraft would fly even higher, raising concerns about their effects on stratospheric ozone and climate.
Stratosphere-Troposphere Coupling
Despite the stratosphere's overall stability, exchanges with the troposphere do occur. Tropopause folds and stratospheric intrusions bring dry, ozone-rich stratospheric air down into the troposphere, particularly near jet streams. Stratospheric warming events — sudden, large temperature changes in the polar stratosphere — can affect surface weather by disrupting the polar vortex and shifting jet stream patterns. The 21-day "sudden stratospheric warming" events that occur every few winters can produce major weather impacts at the surface, including unusual cold outbreaks in the mid-latitudes (the 2018 "Beast from the East" in Europe was associated with such an event). Understanding stratosphere-troposphere coupling is increasingly important for improving long-range weather forecasting.
Importance for Life on Earth
The stratosphere matters fundamentally for life on Earth because of the ozone layer. Without stratospheric ozone, UV-B radiation reaching the surface would be high enough to damage DNA in essentially all surface organisms. Early in Earth's history, before oxygen levels had risen enough to support a thick ozone layer, life was restricted to environments where UV radiation could not reach — underground, underwater, or in protected niches. The development of the ozone layer about 600 million years ago, following the rise of atmospheric oxygen, was a precondition for life to colonise the land surface. Today, the stratosphere continues to provide that protection, and the health of the stratosphere remains a planetary-scale concern that the international community has been actively managing for almost 40 years.