The Earth’s atmosphere is a complex layered system where temperature typically falls with increasing altitude, a phenomenon familiar to anyone who has climbed a mountain or watched a weather report. However, between approximately 12 and 50 kilometers above the surface, this thermal structure dramatically reverses. In this atmospheric layer, known as the stratosphere, the air actually becomes warmer the higher one travels. This counter-intuitive temperature profile stands in direct contrast to the cooler air found higher up in the troposphere below. Understanding this thermal inversion requires examining the stratosphere’s unique chemical composition and how it interacts with solar radiation.
Defining the Stratosphere
The stratosphere is the second major layer of the Earth’s atmosphere, extending from the tropopause up to the stratopause. The tropopause sits at an average altitude of about 12 kilometers but can vary from 7 kilometers at the poles to 20 kilometers at the equator. This layer extends to roughly 50 kilometers, where the stratopause marks its upper limit.
The air density is extremely low, about a thousand times thinner at the stratopause than at sea level. This low density, combined with the temperature increase with height, results in a highly stable atmospheric layer. Warmer, less dense air sits atop cooler, denser air, which suppresses vertical air movement, convection, and turbulence. This thermal stability gives the stratosphere its name, derived from the Latin word “stratum,” meaning layer, and is the reason commercial aircraft often fly in the lower part of this layer.
Ozone The Stratosphere’s Primary Heat Source
The warming of the stratosphere is directly attributable to the presence of ozone (\(\text{O}_3\)), a triatomic molecule of oxygen. While ozone is a trace gas, about 90% of it resides within the stratosphere, concentrated primarily between 15 and 35 kilometers in the ozone layer. This layer is the sole reason for the stratosphere’s unique thermal behavior.
The ozone molecule is an efficient absorber of high-energy ultraviolet (UV) radiation from the Sun. Specifically, ozone absorbs most UV-B and all UV-C radiation. This absorption converts the penetrating radiant energy into thermal energy, making ozone the primary internal heat source for the stratosphere.
The Photochemical Mechanism of Heating
The conversion of solar radiant energy into heat in the stratosphere is governed by the Chapman cycle. This continuous cycle involves the formation and destruction of ozone, both of which generate heat. The process begins when high-energy UV-C radiation strikes an oxygen molecule (\(\text{O}_2\)), causing it to split into two separate, highly reactive oxygen atoms (\(\text{O}\)).
Each single oxygen atom then quickly collides with an intact oxygen molecule (\(\text{O}_2\)) to form an ozone molecule (\(\text{O}_3\)). This recombination reaction is exothermic, releasing energy in the form of heat into the surrounding air molecules. The newly formed ozone molecule is unstable and readily absorbs UV-B radiation, which causes it to dissociate back into an oxygen molecule and a single oxygen atom.
The energy from the absorbed UV-B radiation is converted into the kinetic energy of the resulting fragments, which is another source of heating. The fragments then quickly collide with other atmospheric gases, transferring their kinetic energy and increasing the temperature of the air in situ. This mechanism ensures that as long as solar UV radiation is present, the air in the stratosphere is constantly being warmed directly by these photochemical reactions.
Why Temperature Peaks at the Stratopause
The temperature in the stratosphere does not rise uniformly but reaches its maximum value at the stratopause, the boundary with the mesosphere. At the tropopause, temperatures average about \(-51^\circ\text{C}\) (\(-60^\circ\text{F}\)), warming up to approximately \(-3^\circ\text{C}\) (\(27^\circ\text{F}\)) at the stratopause. This warm peak results from a balance between the vertical distribution of ozone and the intensity of available UV radiation.
Although ozone concentration is highest in the middle stratosphere, the highest temperatures are found higher up because of the solar radiation flux. At the top of the stratosphere, the incoming UV radiation is most intense since little has been absorbed by the atmosphere below. The temperature peak occurs at the stratopause because enough ozone is present at that altitude to efficiently absorb the powerful UV rays, maximizing heat generation. Above the stratopause, the air density becomes too low to absorb significant solar energy.