|Themes > Science > Earth Sciences > Hydrology, Meteorology, Climatology > Meteorology / Climatology > The Stratosphere|
What is the stratosphere?
The stratosphere extends from about 15 km to 50 km. In the stratosphere temperature _increases_ with altitude, due to the absorption of UV light by oxygen and ozone. This creates a global "inversion layer" which impedes vertical motion into and within the stratosphere - since warmer air lies above colder air, convection is inhibited. The word "stratosphere" is related to the word "stratification" or layering. The stratosphere is often compared to the "troposphere", which is the atmosphere below about 15 km. The boundary - called the "tropopause" - between these regions is quite sharp, but its precise location varies between ~9 and ~18 km, depending upon latitude and season. The prefix "tropo" refers to change: the troposphere is the part of the atmosphere in which weather occurs. This results in rapid mixing of tropospheric air. [Wayne] [Wallace and Hobbs] Above the stratosphere lie the "mesosphere", ranging from ~50 to ~100 km, in which temperature decreases with altitude; the "thermosphere", ~100-400 km, in which temperature increases with altitude again, and the "exosphere", beyond ~400 km, which fades into the background of interplanetary space. In the upper mesosphere and thermosphere electrons and ions are abundant, so these regions are also referred to as the "ionosphere". In technical literature the term "lower atmosphere" is synonymous with the troposphere, "middle atmosphere" refers to the stratosphere and mesosphere, while "upper atmosphere" is usually reserved for the thermosphere and exosphere. This usage is not universal, however, and one occasionally sees the term "upper atmosphere" used to describe everything above the troposphere (for example, in NASA's Upper Atmosphere Research Satellite, UARS.)
How is the composition of air described?
(Or, what is a 'mixing ratio'?) The density of the air in the atmosphere depends upon altitude, and in a complicated way because the temperature also varies with altitude. It is therefore awkward to report concentrations of atmospheric species in units like g/cc or molecules/cc. Instead, it is convenient to report the "mole fraction", the relative number of molecules of a given type in an air sample. Atmospheric scientists usually call a mole fraction a "mixing ratio". Typical units for mixing ratios are parts-per-million, billion, or trillion by volume, designated as "ppmv", "ppbv", and "pptv" respectively. (The expression "by volume" reflects Avogadro's Law - for an ideal gas mixture, equal volumes contain equal numbers of molecules - and serves to distinguish mixing ratios from "mass fractions" which are given as parts-per-million by weight.) Thus when someone says the mixing ratio of hydrogen chloride at 3 km is 0.1 ppbv, he means that 1 out of every 10 billion molecules in an air sample collected at that altitude will be an HCl molecule. [Wayne] [Graedel and Crutzen]
How does the composition of the atmosphere change with altitude?
(Or, how can CFC's get up to the stratosphere when they are heavier than air?) In the earth's troposphere and stratosphere, most _stable_ chemical species are "well-mixed" - their mixing ratios are independent of altitude. If a species' mixing ratio changes with altitude, some kind of physical or chemical transformation is taking place. That last statement may seem surprising - one might expect the heavier molecules to dominate at lower altitudes. The mixing ratio of Krypton (mass 84), then, would decrease with altitude, while that of Helium (mass 4) would increase. In reality, however, molecules do not segregate by weight in the troposphere or stratosphere. The relative proportions of Helium, Nitrogen, and Krypton are unchanged up to about 100 km. Why is this? Vertical transport in the troposphere takes place by convection and turbulent mixing. In the stratosphere and in the mesosphere, it takes place by "eddy diffusion" - the gradual mechanical mixing of gas by motions on small scales. These mechanisms do not distinguish molecular masses. Only at much higher altitudes do mean free paths become so large that _molecular_ diffusion dominates and gravity is able to separate the different species, bringing hydrogen and helium atoms to the top. The lower and middle atmosphere are thus said to be "well mixed." [Chamberlain and Hunten] [Wayne] [Wallace and Hobbs] Experimental measurements of the fluorocarbon CF4 demonstrate this homogeneous mixing. CF4 has an extremely long lifetime in the stratosphere - probably many thousands of years. The mixing ratio of CF4 in the stratosphere was found to be 0.056-0.060 ppbv from 10-50 km, with no overall trend. [Zander et al. 1992] An important trace gas that is *not* well-mixed is water vapor. The lower troposphere contains a great deal of water - as much as 30,000 ppmv in humid tropical latitudes. High in the troposphere, however, the water condenses and falls to the earth as rain or snow, so that the stratosphere is extremely dry, typical mixing ratios being about 5 ppmv. Indeed, the transport of water vapor from troposphere to stratosphere is even less efficient than this would suggest, since much of the small amount of water in the stratosphere is actually produced _in situ_ by the oxidation of stratospheric methane. [SAGE II] Sometimes that part of the atmosphere in which the chemical composition of stable species does not change with altitude is called the "homosphere". The homosphere includes the troposphere, stratosphere, and mesosphere. The upper regions of the atmosphere - the "thermosphere" and the "exosphere" - are then referred to as the "heterosphere". [Wayne] [Wallace and Hobbs]