Atmospheric composition and structure
Earth’s atmosphere is a dynamic mixture of gases and aerosols. Some of the gases are approximately “well-mixed” (that is, they are relatively constant in their proportions throughout the horizontal and vertical extent of the atmosphere) and other gases and the aerosols vary in their proportions in both space and time. Aerosols are discussed at some length in a separate article; here the focus is on gases.
In descending order of amount by volume (indicated by percentages), the atmosphere’s gases are molecular nitrogen (N2, ~ 78%) and oxygen (O2, ~ 21%); argon (Ar, <1%); water vapor (H2O, <1 % generally but highly variable); carbon dioxide (CO2, <400 ppm [parts per million]); and a variety of ppm- (and lower) level trace gases such as helium, neon, krypton, molecular hydrogen, methane, the chlorofluorocarbons, ozone, and various oxides of nitrogen. Of these, the space/time distributions of water vapor and ozone (O3, <100 parts per billion generally but highly variable) have important implications for both weather and climate.
Molecules with three or more atoms exhibit highly complex electromagnetic absorption and emission spectra. This complexity includes a variety of energetic frequencies as these molecules absorb and emit radiation, generally called “spectral bands.” When temporal and spatial variability is added to this spectral complexity, the atmospheric heating and cooling associated with absorption of solar energy, and both absorption and emission of thermal energy, becomes significant and complicated.
In the clear (i.e., aerosol-free) atmosphere, most of the absorption of solar energy is associated with stratospheric ozone, while absorption and emission of thermal energy occurs throughout the atmospheric column by the so-called “greenhouse gases.”
Water vapor, the strongest greenhouse gas, is an important part of Earth’s hydrological cycle, and its distribution and behavior in the atmosphere is discussed in that context in other articles.
Ozone is conveniently and appropriately discussed in two separate contexts: tropospheric ozone, a pollutant, and stratospheric ozone, a natural layer of the gas that is critical for life on Earth as we know it and for the maintenance of the climate as well.
Because the atmosphere is relatively transparent to sunlight, it is heated mainly from below. Sunlight heats the Earth’s surface, which radiates energy upward in thermal wavelengths, some of which is absorbed by the greenhouse gases and re-radiated. In addition, via conduction and convection, the surface also transfers energy to the atmosphere by mechanical means.
About 85% percent of the mass of the atmosphere lies below about 12 km. Because Earth’s radius is about 6,400 km, considering the atmosphere to be a thin shell of air surrounding the planet is not unreasonable. (Consider a ball of 20 cm diameter—about the size of a soccer ball—as a model of the Earth. A scale-model atmosphere for this ball would be less than 0.25 mm thick.) However, there is considerable vertical structure within this thin shell that is of relevance to earth science.
Like a pot of boiling water on a stove, heating the atmosphere from below creates instability that manifests itself in turbulence. The atmospheric boundary layer, of variable depth near the surface, is most strongly affected by this turbulence, but the effects of the surface-heating-induced mixing can extend, via thunderstorms, throughout the depth of the troposphere. This layer, noted above as containing approximately 85% of the atmosphere’s mass, is where active weather occurs. It ends at the tropopause, a thin transition zone between the troposphere below and the stratosphere, a relatively stable layer that contains most of the stratospheric ozone, above. Above the stratosphere, beginning at about 50 km, are the mesosphere and thermosphere. These rarified layers, which extend to 100 km or so, contain only about 1% of the atmosphere’s mass.