The following graph shows how various layers of the Earth's atmosphere absorb long-wave radiation. We plot wavelength on the horizontal axis. On the vertical axis we plot the fraction of radiation absorbed along a 3-km path. There are seven absorption plots. Each plot corresponds to a different altitude. The 0-km plot gives the absorption along a 3-km path through a mixture of air and water vapor that is typical at sea-level in the Earth's atmosphere at latitude 30° north. The 3-km plot gives absorption along a 3-km path through a mixture that is typical at altitude 3 km. The final plot gives the absorption along a 3-km path through a mixture typical at 18 km. Click on the graph for a larger version.
We obtained the individual plots of absorption along a 3 km path using the Spectral Calculator. We obtained our typical values for temperature and water vapor content from these plots. We obtained the pressure using the approximate formula for atmospheric pressure with altitude that we derived in Atmospheric Pressure. The following table shows the values we used.
Each plot serves as an estimate of how much radiation will pass through a 3-km layer of the atmosphere from below. The 0-km plot shows that almost all the long-wave radiation emitted by the Earth is absorbed in the first 3 km of the atmosphere, except for wavelengths between 8 μm and 13 μm. Wavelengths between 8 μm and 13 μm pass all the way through the atmosphere. At altitude 9 km, we see the atmosphere starts to become transparent to wavelengths greater than 16 μm. Absorption between 14 μm and 16 μm at altitude 18 km is due to CO2.
In future posts, we will show how these plots allow us to estimate the altitude of the Earth's tropopause and estimate the effect of CO2 concentration upon the Earth's surface temperature.