Molecular vibrations and some energetic rotations have energy level spacings that correspond to energies in the IR region of the electromagnetic spectrum (most rotations are in the microwave range which runs between thermal IR and radio wavelengths). Thus IR radiation absorbed by molecules causes increased vibration. Collisions between these energized molecules and others in the sample transfer energy among all the molecules, which increases the average thermal energy and, hence, raises the temperature. Conversely, molecules that emit IR radiation lose their vibrational energy and their collisions with other molecules decrease the average thermal energy and lower the temperature.
The wavelength unit used here and in most discussions of greenhouse gases is the micrometer, μm, which is usually called a “micron”. Frequencies of radiation in the IR are often given in units of reciprocal centimeters, cm–1, called “wavenumbers” (number of waves per centimeter). To convert microns to wavenumbers, divide the numerical value in microns into 10,000 μm·cm–1.
In order for molecular vibrations to absorb IR energy, the vibrational motions must change the dipole moment of the molecule. All molecules with three or more atoms meet this criterion and are IR absorbers. While the Earth’s (dry) atmosphere is predominantly composed of non-IR absorbers, N2 (78%), O2(21%), and Ar (~0.9%), the 0.1% of remaining trace gases contains many species that absorb IR. The absorptions by CO2, CH4, N2O, and O3 are shown in the schematic diagram in the sidebar below.
Source:
The Engines of Our Ingenuity, Episode 1776, John H. Lienhard, University of Houston; figure was modified by adding the partial charges.
The atmosphere is, of course, actually “wet” and may contain several percent water vapor, as well as liquid and solid water in clouds, from the natural water cycle (evaporation-condensation-precipitation). Water vapor also has strong absorptions in the IR, as shown in the schematic diagram. The majority of atmospheric warming is due to these absorptions by water vapor that occur at both ends of the thermal IR region.
Properties
A rather long article with much good information, written so that the layman of reasonable intellect can understand it. Not that jc will ever read it. Nor any of the other blind denialists here.