Radiative Transfer Concepts Related to Water Vapor Imagery
Water vapor molecules -- the role of vibrational transitions
- Water vapor molecules in the atmosphere absorb outgoing terrestrial radiation in the infrared region of the electromagnetic spectrum (wavelengths between 1 and 100 microns). The energy associated with this absorbed radiation causes the H2O molecules to vibrate and/or rotate with respect to their centers of mass.
- The vibrational transitions (bending, symmetric stretching, asymmetric stretching) are of particular importance to remote sensing of water vapor, since they occur at discrete frequencies. The energy of photons emitted during H2O bending vibrational transitions has a wavelength of 6.3 microns.
- The spectrum of radiation emitted by H2O molecules is broadened by collisions (which distort the molecules and cause them to emit at slightly different wavelengths) and also by various combinations of vibrational and rotational transitions that can occur simultaneously. As a result, satellite radiometers that are designed to detect radiation in the 5.5-7.5 micron band (centered at 6.3 microns) are useful for remotely sensing tropospheric water vapor.
Factors which influence radiation measured at the satellite
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