ABSTRACT
The lines observed with a spectrograph are not infinitely narrow, as the previous discussion might suggest. Spectral lines in the atmosphere have a finite width because of two broadening mechanisms, Doppler28 broadening and pressure broadening. Because these mechanisms are affected differently by changes in temperature and pressure, absorption lines in the earth’s atmosphere are much broader near the surface of the earth than at higher altitudes. This has a substantial effect on the radiance emitted at the top of the atmosphere. Let us examine each one in turn.
