In spectroscopy an absorbed (emitted) line is observed in correspondence to the transition from a lower (higher) to a higher (lower) energetic level within an atom (electronic transitions) or a molecule (electronic, vibrational, rotational transitions). Its characteristic frequency f is related to the amount of the energetic jump from an initial state E(1) to a final one E(2) through the Bohr's relation E(f) − E (i) = hf. As the distribution of the quantized energetic level are specific of each atom (depending on its atomic number N) and molecule (depending on their constituents atoms N, number and dispositions which determine their specific inertia momentum and vibrational properties) even the corresponding atomic and molecular spectra (i.e. the frequencies of the sequences of spectral lines/bands) are specific for each chemical atomic or molecular species. However monochromatic emission just at the frequency f is practically never observed. Always e.m. radiation emitted/adsorbed by atoms or molecules is observed also around the nominal (expected following Bohr's relation) frequency f mostly as a consequence of the following effect: a) changes of quantized energy levels associated to the process of emission/absorption itself: the consequent line broadening around the frequency f is reported as "natural broadening"; b) changes of quantized energy levels due to reciprocal collisions between atoms and molecules ("pressure broadening"); c) the change of the observed f due to the Doppler effect associated to the fact that emitting(adsorbing atoms or molecules are moving toward or far away with different (thermal) velocities ("Doppler broadening"). The natural broadening is practically negligible as compared to that caused by collisions and the Doppler effect. In the upper atmosphere, due to its temperature and pressure, we find a combination of collision and Doppler broadenings, whereas in the lower atmosphere, below about 20 km, collision broadening prevails because of the pressure effect. As far we move far from the central (expected) frequency f as much the contribution of Doppler effect can be neglected compared with the pressure broadening. This fact has important consequences on the possibility to retrieve vertical properties of the atmosphere (vertical sounding) like temperature and concentration of its chemical constituents, exploiting satellite based observations made "off-line" (i.e. at frequencies around but different from f) which relate investigated atmospheric levels as much higher as much far from f are the considered frequencies.