The spread of frequencies of a spectral line as a consequence of the random motions of molecules. Because of the Doppler effect, the observed frequency of radiation emitted by a molecule depends on its motion relative to the observer. Even if a stationary molecule could emit radiation of only a single frequency, a gas of such molecules moving randomly in all directions with a distribution of speeds (see Maxwell–Boltzmann distribution) would be observed to emit radiation over a range of frequencies proportional to the mean molecular speed relative to the speed of light. This mean speed is proportional to the square root of absolute temperature and hence so is Doppler broadening. At normal temperatures and pressures Doppler broadening is dwarfed by collision broadening, but high in the atmosphere Doppler broadening may dominate and, indeed, provides a means of remotely inferring temperatures.