By far the most important of these is molecular relaxation. High frequencies are absorbed more than low. The figures show the variation of the absorption with temperature and relative humidity. Under 'normal' circumstances, atmospheric absorption can be neglected except where long distances or very high frequencies are involved. The speed that sound propagates in a gas depends on the temperature of the gas.
Higher temperatures produce higher speeds of sound.
Since the temperature of the atmosphere is not uniform there are local variations in the sound speed. For example, under normal conditions the atmosphere is cooler at higher altitudes. This will result in the formation of a shadow zone, which is a region in which sound does not penetrate. In reality some sound will enter this zone due to scattering. In environmental noise situations, scattering is caused by air turbulence, rough surfaces, and obstacles such as trees.