John M. Noble
U.S. Army Res. Lab., Battlefield Environment Directorate, White Sands Missile Range, NM 88002
For a long time, atmospheric acoustic propagation has used phases like ``upwind propagation,'' ``downwind propagation,'' and ``crosswind propagation'' to describe the type of propagation conditions present in the atmosphere. For short-range propagation (<3 km), the portion of the atmosphere between 50 and 100 m will primarily influence the propagation of sound. This region of the atmosphere is called the surface layer. Typically, through this region there is not much change in the mean wind direction. Therefore the terms mentioned earlier have some meaning for short-range propagation. This is due to the effective speed of sound for a given direction not varying much due to the small changes in the mean wind direction with height. For longer range propagation, this terminology could have little to do with the actual propagation conditions due to the effect of wind speed and wind direction variations with height above the surface layer. The purpose of this presentation is to discuss why this terminology is not appropriate and for what conditions it is appropriate for use in atmospheric acoustics.