### ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

## 3aPAa12. When is downwind propagation really upwind propagation?

**John M. Noble
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*U.S. Army Res. Lab., Battlefield Environment Directorate, White Sands
Missile Range, NM 88002
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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.