At the greatest distance of 10km in the CNHS the standard deviation may approach 10dB. This is due to the range of values that can occur at larger distances. The ISO (1996) standard assumes spherical propagation with 6dB reduction per doubling of distance. At distances beyond 10 times the source height weather conditions have a stronger influence (ISO, 2007) and an acoustic shadow can occur. Conversely, beyond approximately 1km under favorable propagation conditions, up to a frequency of 70Hz (MG Acoustics, 2014), WTN can propagate cylindrically at 3dB per doubling of distance (Willshire and Zorumski, 1987; Hubbard and Shepherd, 1991; MG Acoustics, 2014).