A vapor cloud explosion occurred at an oil depot near Buncefield in the UK in 2005. Three infrasound arrays in the Netherlands detected various stratospheric phases. Some of these phases appeared with celerities, i.e. horizontal propagation velocity, in the conventional range of 0.28 to 0.31 km/s. Exceptionally fast arrivals, infrasonic forerunners, were identified with celerities of 0.31 to 0.36 km/s. These phases could be explained by head-wave-like propagation in a high velocity acoustic channel between 40 and 50 km height, where stratospheric zonal winds reached values of 120 m/s. The manifestation of infrasonic forerunners is validated by modeling with raytracing through actual atmospheric models and determining the celerity, apparent velocity and back azimuth. One phase occurred with a celerity of 0.25 km/s. Hence, we propose a new celerity range for fast stratospheric phases of 0.31 to 0.36 km/s and to lower the limit of the conventional range to 0.25 km/s.

Raytracing results for energy traveling from the source to DIA at 435 km distance. The sound speed (Vt), effective sound speed (Veff) and surface sound speed (Vs) are given in the right frame. Refractions occurr from regions where Veff becomes larger than Vs, i.e. the stratosphere and thermosphere. The strong velocity gradient in the stratosphere is caused by zonal winds reaching strenghts of 120 to 150 m/s. These winds are directed eastwards during the Northern hemisphere winter. A high velocity acoustic channel is formed for energy traveling from Buncefield to the Netherlands. Exceptionally fast phases (e.g. Is3f) are facilitated by this duct.

Reference:
Evers, L.G. and H.W. Haak, Infrasonic forerunners: Exceptionally fast acoustic phases Geophys. Res. Lett., 2007, 34, L10806, doi:10.1029/2007GL029353.