The geometry of sound rays in a wind
  We survey the close relationship between sound and light rays and geometry. In the case where the medium is at rest, the geometry is the classical geometry of Riemann. In the case where the medium is moving, the more general geometry known as Finsler geometry is needed. We develop these geometries ab initio, with examples, and in particular show how sound rays in a stratified atmosphere with a wind can be mapped to a problem of circles and straight lines. 
G. W. Gibbons and C. M. Warnick
gr-qc math-ph math.MP physics.ao-ph physics.ed-ph physics.flu-dyn