Air-flow birefringence in normal conditions of pressure and temperature is experimentally proved. Measurements of the change of the polarization of a plane wave propagating through a boundary layer are displayed for both laminar and turbulent flow.
G. R. Boyer, B. F. Lamouroux, and B. S. Prade, "Air-flow-birefringence measurement," J. Opt. Soc. Am. 65, 1319-1320 (1975)