Changes in temperature or wind speed with height leads to a change in the direction of a sound wave. This phenomena can be observed when light goes trough a prism. Since different colors (i.e. different frequencies) travel with different speed they change direction in different way inside the prism. As a consequence you can observe the rainbow colors. 

The phenomena is also present in acoustics whenever waves travel from one medium to another (i.e. also for radiation from structures into air). The phenomena can be explained by Snell's law. This is demonstrated for the propagation of sound from one medium to a second medium with different sound speed and density.

Keywords:
Trace wavelength
Wind / temperature gradient

