Interesting! As far as I know the diffraction is caused by the propagation speed of the radio waves being different over water and land. 2daddies writes that this is caused by differences in air densities. Can anyone give som more details on the relationship between density and propagation speed? Are there other variables that affects propagation speed? Let's say density was equal over land and sea; would different amounts of water vapour cause refraction? Would the frequency of the radio signals affect the results?