The refractive index is affected by temperature and humidity. Temperature affects the air density, which affects the dielectric constant of the air. Humidity likewise affects the characteristics of the air, though I'm not sure if it would be the dielectric constant, or conductivity, or what.
In any case, what you need for ducting is a discontinuity in the refractive index of the air. A temperature inversion in the lower atmosphere can create this discontinuity, and the radio
signals "bounce" or refract off the discontinuity, or between two discontinuities, and thus are ducted beyond line-of-sight. In effect, the ducting creates a long waveguide. The air/ocean interface is itself a discontinuity, as we also see in HF multi-hop skip propagation.
A mirage is a similar effect that demonstrates how a discontinuity in air temperature can bend light waves (much higher in frequency that marine VHF
, but the principal is the same). The ripples we see in a distant image on a hot day are caused by changes in the air's refractive index, as are the twinkling of the stars. As a strange example of discontinuities in dielectric constant creating a waveguide, a wooden 2x4 can act as a lossy microwave waveguide.
I have somewhat interchangably used the terms "refractive index" and "dielectric constant" -- I believe that these are essentially the same thing, where "refractive index" is used in optics, and "dielectric constant" is used in electronics
. "Refractive index" is probably the term more recognized by the layman.
By the way, in your initial posting
you write "line of site" -- this should of course be "sight"