Refractive index vs Temperature

[jaumzaum]

I was studying radio waves phenomenons and found this on Wikipedia:

"Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anticyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent."

Shouldn't it be lower refractive index? In my mind higher temperatures have less particles and though lower refraction.

 

[kuruman]

I think it is a matter of total internal reflection. The sine of the critical angle is $\sin\theta_c = \frac{1}{n}$. A signal coming from below at an angle will bend away from the normal at the (cold) - (not as cold) air interface and then away some more at the (not as cold) - (somewhat warm) interface and so on until eventually it reaches the critical angle for total internal reflection and comes back down. It is the same phenomenon as the highway mirage with which you are probably familiar.

 

[tech99]

My understanding is that the refractive index of air increases with pressure and falls with temperature. Overall, the index falls slightly with height, giving slightly downward ray bending. A temperature inversion will give a layer with reduced refractive index, causing energy to be trapped beneath it and giving increased downward ray bending. The radio refractive index is also sensitive to humidity, and so we often have an evaporation duct over the sea.