speed of light and microwave in air
The index of refraction of materials does vary with frequency. The effect looks something like this:



However, for air the variations represented by this curve is tiny, generally much less than 1% variation around an index of 1.0.

The index of refraction of air for optical frequencies is around 1.0002, so the speed of light in air is about \$2.9985\times10^8\$ m/s, as compared to \$2.9979\times10^8\$ m/s in vacuum. 

I haven't found a specific value for the index at microwave frequencies, but I did find the dielectric constant is 1.00 to two decimal places, which means the index can't be higher than 1.0025 (because \$n=\sqrt{\epsilon_R \mu_R}\$, but \$\mu_R\$ only varies in magnetic materials). This Physical Review article claims to have measured the dielectric constant of air at "radio frequencies" to be 1.0005893, giving an index of refraction of 1.00029, but the abstract does not say what frequency was measured.

I'm not sure where you got your figure for the speed of light in air for microwave frequencies, but for the propagation velocity to be 150,000 km/s, the index would have to be about 2, which is nowhere near correct.