How do you make air solid? Without, you know, making it solid through extreme temperature or pressure manipulation?

Now, the reason I mentioned visibility earlier, is because the travel of light through air is imperfect due to diffraction. Now I think (but am unsure, hence why I complained about my lack of memories regarding my physics courses) that higher density in air, even without changing its composition, would increase the hindrance, causing a situation in which the two air densities would cause refraction of the light traveling from one to the other, causing a visible distortion (or even an actual visual effect from the increased diffraction of the Hardened Air).