All the refraction effects come from the change of air refractive index.
The refractive index depends on two things - density and composition.. And density depends on temperature and pressure.
If you look at the air above an open fire, or any heated object, the column will refract light. You can see it in distortion of objects behind, as well as in a shadow which it casts in external light.
The pressure is almost the same, and when the object is heated from its heat capacity, electricity etc., the composition also is the same. Different temperature means different density and refractive index.
But when you deal with air affected by wing - well, the temperature certainly changes. In moist air, fog forms in the parts where air is rarefied and cooled, like above wing and in engine inlets. Obviously the density of air decreases - and in clear dry air, the visual distortion should also be seen. But so long as the speeds are low and airflow subsonic, the distortion is continuous and hard to see.