How come when you open a hot oven, you can see ‘distortions’ of the air – like wavy air?
I wouldn’t have imagined that heat would change the appearance of air.
What causes this?
They’re heat waves.
Hot air is less dense than cold air. These density variations also result in changes the refractive index and this causes light passing through different regions of density to bend, or refract. It’s this refraction which causes distortions of the image behind the air volume containing these density variations.
Hot air and colder air have different densities - when light crosses the boundary between two transparent media of different densities, it is refracted. Because the hot and cooler air are mixing fairly turbulently as you open the oven, the light is bent in all different ways, making it look wavy.
The effect of looking down through the surface of a rippling pool is caused by the same phenomenon - except in that case, one medium (water) is very much more dense than the other, so the effect is more pronounced.
Made for a cool effect in LoTR:FotR when the Balrog roared.
Another random thought: has the OP never been to a desert?
In other words, it is literally, “wavy air”. 
Air is not transparent. It is just very, very translucent to optical wavelengths at standard temperature and pressure.
Stranger
I’ve got it now: those ‘lines’ of air in the desert are America’s famous “Things.”
Or taken a look at the street on a hot day, especially if you’re driving uphill to the top.
That’s the phenomenon known as the fata morgana, I believe. Used to be, in the desert, you’d hear tell of folk seeing cities in the air. Those were generally understood to be real cities far away being reflected by the heat waves coming off the desert.
It’s because heat changes the appearance of air.
Really.
Mirage,
Just simply mirage. That is what a vent rib is mainly for on a shotgun barrel.