Gases come in many colours, air is colourless until you look at the sky and it clearly isn’t. Anyway you can see “waves” from hot air rising. The hot air is less dense and refracts light differently from the air it rises through. Essentially the air is always a lens your brain is ignoring and the rising hot air makes imperfections big enough that you notice
It’s the air being heated. Gas might be transparent, but it is still a solid medium that light passes through. When the air is hot enough, it begins to rise rapidly from the surface. This significant disturbance causes the light passing though it to become distorted. Think how when water is clear and undisturbed it’s easy to see though it, but when it’s disturbed the ripples bend light. Same principle.
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