I’m not sure what you’d call this phenomenon, but on some really hot days, you can open (say) a car and see the hot air swirling around inside. It almost looks like turbulence, but with plain, uncolored air.
I wish I had a pic, but I can’t find one because I don’t know what it’s called and I don’t live in a hot enough environment.
If you know what I’m talking about… what causes it? How come the air is visible during those moments but not normally? Is it even “air” that I see?
The hot air is very slightly less dense than the surrounding air. This gives the hot air a different index of refraction (essentially, the speed of light traveling through it is slightly different than the surrounding air). Because the light is being bent when it travels through the heated region, your eye can see the hot air. This is also how mirages form, and why stars twinkle.
Also, note that you are in fact seeing turbulence — on a hot day, the sun heats the ground and the ground heats the air nearest to it. This causes a situation in which you have hot, less dense air supporting cool, denser air; the hot air therefore “wants” to rise, and when it does it causes turbulence. The technical name for this process is a Rayleigh-Taylor instability — in particular, check out the computer simulations depicted about halfway down the page.
Fascinating, thanks!
Hmm. Does this work the other way around? How come you can’t see the airflow (for example) when you open a freezer during the middle of summer, or when you spray an aerosol can?
Actually, you can. However, you generally need a very intense, directional light source to see this effect, which is why you can see it outside. Most people don’t keep their freezers outside. 
Try it with a can of “compressed air” - take it outside on a sunny day and watch the shadow as you squeeze the trigger. (Note that this isn’t exactly the same thing, because these products don’t actually contain air - they use a fluorocarbon gas.)
You can see the same effect in water. Put 1-2 inches of water in a saucepan (better with a shiny interior, not colored or nonstick). Put it on the stove on high heat & wait a couple minutes.
Long before you get any bubbling you’ll easily see light & dark areas swirling around in there. Same exact process as MikeS described.
I used to see dust devils when I lived in West Texas. They’re like miniature tornadoes.
NSFW (soundtrack “That is so f—ing cool!”) Broken link, fix http.
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tp://www.youtube.com/swf/l.swf?swf=http%3A//s.ytimg.com/yt/swf/cps-vfl64504.swf&video_id=w4b_yTJkPI8&rel=1&showsearch=1&eurl=&iurl=http%3A//i4.ytimg.com/vi/w4b_yTJkPI8/hqdefault.jpg&sk=0daTQ2fbVdmFixA4uOV6o9iYSWoBw44TC&use_get_video_info=1&load_modules=1
Was this meant for a different thread?
Went back and re-read the OP.
My bad. :smack: I thought the OP was about strange swirling air outside, not inside, the car.