Can I see vehicle wakes?

June 20, 2010 5:11 PM

I think I can sometimes see the wakes behind cars. Am I deluding myself, or can one occasionally see the turbulence behind a vehicle?

Under certain circumstances - usually an overcast, humid but not raining day - I think I see the wakes of moving cars crossing past me. If I focus more distantly than on the cars themselves, the wake-like blurs are clearer. It's not spray; the road is dry.

I realise that this is deep within special confirmation bias territory, but really, would the pressure disturbances of road traffic moving at normal speeds be able to cause something one could see?

I've been able to ‘see’ this for a long time; more than 20 years. I have 20/20 vision and no history of vision distortion.

posted by scruss to science & nature (13 answers total)

Could be heat waves from the exhaust, I'd think.
posted by xingcat at 5:14 PM on June 20, 2010

What you're probably seeing and what I sometimes see are waves from the heat of the exhaust pipe. Someone can probably give a better explanation than me.
posted by MaryDellamorte at 5:15 PM on June 20, 2010

I meant to add - this is from the roof. Nowhere near the exhaust.
posted by scruss at 5:18 PM on June 20, 2010

You can see heat waves off of the road, so you'd be able to see them off of the roof of a car too.
posted by MaryDellamorte at 5:21 PM on June 20, 2010

Heat from various surfaces on the car.
posted by Inspector.Gadget at 5:51 PM on June 20, 2010

I not-too-rarely perceive a little trailing distortion, no longer than the car/van producing it, a fleeting low contrast whiteness (but "white" is too strong a word), especially when I'm staring off into the distance, with the cars moving horizontally across my field of vision, in focus but not the focus of my gaze. I take it to be a vision artifact, not a part of objective reality.
posted by NortonDC at 5:57 PM on June 20, 2010

You can absolutely see heat distortion from the roof of a car. The hood, too.

It is vaguely possible that you see that, and then through it discern also the wake. You may be noticing an effect too subtle for most people.

I really don't think that you can see the pressure differentials directly. They just aren't visible phenomena in this speed range, without having some sort of smoke, fog, or other aerosol in the air.
posted by Netzapper at 5:59 PM on June 20, 2010

You can occasionally see condensation forming from the shock off the wings of open-wheel race cars. I assume it's some variant of a prandtl singularity like you see in the "sonic boom" videos from air shows (that almost universally are not sonic booms).

But you can only see it on very humid days, and you can only see it occasionally on a track during those humid days (usually IIRC on 200+ mph stretches), and the wings on F1 and CART/IRL cars torture the airflow in ways that road cars just don't.

I would put my utterly-uneducated-in-physics-and-aero money on it being regular workaday heat shimmer, but you're not able to pick it out except in the relatively uninterrupted, steady view over the cars instead of at them.
posted by ROU_Xenophobe at 6:06 PM on June 20, 2010

It seems unlikely to me that it would be heat shimmer, if the car is actually moving. The air coming off the top of the car wouldn't be much warmer than ambient, unless the car had just started moving.

My guess is it's a vision artifact ,like NortonDC says— certain combinations of lighting and movement can produce fleeting gray "afterimages" like that.

Possibly it could be a brief condensation, kind of like ROUX alludes to with the supersonic cones, except obviously the air coming off a car is very subsonic so the physics aren't really the same. Still, the sudden pressure drop as the airflow rounds the corner of the top of a squared-off car and possibly goes into a vortex, could briefly cause some visible fog if the air is saturated with moisture, as it would be on an overcast, humid day. Short-lived visible clouds do form in the low pressure zones around subsonic aircraft. But they usually seem to have relatively sharp edges and stay in a well-defined position w.r.t. the aircraft; if this were what you're seeing, I think you'd have noticed more structure than you've described here.
posted by hattifattener at 7:10 PM on June 20, 2010

Honestly, there is no way that you are able to see an aerodynamic wake off of a car. None at all. You may be seeing some other effect - a heat haze is possible - but certainly not the air movements from a wake.

You can sometimes see venturi after aircraft and very fast cars (F1 cars for instance in humid air from the outside edges of the rear wing and around the wing tips on a military jet) but you need the right humidity for this to be discernible with the naked eye.

However, even when these conditions are right for viewing this effect, all the other aerodynamic movements (which are massive around an F1 car or a jet aircraft) are still not visible.
posted by Brockles at 11:25 PM on June 20, 2010

> If I focus more distantly than on the cars themselves, the wake-like blurs are clearer.

I see these all the time, they're wave-patterns of 'edge detectors' being triggered on my retina. A variety of "visual trails" or palinopsia. For me they appear as a vertical stripe extending upwards from the upper rear corner of, say, a big UPS truck moving across a bright overcast background sky. It's in my brain, not out in the world. It works best if my eyes are almost still, staring into the distance, while the dark object is moving.

If you follow the car with your eyes, the "wake" disappears. For me, the wakes are darker gray against the lighter background. I find I can generate the same phenomenon by staring at the overcast sky, then stretching out my arm and moving my vertical finger rapidly from right to left across my visual field.
posted by billb at 11:45 PM on June 20, 2010 [1 favorite]

Seconding the "edge detectors." On a bright day, against a bright background, I can see a trailing edge of darkness behind cars. It freaked me out until I realized it was, as billb says, my retina adjusting not-so-swiftly to the swift movement of dark objects against light backgrounds. Now I look for it when it happens and it makes me think about how eyes work, rods and cones, why prey animals freeze while predators stare fixedly at them waiting for the slightest movement... Our eyes and brains are very cool. Enjoy the effect.
posted by woot at 3:06 AM on June 21, 2010

Possibly it could be a brief condensation, kind of like ROUX alludes to with the supersonic cones

Most of those videos are from air shows, where I assure you the planes are very firmly subsonic, which was why I noted that the "sonic boom" videos aren't from sonic booms.
posted by ROU_Xenophobe at 12:18 PM on June 21, 2010