I know they are not pulse jets causing it but why do some contrails have evenly spaced little “fingers” sticking out at a 90 degree angle? I saw this last evening. The fingers were very pronounced on what I assume was the downwind side of the contrail. They looked almost like saw teeth, very uniformly spaced and a little further apart (2x?) than they were wide. Not one or two but maybe 15 or more which, at that altitude and distance, covered many miles. I wasn’t taking detailed notes or counting. It just made me wonder what could be causing it.
I’m not sure I’ve ever seen exactly the phenomenon you’re describing. But here’s a semi-informed WAG:
Contrails are formed from engine exhaust. As such, they trail directly behind the engines. Aircraft also leave invisible wake vortices which trail from the wingtips. These vortices expand in diameter until each is about 2/3rds of the total wingspan of the airrcraft. They also slowly descend below the flight path whereas contrails tend to stay at the altitude they were formed.
I can readily believe that a small instability in the expanding wake vortices could cause them to trail back in a corkscrew fashion. I mean the whole vortex body cones a little, not just that the air within the vortex is swirling; it’s always doing that too.
For an airplane with wing mounted engines, the overall result would be that the vortices impinge on the contrail at regular intervals corresponding to one rotation of the corkscrew coning. The visual effect would be a periodic tear or smear of the contrail transverse to the long axis.
A totally different possibliity is simply that the aircraft is operating in choppy air. “Chop” is really just a periodic more or less stationary vertical ripple in air pressure over distance. As the airplane punches through the ripples rapidly, it’s perceived much like surface chop on a lake is perceived by a boat.
As the smooth continuous contrail is laid across this non-smooth surface, almost any periodic disturbance in dispersion and hence appearance from the ground is plausible.
We fly though deteriorated contrails all the time. From up close they usually look hollow; we see a 200 foot diameter “smoke ring” with clear air outside and much less dense but still obvious “smoke” inside it. It’s a long tube of cloud filled with thinner cloud inside.
I’m pretty sure you’re mistaken there. The contrails are just water vapor. There’s a low pressure area created above a lifting wing. If that lower pressure drops the air below vapor pressure line, the water vapor condenses and creates a sort of “cloud” that trails behind the plane.
No idea on the teeth, but that’s what causes the trails.
Contrails are formed from jet exhaust. The hot exhaust produces a thin tight contrail leaving the aircraft. That can widen and and even seed further condesensation over time causing the contrail to appear much larger. No idea about teeth, but they probably form well after the exhaust from the jet engine.
Given that airplane fuel is a mix of various Hydrocarbons, when they are burned, or Oxidized, they form Carbon Dioxide and Water Vapor (Dihydrogen Monoxide, or DHMO). So, Yes, contrails are formed from engine exhaust.
I’ll admit that now that I think about it, my low pressure above the wing theory was just that - my theory.
Really? you wanna call it dihydrogen monoxide? Do you routinely call salt sodium chloride too? I guess, you’d have to change your name to NaCl. Thanks for the italics, though. Those were pretty.
You’re sorta right in that there absolutely *are *circumstances where you can see visible condensation due to either wingtip vortices or simply the low pressure airflow over the wing top. These are very real, but they aren’t persistent. In sufficiently humid air, the pressure interactions around the aircraft can produce visible cloud. But a few seconds later after the disturbance has passed and ambient pressure, temperature, & humidity reassert themselves, the visible cloud decoheres back into invisible water vapor. Just as it was before the airplane went by. This decoherence takes 5-10 seconds tops.
But that’s not the driving factor in contrail formation. The key thing in contrails is (as others have said) we’re injecting relatively concentrated water vapor into what’s otherwise a very cold (-30F or colder) and very dry environment. If we add enough water vapor concentrated in one area, the local humidity goes up enough that it condenses into visible cloud. Over time the cloud will disperse and the local humidity will decline until the cloud becomes invisible. As yo’ve doubless seen, sometimes contrails drag only a couple of miles behind a jet. In other words, they disperse within 20-30 seconds. Other times they’ll stretch from horizon to horizon. The key difference is how close the ambient humidity is to the dew point (AKA cloud formation temperature) for the particular altitude (AKA air pressure).
Where conditions are *almost *such that clouds could form, contrails will be long lived. Where conditions are well away from those needed for cloud formation, contrails will form and quickly disperse, or not form at all.
Finally, the other components of jet exhaust provide condensation nuclei for the ambient moisture to coalesce around. It’s effectively cloud seeding, but at the very microscopic scale which produces visible cloud from invisble vapor, not at the fairly macroscopoic scale which produces raindrops from visible cloud.
If you look carefully at the contrails, you can readily tell the difference between 4 engine jets, 2 engine jets with wing-mounted engines, and 2-engine jets with tail-mounted engines.
Late add: Here’s a decent example of wing-top vapor starting at about 30 seconds in. landing at edinburgh--including wing vapour! - YouTube Note the vapor decoheres just behind the wing. In other words, any given hunk of air turns to cloud and back in much less than one second. Although it can be created continuously, it’s not a long-lived phenomenon.
Perhaps, but I doubt it’s common.
Usually everything ends up entrained into a pair of fairly wide counterrotating vorticies. The famous photo of the contrail-creation process is:
http://anandgreenwell.com/?p=74
Seeing the above image it’s difficult to imagine how separate engine trails could survive and remain visible after being entrained into the rotating “downwash wake” far behind the plane.
Ah, here’s another one showing four jet exhausts becoming two wing vortices. Too bad it isn’t longer.
750px-Qantas_and_contrails.jpg
Here’s an explanation of “contrail pendules” on an anti-chemtrails site:
http://contrailscience.com/contrail-gaps-and-other-questions/
I think perhaps we’re talking different timelines. Look at your link http://upload.wikimedia.org/wikipedia/commons/thumb/c/c6/Qantas_and_contrails.jpg/750px-Qantas_and_contrails.jpg , which is certainly the classic arrangement.
Someone who was watching that airplane lay down that contrail could instantly see it’s a 4-engine jet. Which is what I was talking about. They would also immediately apprehend that contrails are formed by engine exhaust, not from wingtip vortices or overall wing downwash effects as some folks claimed upthread.
Now someone looking at the same contrail 1 minute = 8 miles behind the jet would be hard-pressed to tell whether it was generated by a 4- or 2-engine jet. Which I understand to be your claim. And that too is correct.
If you look at enough of them often enough, you’ll learn that the spacing difference between wing-mounted and tail-mounted engines remains obvious for some time. Say 5 minutes or 40-ish miles behind the generating aircraft.
At some point the two trails merge into one.
And at some other point they dissipate completely. Which may be as soon as before a 4-trail merges into a 2-trail or as late as several hours later after the trail has simply diffused, along with another hundred like it, into a generalized thin layer of cirrostratus.
Vortices it is. Make sense.
Ha! Contrails can be caused by low pressure as a result of lift OR Jet exhuast. Have a look at the recent shuttle landing. There’s no engines running at all - still we have contrails:
And, now I see that** LSLguy** has already covered that.
So, hey! Look at the cool picture of the shuttle!
Don’t you just hate, when somebody “beats you to the punch”, like that? 
contrail pendules are explained in a little more detail here, and provides links for additional info:
Not only areoplanes 
http://www.microstockgroup.com/off-topic/is-it-real-or-is-it-photoshop/
Me, I like jet tutus the best.
I had no idea what the hell “conspiracy” meant in your header. Live and learn.
I don’t think those are contrails in that picture. I think those are lights on a time-lapse photo, and some dust from the runway.
I’m not a photographer, so I’m not sure how that picture was taken (could be multiple shots edited together).
The shuttle trails are not “contrails”. They are condensation trails dragging off the wing-top and wingtips.
And a bunch of tire smoke just after touchdown under the left (right to us) wing.