Very interesting. I suspect it's flying just above a region that's suitable for persistent contrail growth, the contrail sinks into it, sublimating to sub-visible ice crystal size, then as it sinks into the more humid air the crystals start to grow again as it switches from sublimation to deposition.
skephu
Senior Member.
The ice crystals themselves are sub-visible in visible contrails as well, as they are ~10 to 100 micrometers in size as far as I know. I'm not sure how small they must be so that the entire contrail becomes invisible.
Besides, the smaller the crystals the slower they fall, so if they are so small that the contrail is invisible, they must fall extremely slowly.
Considering these issues, it seems unlikely that they start to sublimate but they fall into a more humid region before completely sublimating.
Maybe only the soot remains? But soot should also fall extremely slowly.
Whitebeard
Senior Member.
looks like that video is time lapse anyway given the way the contrail drifts sideways. Maybe over the few seconds / minutes (its hard to tell without a time reference on the vid) it drifted on the wind out of one air mass not suitable for contrail persistence into one that was. just a thought
Given the timeframe, it's not going to be the ice crystals sinking due to gravity, but due to the sinking of the wake vortex.
skephu
Senior Member.
I see, so how fast does the wake vortex sink, and for how long?
http://www.skybrary.aero/bookshelf/books/184.pdf
The main characteristics of aircraft wake vortices are:
Sink rate: 300 to 500 feet/minute
Stabilization at 500 to 900 feet under the aircraft at the origin of the vortices
Lateral movement at 5 knots, when reaching the ground
Life span: − Approximately 30 seconds, with a wind speed between 5 and 10 knots −
Up to 85 seconds, when the wind speed is less than 5 knots −
Up to 100 seconds in still air. Figure 2 Aircraft Wake Vortices
skephu
Senior Member.
Thanks. Well, the wind speed can hardly be very small at cruise altitude, it is certainly not between 5 and 10 knots, more like 40 knots. So the life span of the vortex is certainly less than 30 seconds, maybe 5-10 seconds. It sinks maybe about 50 feet during that time. Still a very slim chance that it sinks into a humid layer.
Trailspotter
Senior Member.
The lifetime of wake vortices is certainly more than 30 seconds. A well-known manifestation of wake vortices is the Crow instability that lasts a few minutes. I would have given links and examples, but cannot do it right now. I'm on holidays travelling without my laptop.
Edit
I think that @Mick West external quote above may predate the appearance of super heavy planes with more powerful wake vortices.
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So if there's only a slim chance, then we would expect to see this type of thing very rarely?
Out of the thousands of contrails I've watch forming, I've only seen this once. So that fits the theory.
NoParty
Senior Member.
I too have only witnessed it once.
But then, 99.99% of the time, I'm glancing at the sky in real time...
and, of course, time-lapse makes real time look different.
I think that it's interesting...and yeah, probably pretty rare...but none of that makes
Roddie's pseudo-scientific (turbo encabultor?) "deduction" any more palatable/plausible
But then, 99.99% of the time, I'm glancing at the sky in real time...
and, of course, time-lapse makes real time look different.
I think that it's interesting...and yeah, probably pretty rare...but none of that makes
Roddie's pseudo-scientific (turbo encabultor?) "deduction"
any more palatable/plausible
I'm not convinced by my "sinking into a humid layer" explanation though. It might be the ambient air is only just ice supersaturated, and there's two sets of mixing going on, to do with how the exhaust interact with the wake vortex. In one set the water condenses rapidly, but also passes through a stage where it's not supersaturated. In another it ends up supersaturated, but takes longer to get there.
This also seems unlikely to me, just throwing it out there.
This also seems unlikely to me, just throwing it out there.
deirdre
Senior Member.
his written description doesnt match what the video is saying. but that aside, DOES it disappear?
that sun ray in the middle that kinda looks like cloud, is just some weird sun ray smear from the glass and maybe fisheye lens, i guess, since it never moves or changes. Perhaps as the contrail blows sideways its just the sunlight affecting the view of it?
of course i see no reason it cant be blowing INTO a higher humidity area, thereby growing so we can visually see it again. especially since the cloud at 2:22 appears out of nowhere too.
either way, i have no idea how aluminum or any other metal would 'disappear' then 'reappear'. esp since the whole thing reappears at the same time basically. the full length. a 'delay' would mean the building end would reappear first, but it doesnt.
that sun ray in the middle that kinda looks like cloud, is just some weird sun ray smear from the glass and maybe fisheye lens, i guess, since it never moves or changes. Perhaps as the contrail blows sideways its just the sunlight affecting the view of it?
of course i see no reason it cant be blowing INTO a higher humidity area, thereby growing so we can visually see it again. especially since the cloud at 2:22 appears out of nowhere too.
either way, i have no idea how aluminum or any other metal would 'disappear' then 'reappear'. esp since the whole thing reappears at the same time basically. the full length. a 'delay' would mean the building end would reappear first, but it doesnt.
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The air and the clouds and contrails in it move as one, so when a contrails is being blown sideways, it's not really going into a new parcel of air.
But then the entire air mass can still change if it's being raised or lowered.
Ross Marsden
Senior Member.
I think this is about what is visible and what is sub-visible.
The contrail may be forming in just marginal conditions for the exhaust/environment mixture reaching water saturation enough to be visible. It is visible for a time but faintly, and slowly seems to dissipate and cease to be visible.
It is in a region of ice-supersaturation, and the sub visible trail particles are slowly increasing in size as the surplus water vapor deposits on them. Soon the cloud is optically thick enough to be visible again, and it behaves as you would expect a persistent contrail to behave... drift, spread, remain optically thick enough to be visible (due to deposition) despite the spreading.
I have not seen this before either. Interesting.
The Appleman-Schmidt Criterion, if I recall correctly, says not only reach water saturation, but reach water saturation enough to create a visible cloud with a certain liquid water concentration. Actually that may not be true, but in attempting to verify it I found this interesting book:
https://books.google.co.nz/books?id=q45NjC4ljAMC&pg=PA244&lpg=PA244&dq=schmidt+appleman+criterion&source=bl&ots=35VJbWSA6Q&sig=px62Z03elvbYAzYvjkuf0MDM0XY&hl=en&sa=X&ei=RnrYVLf5Es-j8AXO14JY&ved=0CGAQ6AEwCQ#v=onepage&q=schmidt appleman criterion&f=false
Atmospheric Physics: Background – Methods – Trends
edited by Ulrich Schumann
The contrail may be forming in just marginal conditions for the exhaust/environment mixture reaching water saturation enough to be visible. It is visible for a time but faintly, and slowly seems to dissipate and cease to be visible.
It is in a region of ice-supersaturation, and the sub visible trail particles are slowly increasing in size as the surplus water vapor deposits on them. Soon the cloud is optically thick enough to be visible again, and it behaves as you would expect a persistent contrail to behave... drift, spread, remain optically thick enough to be visible (due to deposition) despite the spreading.
I have not seen this before either. Interesting.
The Appleman-Schmidt Criterion, if I recall correctly, says not only reach water saturation, but reach water saturation enough to create a visible cloud with a certain liquid water concentration. Actually that may not be true, but in attempting to verify it I found this interesting book:
https://books.google.co.nz/books?id=q45NjC4ljAMC&pg=PA244&lpg=PA244&dq=schmidt+appleman+criterion&source=bl&ots=35VJbWSA6Q&sig=px62Z03elvbYAzYvjkuf0MDM0XY&hl=en&sa=X&ei=RnrYVLf5Es-j8AXO14JY&ved=0CGAQ6AEwCQ#v=onepage&q=schmidt appleman criterion&f=false
Atmospheric Physics: Background – Methods – Trends
edited by Ulrich Schumann
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scombrid
Senior Member.
I think it is drifting across a "standing wave" in the flow of air across which the plane flew. Look at the way the cirrus clouds behave later in the video particularly around 2:30 when a cloud just pops into existance. You have a relatively moist flow at flight level and cirrus appear or disappear as the flow moves up and down the peaks and troughs in the flow.
So the airplane exhaust stream is there but in visible until is lifts up a wave in the flow where the pressure changes and it poofs into visibility. That's my hypothesis anyway. Water would certainly behave like that more than some solid sprayed as a sun shield.
So the airplane exhaust stream is there but in visible until is lifts up a wave in the flow where the pressure changes and it poofs into visibility. That's my hypothesis anyway. Water would certainly behave like that more than some solid sprayed as a sun shield.
NoParty
Senior Member.
No doubt it doesn't really offer evidence of "chemtrails."
But I think that Roddie would claim that--since chemtrailers know, and others are catching on to them--
.0001% of the newest "chemtrails" will now be these temporarily invisible ones, so that it's harder for genius spotters
on the ground to connect the "chemtrail" to the criminal plane that "sprayed" it.
Trailspotter
Senior Member.
Actually, this is not a rare event, I've seen it many times before. It is just the OP video creates a false perspective of it. It wasn't a trail that disappeared and then reappeared again. There were TWO trails: one became dense very quickly but dispersed soon afterwards, the other grew slowly and eventually spread out.
The former was the hybrid contrail formed by the interaction of a part of exhaust contrail with the wake vortices. Its lifetime is limited by the Crow instability to a few minutes which is enough to create a long trail of fairly constant length behind the plane as seen in the OP video.
The latter was formed of the remaining part of exhaust contrail. Depending on the humidity, its initial density could have been very low to be noticeable in a wide angle shot of the OP video, but, as it grew up, it was picked up by the camera.
There are several earlier discussions of split and hybrid contrails here on Metabunk. I would give some links if I remembered how to do it on my iPhone, sorry.
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This does sound like a plausible explanation. The contrail dissipates on the left of the screen (when it's essentially coming down the other side of the wave).
Trailspotter
Senior Member.
I finally got the opportunity to examine the OP video thoroughly after downloading it on my computer. First of all, the contrail in the beginning is not a short one that evaporates quickly. It is a time lapse movie made with very wide 'fish eye' lenses, suggesting that the trail actually was much longer and lasted quite a while. In fact, its length and duration can be deduced from the video and related information.
The camera faces South; at the solar noon the Sun is right above the building in the middle. The Sun elevation at this point on the date (28-Jan-2015) was 34°. There are more time lapse videos with the same set up on the author's channel. Some of them captured 22° halo, for example, the one taken on 11-Feb-2015:
The halo provides the 'scale bar' for angular dimensions. In particular, the angular width of the tall building in the middle is also about 22°. It takes approximately 1.5 hours for the Sun moving over the building from the left side to the right. In the OP video, the Sun does it in about 17 seconds. It means that the time has been speeded up about 300x, or about 5 minutes of real time per one second of the video. As the movie rate is 24 fps, it corresponds to 12.5 seconds per frame.
It takes 3 to 4 frames for the contrail moving along by its full length, meaning the trail persisted for 40-50 seconds after it was formed. The screenshot below shows the contrail 4 frames = 50 seconds after it first appeared from behind the building:
I've used the Sun elevation at the solar noon (34° on 1-28-15, 38° on 11-2-15) as a reference for the determination of the angular positions of the contrail ends. The bottom end is at about 24° above the horizon, the top one is at 39° above the horizon and about 22° further to the right. In the assumption of a level flight at the altitude of 10 km, the calculated contrail length is about 12 km. Thus the estimated plane speed is about (12 km/50 sec)x3600 = 860 km/h, which is in the range of the cruise speed values of commercial jets.
The contrail lifetime is significantly longer than the lifetimes of short contrails (<10 sec), as seen in this real-time video:
but it is comparable with the lifetime of "hybrid contrails", limited by the Crow instability:
This analysis supports my suggestion in the previous post (#25), that the trail in the beginning of the OP movie was (a pair of) the hybrid contrail(s). As the above link show, at certain atmospheric conditions the hybrid contrails co-occur with ordinary exhaust contrails that may have different initial densities. The "reappearing" trail was most likely such an exhaust trail, the initial density of which was to weak to be picked up by the wide angle camera lenses. However, enhancing a screenshot from the video in full HD on a big screen suggests its presence where the initial, hybrid contrail has already dissipated:
The camera faces South; at the solar noon the Sun is right above the building in the middle. The Sun elevation at this point on the date (28-Jan-2015) was 34°. There are more time lapse videos with the same set up on the author's channel. Some of them captured 22° halo, for example, the one taken on 11-Feb-2015:
The halo provides the 'scale bar' for angular dimensions. In particular, the angular width of the tall building in the middle is also about 22°. It takes approximately 1.5 hours for the Sun moving over the building from the left side to the right. In the OP video, the Sun does it in about 17 seconds. It means that the time has been speeded up about 300x, or about 5 minutes of real time per one second of the video. As the movie rate is 24 fps, it corresponds to 12.5 seconds per frame.
It takes 3 to 4 frames for the contrail moving along by its full length, meaning the trail persisted for 40-50 seconds after it was formed. The screenshot below shows the contrail 4 frames = 50 seconds after it first appeared from behind the building:
I've used the Sun elevation at the solar noon (34° on 1-28-15, 38° on 11-2-15) as a reference for the determination of the angular positions of the contrail ends. The bottom end is at about 24° above the horizon, the top one is at 39° above the horizon and about 22° further to the right. In the assumption of a level flight at the altitude of 10 km, the calculated contrail length is about 12 km. Thus the estimated plane speed is about (12 km/50 sec)x3600 = 860 km/h, which is in the range of the cruise speed values of commercial jets.
The contrail lifetime is significantly longer than the lifetimes of short contrails (<10 sec), as seen in this real-time video:
but it is comparable with the lifetime of "hybrid contrails", limited by the Crow instability:
This analysis supports my suggestion in the previous post (#25), that the trail in the beginning of the OP movie was (a pair of) the hybrid contrail(s). As the above link show, at certain atmospheric conditions the hybrid contrails co-occur with ordinary exhaust contrails that may have different initial densities. The "reappearing" trail was most likely such an exhaust trail, the initial density of which was to weak to be picked up by the wide angle camera lenses. However, enhancing a screenshot from the video in full HD on a big screen suggests its presence where the initial, hybrid contrail has already dissipated:
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skephu
Senior Member.
Thank you for the great analysis! This is indeed plausible.
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