As I said a while back:

So we now agree!! Relative humidity is the defining factor for length of contrails.

Yes, the aeroplane emits a wee bit of water, but not all of the water for those contrails that run from one end of the sky to the other.

(About the flight levels, being several hours apart would have been the factor, they would have been fairly similar flight levels.)

Out of interest, I have had a contrail forming behind me in a Piper Aztec! Aeroplanes at different levels (up and down) told me. I tried to 'turn back' to see it, but as I was at a level that was humid enough for the contrail to form, the 'background' patches of cloud at the same level meant that my contrail was 'camouflaged' against them and I couldn't see it. The other aeroplane said that it was absolutely clear though and my wife has just this minute confirmed that as she was in one of those aeroplanes. (Just a wee bit of weather research that we did once where we learnt a thing or two about the atmosphere first hand...)

You are confusing two different phenomena. The classic contrails are formed when the water in the exhaust forms ice crystals and the subsequent longevity is determined by the atmosphere's relative humidity.

Contrail formation has been around since WWll...in fact predicting contrail formation level was a vital skill for wartime meteorologists.

https://cimg7.ibsrv.net/gimg/pprune....0974ba9cb.jpeg
Medium level trails formed by disturbance of the very high humidity air (in this case my aircraft filmed by a colleague in the sub-tropics)
https://cimg2.ibsrv.net/gimg/pprune....23ded333d.jpeg
The high level engine exhaust seeded ice trails, In this case over the western Pacific. By the way...contrails form as readily at night as seen under moonlight, its just that there are fewer of us aloft then.

British Geological survey


Water vapour, as a gas, not droplet, has the greatest contribution to the 'greenhouse effect' and it is present throughout the whole depth of the atmosphere.


​​​​As an illustration of the relative importance of clouds compared to gasses in the terrestrial energy balance :
https://cimg0.ibsrv.net/gimg/pprune....0f8e5d6c1b.gif
Note clear sky emissions to surface compared to {319}compared to cloud {26.6}

What then happens to the H2O the engine does emit?


https://cimg0.ibsrv.net/gimg/pprune....43ef4e9c8d.gif

I don’t know why I didn’t think of this before, but back in the 70’s I was taxiing behind a DC-8 at CYWG...temps around -40°. He was making contrails down the taxiway, and they were spreading in the almost calm air. He turned a clear morning into zero visibility in ice fog. All of that of course came from engine exhaust water.

The moisture in the air could have been super-cooled.

Initially my thoughts too. But, at - 40°c the amount of water in the atmosphere would probably not have led to a dense fog.
I witnessed a similar event when a pair of F4s took off in clear, still air at about - 3°c from a frosty airfield, the fog formed from the exhaust plume and rolled across the airfield from the runway. It stayed all day, the pair diverted.

Some papers on contrails, aerodynamically and engine caused.

https://www.faa.gov/about/office_org...crophysics.pdf

https://journals.ametsoc.org/view/jo...-14-0362.1.xml

Re posts # 47 to 49, possibilities?

https://glossary.ametsoc.org/wiki/Steam_fog

https://glossary.ametsoc.org/wiki/Ice_fog

In brief, aerodynamic 'fog' or visible vapor are caused when the local acceleration of the air around the aircraft drops the static air temperature - if the relative humidity is already high, that drop in SAT can cause moisture in the air to condense forming fog. Aerodynamic fog is visible immediately behind the aero surface that is creating it. Further, although there are exceptions, aerodynamic fog normally dissipates quite quickly - as the air settles down the SAT returns to it's original warmer value and the moisture can 'evaporate' back into the air.
Engine 'fog' - commonly known as contrails - are due to water added to the air from the engine exhaust as a product of combustion - as megan posted the combustion of a ton of jet fuel produces over a ton of water. Engine fog is generally not visible immediately behind the engine since the exhaust is hot - only becoming visible as the exhaust cools to the surrounding temperature. Since engine fog is generated by water being added to the surrounding air, in the right conditions engine fog can persist a long time - sometimes for hours.

Of course engine fog isn't just formed behind aircraft - cars and trucks can generate it (usually when it's really cold). Years ago I was driving across central Utah in the middle of winter when it was extremely cold - around -35 deg. C. At one point when I looked in the mirror, all I could see behind was a white cloud emanating from my car. I was briefly worried that something had just gone seriously wrong with the engine, but a quick check of the gauges showed everything normal - it was just the moisture in the engine exhaust turning to fog in the bitter cold. A few miles further on the ambient conditions changed enough that the fog pretty much disappeared.

A sad thought occurs to me. Since water produced by combustion is important for climate change, hydrogen fuel is not such an attractive alternative to hydrocarbons. The exhaust of hydrogen combustion is only water, which on the ground is a win, but in the upper atmosphere, apparently, not so much.

That depends on where the hydrogen comes from. If it is part of a closed cycle then it is already part of the balance, if it is introduced into the cycle (as carbon from combustion is) then it is a problem. On balance I think that it will be a problem since the evaporation part of the current cycle is principally a surface phenomenon.

A lot of that first image looks to be forming over the wing root upper surface... i.e., aerodynamic depression fog.

tdracer

A very clear account of aerod fog (depression fog) Vs Hydrocarbon burning engine fog

Aircraft cause snow.

https://simpleflying.com/chicago-snow-landing-planes/

When I was trainng on Harvards in Canada in 1956 I recall that once, when the first few started up in the morning the airfield was effectively "blacked" out for an hour or two. When the temperature had risen to about minus 25F things became workable. I guess that settles an argument about disturbing the airmass.

Down here in southern Germany contrails have been mainly conspicuous, for many months, by their absence.
Before the pandemic however, our neighbour - an ex Luftwaffe ground engineer - used to regale me with conspiracy theories about bauxite being thrown out to creat rain...
After several failed learned discussions with him I tried asking why then, after weeks of blue skies and mucho “bauxite”, it hadn’t rained - and also why, following days of floods and downpours, “bauxite” was being chucked out on the first clear day. He has since been a bit quiet on the subject.

Pontoflex,
That happened at Benson (also one morning) with dew point at ambient temperature but still clear sky. An Argosy driver started his first engine and a small patch of fog formed. By the time he got all four going the field was socked in and he could shut down again... All it needs is a trigger.

billovitch

It is not 'bauxite' that us used in attempts to create rain. In the very early days they tried soot. In the days that I was involved they were using silver iodide or dry ice (the silver iodide was from burning flares impregnated with it, the dry ice was in centimetre cubed blocks), I have heard that later trials found much more success using salts. It is all about trying to find the most effective hygroscopic nuclei, all of those mentioned were just that, but it appears that the salt was most effective. Although, none of them were really that effective, but it was all valuable research. (And quite good fun!!)