Many among the chemtrail community do not distinguish between these aerodynamic condensation trails and the more common exhaust condensation trails.
Aerodynamic condensation trails typically occur well below normal cruise altitude (but somewhat above freezing level) which is why they are characterized as "low altitude". I suppose they mean 'low altitude, in comparison to exhaust condensation trails'.
Many among the chemtrail community do not distinguish between these aerodynamic condensation trails and the more common exhaust condensation trails.
Aerodynamic condensation trails typically occur well below normal cruise altitude (but somewhat above freezing level) which is why they are characterized as "low altitude". I suppose they mean 'low altitude, in comparison to exhaust condensation trails'.
Ross . . . Just how persistent can they be . . . and can they spread into haze or low level cloud banks? I use to watch the F16 go through their test flights and they on a humid day in Fort Worth could produce such trails below 5,000 feet but they didn't last very long at all . . . experienced the same thing with F15s in St Louis and at Langley AFB in Hampton, Va . . .
Ross . . . Just how persistent can they be . . . and can they spread into haze or low level cloud banks? I use to watch the F16 go through their test flights and they on a humid day in Fort Worth could produce such trails below 5,000 feet but they didn't last very long at all . . . experienced the same thing with F15s in St Louis and at Langley AFB in Hampton, Va . . .
They can be quite persistent. I think they evolve differently because they are not so intimately involved with the exhaust and the wake vortices. For instance, you don't see pendules in these trails.
They are not really "low level". They are usually above about the -10°C isotherm.
They can be quite persistent. I think they evolve differently because they are not so intimately involved with the exhaust and the wake vortices. For instance, you don't see pendules in these trails.
They are not really "low level". They are usually above about the -10°C isotherm.
Ross, It seems rather complex . . . let me list the different species and see if you agree. . .
1) The traditional misidentified "Chemtrail" . . . higher altitude persistent contrail mediated by the ice supersaturated layers at usually 30,000 feet plus and -40 degree or lower temperatures . . . exhaust and/or vortices, aerodynamic in origin . .
2) A mid level aerodynamic mediated species where air temperatures are -10 degrees C or lower potentially persistent . . .
3) Low level aerodynamic non-persistent trails in air which is humid but at almost any temp . . .
Ross, It seems rather complex . . . let me list the different species and see if you agree. . .
1) The traditional misidentified "Chemtrail" . . . higher altitude persistent contrail mediated by the ice supersaturated layers at usually 30,000 feet plus and -40 degree or lower temperatures . . . exhaust and/or vortices, aerodynamic in origin . .
2) A mid level aerodynamic mediated species where air temperatures are -10 degrees C or lower potentially persistent . . .
3) Low level aerodynamic non-persistent trails in air which is humid but at almost any temp . . .
Close.
1) The traditional misidentified "Chemtrail" . . . higher altitude persistent exhaust condensation trail at usually 30,000 feet plus and -40 degree or lower temperatures where the air is ice supersaturated... involved with downdraft and wing vortices, also exhaust heat and latent heat of condensation and freezing. Relatively large water mass involved. Initial condensation occurs during an isobaric mixing process.
2) A mid level aerodynamic mediated species where air temperatures are typically -10 degrees C or lower in and ice supersaturated region. Initial condensation occurs in a process of adiabatic cooling by reduction of pressure. Relatively small water mass involved so latent heat of condensation and freezing are not as important.
Close.
1) The traditional misidentified "Chemtrail" . . . higher altitude persistent exhaust condensation trail at usually 30,000 feet plus and -40 degree or lower temperatures where the air is ice supersaturated... involved with downdraft and wing vortices, also exhaust heat and latent heat of condensation and freezing. Relatively large water mass involved. Initial condensation occurs during an isobaric mixing process.
2) A mid level aerodynamic mediated species where air temperatures are typically -10 degrees C or lower in and ice supersaturated region. Initial condensation occurs in a process of adiabatic cooling by reduction of pressure. Relatively small water mass involved so latent heat of condensation and freezing are not as important.
Thanks . . . Ross . . . while the high level type (1 above) are a form of cirrus clouds . . . what type of clouds could result or propagate from the mid level aerodynamic trails (2 above) ???
Thanks . . . Ross . . . while the high level type (1 above) are a form of cirrus clouds . . . what type of clouds could result or propagate from the mid level aerodynamic trails (2 above) ???
Hmmmmmm . . . seems rather complex and more relevant to the tropics and subtropics . . . we still seem to be talking about cirrus clouds . . . seems -41C is the upper temperature limit for persistent aerodynamic contrails which is also the highest temperature at which exhaust mediated persistent contrails propagate and both seem to form at or near cruising altitudes. . . simply stated the higher and colder . . . the persistent contrails are primarily exhaust mediated and the lower but still high altitudes are where temperatures are higher (above -41C) the persistent contrails are mediated by aerodynamic processes. . . My question is: what is the lowest altitude at which persistent contrails of any species may form?
Well, it is rather complex, and rather simple.
Simple bit: It has nothing to do with altitude, as such.
Complex bit: It has a lot to do with temperature and Relative Humidity.
Persistent contrails of any type can occur at any altitude provided the required temperature and relative humidity conditions are met.
We've seen photos of exhaust condensation trails on take-off at ground level in, where was it? Alaska, Canada?
Well, it is rather complex, and rather simple.
Simple bit: It has nothing to do with altitude, as such.
Complex bit: It has a lot to do with temperature and Relative Humidity.
Persistent contrails of any type can occur at any altitude provided the required temperature and relative humidity conditions are met.
We've seen photos of exhaust condensation trails on take-off at ground level in, where was it? Alaska, Canada?
I understand what you are saying and I know there are significant exceptions to almost any weather/atmospheric related question; however, I was trying to establish with 90% plus probability for the vast majority of persons viewing persistent contrails what the lower limit of their altitude would be (based on likely temperature layering) . . . even in Alaska where I lived for several years persistent contrails were not that common and they were not seen at low altitudes because the RH at ground level at -35F was usually around 2% . . . much higher RH and ground fog would cover the visibility of any contrails . . .
Hmmmmmm . . . seems rather complex and more relevant to the tropics and subtropics . . . we still seem to be talking about cirrus clouds . . . seems -41C is the upper temperature limit for persistent aerodynamic contrails which is also the highest temperature at which exhaust mediated persistent contrails propagate and both seem to form at or near cruising altitudes. . . simply stated the higher and colder . . . the persistent contrails are primarily exhaust mediated and the lower but still high altitudes are where temperatures are higher (above -41C) the persistent contrails are mediated by aerodynamic processes. . . My question is: what is the lowest altitude at which persistent contrails of any species may form?
Holy Moley, you are over thinking it, man.
Guestimating, 90% of the time...
Persistent exhaust condensation trails: above about 28,000 feet (provided temperature and RH conditions are met), below tropopause 45,000 feet (varies a lot).
Persistent aerodynamic condensation trails: above 10,000 to 15,000 feet (provided temperature and RH conditions are met), below tropopause 45,000 feet (varies a lot).
Persistence depends of the moisture content of the air, usually quantified by the Relative Humidity with respect to water, which needs to be above about 63% at temperatures around -48°C.
I made this handy table... Altitude Press Temp RHw
25000 376.0 -34.5 71%
26000 359.9 -36.5 70%
27000 344.3 -38.5 69%
28000 329.3 -40.5 68%
29000 314.9 -42.5 66%
30000 300.9 -44.4 65%
31000 287.4 -46.4 64% 32000 274.5 -48.4 63%
33000 262.0 -50.4 62%
34000 250.0 -52.4 61%
35000 238.4 -54.3 60%
36000 227.3 -56.3 59%
37000 216.6 -56.5 59%
38000 206.5 -56.5 59%
39000 196.8 -56.5 59%
40000 187.5 -56.5 59%
This relates the altitude, pressure and temperature in the International Standard Atmosphere with the RH (with respect to water) at the ice-saturation point. (Also at this RH the Frost Temperature is the same as the ambient Temperature - in case you want to conceptualise it that way.)
At higher RH than these listed at those temperatures, the air is ice-supersaturated, and this is the condition for persistence of contrails - ice cannot evaporate with this much water vapor already in the air at this temperature.
(The table doesn't extend down into the aerodynamic contrail zone - it was constructed for the exhaust contrails case.)
These amounts of water vapor in the air are tiny. For the level bolded there, the water content at ice-saturation is 0.1155 grams per kilogram of dry air, which (at that pressure and temperature) occupies 2.4 cubic metres. That's a cube 1.33 metres on an edge (4 feet 4 inches). One teaspoon of water is 5 grams, so it's a couple of drops in a 4 foot 4 inch on-a-side cube.
It's no wonder cheap disposable automatic (radiosonde) instruments have difficulty measuring it accurately.
Holy Moley, you are over thinking it, man.
Guestimating, 90% of the time...
Persistent exhaust condensation trails: above about 28,000 feet (provided temperature and RH conditions are met), below tropopause 45,000 feet (varies a lot).
Persistent aerodynamic condensation trails: above 10,000 to 15,000 feet (provided temperature and RH conditions are met), below tropopause 45,000 feet (varies a lot).
Persistence depends of the moisture content of the air, usually quantified by the Relative Humidity with respect to water, which needs to be above about 63% at temperatures around -48°C.
I made this handy table... Altitude Press Temp RHw
25000 376.0 -34.5 71%
26000 359.9 -36.5 70%
27000 344.3 -38.5 69%
28000 329.3 -40.5 68%
29000 314.9 -42.5 66%
30000 300.9 -44.4 65%
31000 287.4 -46.4 64% 32000 274.5 -48.4 63%
33000 262.0 -50.4 62%
34000 250.0 -52.4 61%
35000 238.4 -54.3 60%
36000 227.3 -56.3 59%
37000 216.6 -56.5 59%
38000 206.5 -56.5 59%
39000 196.8 -56.5 59%
40000 187.5 -56.5 59%
This relates the altitude, pressure and temperature in the International Standard Atmosphere with the RH (with respect to water) at the ice-saturation point. (Also at this RH the Frost Temperature is the same as the ambient Temperature - in case you want to conceptualise it that way.)
At higher RH than these listed at those temperatures, the air is ice-supersaturated, and this is the condition for persistence of contrails - ice cannot evaporate with this much water vapor already in the air at this temperature.
(The table doesn't extend down into the aerodynamic contrail zone - it was constructed for the exhaust contrails case.)
These amounts of water vapor in the air are tiny. For the level bolded there, the water content at ice-saturation is 0.1155 grams per kilogram of dry air, which (at that pressure and temperature) occupies 2.4 cubic metres. That's a cube 1.33 metres on an edge (4 feet 4 inches). One teaspoon of water is 5 grams, so it's a couple of drops in a 4 foot 4 inch on-a-side cube.
It's no wonder cheap disposable automatic (radiosonde) instruments have difficulty measuring it accurately.
Wow! Ross, you are rather thorough . . . so if I read your response correctly one could expect persistent contrails from 10,000 to 15,000 feet (aerodynamic) under the correct conditions and persistent contrails between 28,000 and the Tropopause (exhaust mediated) . . . which means between 15,000 and 28,000 feet persistent contrails should be rather rare . . . would that be a fair reading? Therefore, since most cruising altitudes are in the 28,000 feet plus levels few persistent contrails would be aerodynamically mediated unless descending or ascending on takeoff or landing . . . would that be a proper analysis?
Wow! Ross, you are rather thorough . . . so if I read your response correctly one could expect persistent contrails from 10,000 to 15,000 feet (aerodynamic) under the correct conditions and persistent contrails between 28,000 and the Tropopause (exhaust mediated) . . . which means between 15,000 and 28,000 feet persistent contrails should be rather rare . . . would that be a fair reading? Therefore, since most cruising altitudes are in the 28,000 feet plus levels few persistent contrails would be aerodynamically mediated unless descending or ascending on takeoff or landing . . . would that be a proper analysis?
No, sorry, I wasn't clear about the 10,000 to 15,000. That is a range for the bottom boundary for the aerodynamic trails. You need to be several degrees (~10°) above the freezing level so that the dew point and frost point are far enough separated, otherwise you are not only humid at high RH but in cloud or close to a cloud deck. The height of the freezing level varies a lot.
Yes, I think that is where you see the aerodynamic contrails - on approach or climb out. Even so, above 15,000 feet and you are quite a distance away from the airport.
No, sorry, I wasn't clear about the 10,000 to 15,000. That is a range for the bottom boundary for the aerodynamic trails. You need to be several degrees (~10°) above the freezing level so that the dew point and frost point are far enough separated, otherwise you are not only humid at high RH but in cloud or close to a cloud deck. The height of the freezing level varies a lot.
Yes, I think that is where you see the aerodynamic contrails - on approach or climb out. Even so, above 15,000 feet and you are quite a distance away from the airport.
Ross. . . thanks for the clarification . . . so we have instead a two layer cake where one could start seeing persistent contrails in the lower one, say at around 15,000 feet (aerodynamically mediated) transitioning into the higher layer at around 28,000 feet (exhaust mediated persistent contrails) up until one reaches the tropopause . . . and again because cruising altitudes are normally above 30,000 feet aerodynamically mediated persistent contrails would be expected to be rather rare . . .
Actually you can probably get aerodynamic contrails all the way up to the trope.
Perhaps the important thing with those is the absence of a temperature criterion (apart from needing to be somewhat below freezing). The air needs to be well ice-supersaturated, within a few % of water saturated, so that the few-degree aerodynamic temperature drop over the wing pushes the air into water saturation, and you're laughing... "wingsprayer"!
Actually you can probably get aerodynamic contrails all the way up to the trope.
Perhaps the important thing with those is the absence of a temperature criterion (apart from needing to be somewhat below freezing). The air needs to be well ice-supersaturated, within a few % of water saturated, so that the few-degree aerodynamic temperature drop over the wing pushes the air into water saturation, and you're laughing... "wingsprayer"!
I think I get what you are saying; however, because the temperatures normally cool as one ascends to higher cruising altitudes would not the opportunity to form aerodynamic persistent contrails begin to reduce significantly?
I think I get what you are saying; however, because the temperatures normally cool as one ascends to higher cruising altitudes would not the opportunity to form aerodynamic persistent contrails begin to reduce significantly?
I would think the characteristics and identification of the trails left by the photographed aircraft is appropriate; however, I have no problem with a separate Thread about the cause of trails at differing altitudes, etc . . . . not sure what to title it or is it just additional information . . . appropriate to this Thread and others as well . . . ?
You are correct but there are more details to discuss. I am attempting to move posts from a different thread . . . however, I am encountering some technical difficulty . . . Please go to the top of this Thread to understand the discussion . . . posts were moved over from previous Thread . . .
I think I get what you are saying; however, because the temperatures normally cool as one ascends to higher cruising altitudes would not the opportunity to form aerodynamic persistent contrails begin to reduce significantly?
No, it's OK, I have your question.
I had a quick look through the paper, "Aerodynamic Contrails: Microphysics and Optical Properties" linked by Jay above, but I couldn't see a value for the typical pressure drop over the wing. I wanted to use that to calculate what that meant in terms of a temperature drop at a range of pressures (altitudes).
The paper does indicate that the temperature drop at 300 hPa was 15 degrees close to the wing root.
For the selected trajectory closest to the wing (y = 3.5 m), T decreases by delT = 15 K below the ambient value T = 220 K.
So, applying the equation of state for air, P = ρRT, pressure is proportional to temperature so the changes in pressure are also proportional to changes in temperature.
This means that regardless of the ambient pressure, a pressure change of a given amount will result in the same temperature change.
I don't see how you conclude that aerodynamic persistent contrails are less likely or more difficult to form at higher altitude (all other things, like RH being equal).
In that paper, there is this handy table that compares various measures of the two types of contrail. Si is the ice saturation ratio.
No, it's OK, I have your question.
I had a quick look through the paper, "Aerodynamic Contrails: Microphysics and Optical Properties" linked by Jay above, but I couldn't see a value for the typical pressure drop over the wing. I wanted to use that to calculate what that meant in terms of a temperature drop at a range of pressures (altitudes).
The paper does indicate that the temperature drop at 300 hPa was 15 degrees close to the wing root.
For the selected trajectory closest to the wing (y = 3.5 m), T decreases by delT = 15 K below the ambient value T = 220 K.
So, applying the equation of state for air, P = ρRT, pressure is proportional to temperature so the changes in pressure are also proportional to changes in temperature.
This means that regardless of the ambient pressure, a pressure change of a given amount will result in the same temperature change.
I don't see how you conclude that aerodynamic persistent contrails are less likely or more difficult to form at higher altitude (all other things, like RH being equal).
In that paper, there is this handy table that compares various measures of the two types of contrail. Si is the ice saturation ratio.
Ross, under what conditions would one see aerodynamic persistent contrails but no exhaust persistent contrails and vice versa ????? Or are they always a combination of the two??
You can trivially calculate the average pressure drop if you know the weight of the a/c and the plan area of the wing, since lift = weight in steady flight, and lift = pressure difference x area
You can trivially calculate the average pressure drop if you know the weight of the a/c and the plan area of the wing, since lift = weight in steady flight, and lift = pressure difference x area
Thanks . . . but I need the answer in simple terms that an average novice to aerodynamics and Boyle's Law in the open atmosphere like myself can understand . . . simple . . .
1) What is the range of and how low can predominately exhaust mediated persistent contrails form ? . . probably as low as 28,000 feet and above, until you reach the troposphere. . .
2) What is the range of and how low can predominately aerodynamically mediated persistent contrails form . . . from 15,000 feet to the troposphere.
3) Is there a range of altitudes where one species of persistent contrails is more likely to form than the other . . . between 15,000 feet and the troposphere?
George, I don't think what you want can be done properly. Any generalisation about this has to have some qualifications to prevent inappropriate application.
As that table above suggests, the formation mechanisms are different. The persistence condition is the same for both - ice-supersaturation.
The 3 types are easily identified by the appearance at or soon after formation time; there is usually a photo or video under discussion; there is plenty of reference sources to back up the existence and occurrence conditions of all three, so I don't see what the problem is.
The subject is a scientific one, and anyone who wants to engage in the discussion needs to suck it up to some extent.
I strongly think that it would be misleading to try and generalise/define these phenomena in therms of altitude alone. In fact when you do define them in their proper terms, the mention of altitude reduces to being done in qualitative terms.
George, I don't think what you want can be done properly. Any generalisation about this has to have some qualifications to prevent inappropriate application.
As that table above suggests, the formation mechanisms are different. The persistence condition is the same for both - ice-supersaturation.
The 3 types are easily identified by the appearance at or soon after formation time; there is usually a photo or video under discussion; there is plenty of reference sources to back up the existence and occurrence conditions of all three, so I don't see what the problem is.
The subject is a scientific one, and anyone who wants to engage in the discussion needs to suck it up to some extent.
I strongly think that it would be misleading to try and generalise/define these phenomena in therms of altitude alone. In fact when you do define them in their proper terms, the mention of altitude reduces to being done in qualitative terms.
Sorry to hear that . . . it makes it very difficult to have a discussion with a Chemtrail buff who insists they saw a persistent trail at low altitude (below normal cruising altitudes) and you cannot give a solid probability of how it may have happened or if it happened at all . . .
Sorry to hear that . . . it makes it very difficult to have a discussion with a Chemtrail buff who insists they saw a persistent trail at low altitude (below normal cruising altitudes) and you cannot give a solid probability of how it may have happened or if it happened at all . . .
It seems to me that the key issue to focus on in that case is not conditions for formation of aerodynamic vs. exhaust contrails (since both can form at low altitude, even ground level), but rather the ice-supersaturated conditions required for persistence of either one. But even then, it's not a question that can be answered with a simple rule.
Besides that, I often see chemtrails activists say that they've seen low-altitude persistent trails, but I haven't seen them substantiate the claim, or explain how they determined the altitude. Given that it's not something one can reliably measure "by eye."
It seems to me that the key issue to focus on in that case is not conditions for formation of aerodynamic vs. exhaust contrails (since both can form at low altitude, even ground level), but rather the ice-supersaturated conditions required for persistence of either one. But even then, it's not a question that can be answered with a simple rule.
Besides that, I often see chemtrails activists say that they've seen low-altitude persistent trails, but I haven't seen them substantiate the claim, or explain how they determined the altitude. Given that it's not something one can reliably measure "by eye."
I think one can say there is a much greater chance of persistent contrail formation at cruising altitudes because of the frequency of the flights in those regions as well as the higher likelihood of experiencing temperatures and RH need to propagate them . . . while conditions may exist that could propagate them at lower altitudes the number of jet engine aircraft flying in those environments are usually much less and the number of significant contrails on takeoff and landings are minimal . . . also the lower contrails form the fewer people are able to see them . . .
My original thought was that aerodynamic mediated persistent contrails are more likely at lower altitudes, especially below normal cruising altitudes . . . so I will dismiss that concept . . .
Ice saturation occurs over 17% of the surface area of the planet at any one time. Air travel isn't so plentiful as to significantly affect this figure, especially as precipitated ice is forever increasing its downward velocity as it accretes, and falls out of the affected strata. Although where trails have combined (at a lower altitude) it is obviously wetter - locally, and temporarily - as they are in effect clouds. Above them, the atmosphere has been made dryer by the departure of these trails.
Contrailscience can source you the relevant research findings. The carbon dioxide the plane makes is another far more important issue.
Ice saturation occurs over 17% of the surface area of the planet at any one time. Air travel isn't so plentiful as to significantly affect this figure, especially as precipitated ice is forever increasing its downward velocity as it accretes, and falls out of the affected strata. Although where trails have combined (at a lower altitude) it is obviously wetter - locally, and temporarily - as they are in effect clouds. Above them, the atmosphere has been made dryer by the departure of these trails.
Contrailscience can source you the relevant research findings. The carbon dioxide the plane makes is another far more important issue.
Jazzy, sorry I don't get your point? 17% ice saturation over the surface of the earth at anyone time means . . . if I fly in any random direction at any hypothetical altitude I can expect a persistent contrail to propagate 17% of the time . . if that is your point . . . then are you saying aircraft flying above 28,000 feet have no greater chance of producing persistent contrails than those flying at 5,000 feet or above the troposphere?
Jazzy, sorry I don't get your point? 17% ice saturation over the surface of the earth at anyone time means . . . if I fly in any random direction at any hypothetical altitude I can expect a persistent contrail to propagate 17% of the time . . if that is your point . . . then are you saying aircraft flying above 28,000 feet have no greater chance of producing persistent contrails than those flying at 5,000 feet or above the troposphere?
Jeez, George, I'm surprised at you! Have you forgotten everything you have read and learned about contrail formation conditions? Your reaction there is exactly what is to be exoected when you simplify an essentially complex concept.
You will never convince a chemtrail believer with a single sweeping generalization. ... Except perhaps, "The trails are contrails", which of cause is an invitation for further questions and explanation.
Jazzy, sorry I don't get your point? 17% ice saturation over the surface of the earth at anyone time means . . . if I fly in any random direction at any hypothetical altitude I can expect a persistent contrail to propagate 17% of the time . . if that is your point . . . then are you saying aircraft flying above 28,000 feet have no greater chance of producing persistent contrails than those flying at 5,000 feet or above the troposphere?
You will get ice supersaturation wherever the temperature is below the freezing temperature of water, and you won't get this condition wherever there are insufficient water vapor molecules per unit volume to achieve supersaturation. The former condition can occur at ground level at the poles, and the latter condition applies above 75,000 feet at the equator, and 25,000 feet at the poles - approximately.
Once these conditions are met (or not, and fanjet engines really help to pick this out) the consequences (persistent spreading contrails) are inevitable, and not a matter of chance at all.
Earth's atmosphere has a wide range of humidity. Clouds help us to see that which is visible, but the proportion of water vapor in the atmosphere is variable even in a cloudless blue sky. This variation is almost invisible, and the 17% figure reflects this proportion of a sky not visible to us.
You will get ice supersaturation wherever the temperature is below the freezing temperature of water, and you won't get this condition wherever there are insufficient water vapor molecules per unit volume to achieve supersaturation. The former condition can occur at ground level at the poles, and the latter condition applies above 75,000 feet at the equator, and 25,000 feet at the poles - approximately.
Once these conditions are met (and fanjet engines really help to pick this out) the consequences are inevitable, and not a matter of chance at all.
Then you have totally lost me . . . seems the higher altitudes have a greater chance of below freezing temperatures . . . so it is logical persistent contrails have a much better chance to form at 28,000 feet than at 5,000 feet . . . assuming RH is the same in both places . . .
Jeez, George, I'm surprised at you! Have you forgotten everything you have read and learned about contrail formation conditions? Your reaction there is exactly what is to be exoected when you simplify an essentially complex concept.
You will never convince a chemtrail believer with a single sweeping generalization. ... Except perhaps, "The trails are contrails", which of cause is an invitation for further questions and explanation.
According to my friends here Mick . . . if I understand them correctly, I should not mention the higher altitude issue . . . I don't think anyone was talking about atmospheric changes . . .
I don't think there are significant atmospheric changes for contrail formation or persistence.
Temperature is up by about 0.8°C, mostly in recent decades. I don't think this means much in terms of the height of the -40°C isotherm.
As far as I know, there is no change in the RH (not the same as water vapor content).
The question involves memory, storage efficiency, recollection accuracy, conscious interest, and other qualitative influences.
When you started this line of questioning, you didn't mention we were dealing with immeasurable organic factors like memory.
As Mick says, the reasons are about changes in the aviation industry. Memory for this type of thing is very unreliable, as we have seen. For instance, the first persistent contrail I recall noticing was in about 2002. I started in meteorology in 1977. You would think that I would recall seeing one before 2002, but I don't. No shame; not unprofessional; poor memory. Pfft. There is plenty of reliable actual documentation to rely on, though.
Not so logical when you know the earth is orbiting the sun with a spin axis angle range of 23.5 degrees, causing seasonal variations in atmospheric temperature (at any height), making a partial nonsense of those stated figures.
It is by chance one meets those conditions, but the consequences of these conditions require no element of chance. Best not use the word "chance" like that.
PS contrails will always form when their conditions are met, for instance, within pre-existing ice clouds (such as cirrus, jet streams, thunderheads)*. But the conditions can also be met in clean pure blue sky…
* This sometimes causes distrails. as the ice falls out.
