My parents insist they remember contrails being significantly shorter around 1980s.
From my research it seems this might be the result of several factors such as climate change, advancements in aviation technology (engine design, fuel types) or changes in common aviation procedures. Of course, this may also be bad memory.
Can that be factually true?
There could be a few things:
They are not remembering contrails, they are remembering exhaust. Old jet engines, particularly the early turbo fans and low-bypass turbo jets, as well as early water injection systems were prone to be much smokier. Advancements in understanding of engines as well as high bypass turbo fans have mitigated this quite a bit. So what your parents may be remembering are actually exhaust trails.
Depending on how old your parents are, aircraft have cruised at various altitudes over the years as structural technology has allowed better pressurization, and jet technology has allowed higher cruise altitudes to take advantage of trade winds. Since contrails are based on ambient moisture content, a given aircraft's cruise altitude will affect their behavior and appearance due to various environmental factors.
Humidity matters; as the FAA notes there are both short lived and long lived contrails. So unless your parents have been observing standard aircraft routes over a fixed point, through all seasons, for years, varying conditions will change the formation and life of contrails as ambient humidity fluctuates.
After the initial formation of ice, a contrail evolves in one of two ways, again depending on the surrounding atmosphere’s humidity. If the humidity is low (below the conditions for ice condensation to occur), the contrail will be short-lived. Newly formed ice particles will quickly evaporate as exhaust gases are completely mixed into the surrounding atmosphere. The resulting line-shaped contrail will extend only a short distance behind the aircraft (See Figure 2). If the humidity is high (greater than that needed for ice condensation to occur), the contrail will be persistent. Newly formed ice particles will continue to grow in size by taking water from the surrounding atmosphere. The resulting line-shaped contrail extends for large distances behind an aircraft (See Figures 2 and 3). Persistent contrails can last for hours while growing to several kilometers in width and 200 to 400 meters in height. Contrails spread because of air turbulence created by the passage of aircraft, differences in wind speed along the flight track, and possibly through effects of solar heating.
I would read up on the linked brief from the FAA, which has a lot of interesting contrail info.
Case in point, it's pretty dry where I am today; here's a short contrail I just observed from my front porch:
(source: own photo)
Years and locations may be relevant here. I have distinct recollections of 1982 where El Nino weather patterns were severe world-wide.
In the eastern U.S., it was the warmest winter in 25 years. source
And air temperatures have a direct relationship to condensation and dew point.
I'm wondering whether modern high-bypass jet engines are a factor. They create a core of exhaust gases surrounded by a swirl of unmodified atmosphere. Two benefits are increased fuel efficiency, and reduced noise (this latter because the exhaust noise is trapped by the swirling air).
Possibly, this swirl also confines the water in the core exhaust for longer, wgich would make for a longer con-trail in an atmosphere where the water is evaporating, or a denser one where water is condensing on combustion-generated particles.
Comparing 1980 to 1990, satellite photos reveal a
significant decrease in contrail cloudiness over the [sic] western Europe and a significant increase over the [eastern] North Atlantic between March and July. ... Causes, possible errors and consequences are discussed.
Volgograd is distant from both of those, and the compared time periods match, so your parents' report isn't implausible. But unless they kept written records, their recollection is hard to reconcile with others' recollection, in Volgograd or elsewhere.
I live on East coast of the UK, right under a bunch of major airways that connect middle East, mainland Europe with North America. On a clear day we can usually see several aircraft in the sky at any time.
Roughly 10-15 years ago we had never seen a persistent contrail over here. I had seen photos of them from places like America. We either had NO trail, or just a few seconds worth.
One day the sky was covered with trails. It was the first time we ever saw them. Facebook (which was new) had tens of pictures from people all over town, posting what none of us ever saw before.
As the days passed by, nobody ever mentioned it again, the trails remained every other day from that point onwards, and it soon turned into "normal" and now I think it's "always been that way" in many peoples mind.
The default AI Google responses simply don't apply here. We always had tons of air traffic, and the weather (besides that effect what trails hiding the sun bring) has not changed. The high bypass engines which apparently cause worse trails, has been a thing for decades, so that doesn't explain it either.
Remember we went from none at all, never seen, to everywhere, every day. My best guess, is that somehow the humidity up there is higher than it used to be, so the conditions to form the trails are ripe virtually every day.
