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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?

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    $\begingroup$ Thank you for the links. Are there any specific historical changes that are generally understood to have affected contrails? $\endgroup$
    – OLEGSHA
    Commented Mar 29, 2023 at 9:40
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    $\begingroup$ Well, according to those answers and the referenced paper a turbofan, i.e. a more modern engine, generates more contrail than a turbojet, i.e. an older engine. $\endgroup$
    – sophit
    Commented Mar 29, 2023 at 12:04
  • $\begingroup$ WWII Allied bomber crews, with their trails screaming "Here we are!" to Axis defenses, might beg to differ... $\endgroup$
    – GB540
    Commented Mar 29, 2023 at 21:41
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    $\begingroup$ I remember contrails in the 1970s arcing across much of the sky. $\endgroup$
    – RonJohn
    Commented Mar 30, 2023 at 15:39

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There could be a few things:

  1. 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.

  2. 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.

  3. 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:

short contrail observed from porch (source: own photo)

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  • $\begingroup$ Unrelated to the topic, but in #2 you mention trade winds, is there a reason you say this and not jet streams? I was under the impression that trade winds were relatively close to the surface of the earth as they were used by sailors for trade routes. $\endgroup$
    – Tanenthor
    Commented Mar 30, 2023 at 0:34
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    $\begingroup$ @Tanenthor I sail a lot as well as flying, must have been top of mind. $\endgroup$
    – Dave
    Commented Mar 30, 2023 at 1:10
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    $\begingroup$ I strongly think it's the humidity thing. In my experience, contrails looked basically the same in the 80's as they do now, and they've always been quite variable, sometimes short, sometimes long, and sometimes absent entirely. $\endgroup$ Commented Mar 31, 2023 at 3:55
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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.

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    $\begingroup$ My parents are from Volgograd. As far as I can see basically no soviet climate data exists online, so I'm looking at NASA datasets to find out if there were any significant climate changes in that area $\endgroup$
    – OLEGSHA
    Commented Mar 30, 2023 at 8:50
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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.

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  • $\begingroup$ That was my thinking. The bypass ratio of the CFM56 family used in B737s of the 1980s was 6.0 or less, while it is 9 or more for modern engines. This effectively reduces the water vapor contents by a factor of almost 2 and probably leads to lower exhaust temperatures. It would be surprising if the condensation patterns were the same, actually. $\endgroup$ Commented Mar 31, 2023 at 21:31

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