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.
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.
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
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.