We all see trails growing in clear blue skies, and we know the importance of the saturation point.
We know highly supersaturated, 125% – 140%, air allows for clouds to form, and subsaturation results in clear blue skies.
We have learned that jet engines are flying water-making machines.
We know that the word “chemtrails” has been socially stigmatised with conspiracies that make it an impossible subject to investigate or talk about.
We have learned that “trace” amounts are actually astronomically large numbers. We know the metallic elements and the high sulfur content of the fuel make the trails grow logarithmically, directly behind the engines.
Now we will learn why the trace elements are so important.
Gysel (2003) measured how jet engine exhaust particles take up water (their hygroscopicity) depending on fuel sulfur content (FSC) and particle size.
They found that higher FSC produced sulfuric acid or sulfate-rich coatings on particle surfaces, increasing water uptake, especially for very small particles.
Small particles and high sulfur equal more trails.
Overall, this effect was found to enable visible contrails to form in subsaturated, clear blue skies.
These findings agree with all theoretical models of particle hygroscopicity.
The graph on the left shows experimental evidence of the enhancement effect which high FSC has on trail growth.
It can be seen that the rise in growth rate is directly related to the amount of sulfur in the fuel.
This table summarises the Gysel study:
There is no significant difference between old and new engines.
Smaller, spherical particles with high fuel sulfur content are the most hygroscopic.
Hygroscopic growth under subsaturated conditions will modify trails, natural cirrus clouds, and contribute to additional cloud formation.
The Gysel study shows that small changes in the mixture of the fuel can have big effects on the trails we see. Trace amounts are very important.
This graph by Jensen (1998, NASA SUCCESS campaign) shows the trail made on 12th May 1996 plotted in thick black using triangles.
They watched it gain height at a rate of 10 cms per second, and suggested this was from the warming of the sun.
But notice how that trail grew with a logarithmic profile, fast at the beginning of its life.
This did not agree with the models on the same graph.
These observations from another NASA SUCCESS study confirmed metallic particles made up the ice nuclei of the 12th May 1996 trail which grew with a logrythmic profile.
This logarythmic growth pattern is not natural and does not match with previous models or understandings.
All laws governing the growth pattern for ice crystals and snowflakes predict an exponential growth pattern.
Exponential Growth:
Exponential growth occurs when something continuously builds on itself. As it grows, the growth rate itself also increases. The surface of the object is growing, as the surface gets bigger, more can grow on it. This is how snow and ice grow. As they accumulate, they grow faster because more ice is able to build on top of what’s already there.
Logarithmic Growth:
Logarithmic growth, on the other hand, starts off quickly and then slows down over time. It’s like getting a big push in the beginning, but eventually, the rate of growth decreases. This is common in biological systems like the growth of animals and plants. At first, they grow rapidly, but as they get bigger, their growth slows down as they reach maturity.
The way modern trails grow is logarithmic, as discribed in the NASA SUCCESS campaign.
We all see that they are thick directly behind the plane. They are getting a push from the hygroscopic metallic elements in the sulfur rich fuel.
The data for this graph came from 10 science studies that recorded the growth of trails since the 1990s.
The humidity needed for trails to grow has gradually reduced, so they are now able to grow in subsaturated air. Clear blue skies.
Wang (2023) measured the size of fresh contrails, contrail cirrus, and natural cirrus ice crystals and found contrails to be much smaller than natural cirrus.
Fresh contrails = 8 µm
Contrail cirrus = 22 µm
Natural cirrus = 28 µm
This difference in size is directly related to the smaller size of the ice nuclei which the ice crystals grow on.
All of this is further proves the ice nuclei from jet engines are very small.
From this information, it can be seen that even though they are present in trace amounts, the metallic elements in the fuel play a big role in making small ice nuclei.
Jet A1 fuel is used throughout the airline industry, and the super-small particles are made from the aluminium, iron, chromium, titanium, and silicon elements in the fuel.
After which, calcium, sodium, manganese and zirconium attach to these small particles to make them grow to just the right size, 30 nm.
These small particles are then coated with sulfur making them extremely hygroscopic and able to form trails in subsaturated air, with a logarithmic profile directly behind the engine.
By altering the ratio of small nucleating metals to growth metals, the size of the finished nucei can be tightly controlled. Just like changing a cake mixture to produce a different texture cake.
All atmospheric science agrees with this interpretation, even though no single scientific study puts the whole scenario together as one unified understanding. Once it is laid out clearly, it becomes apparent.
All of the science lines up, and it fits perfectly with our observations. Trails now grow in clear blue skies because their growth is enhanced by metallic elements and sufur in the fuel.