An open discussion about contrails

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Introduction

I believe the addition of contrails to War Thunder has been a very welcomed, and much needed addition for the sake of realism, and immersion. However, I would like to make the argument that what we currently have in game, does not line up with real world observations. In particular, they aren’t being constrained to a small range of altitudes like they are in the real world. With the help of some prominent literature, I would like to make the case for what I think would be a better implementation of contrails in War Thunder. Please let me know your thoughts in the comments.

Observed in-game Contrail formation characteristics.

  1. Formation of contrails happens suddenly, starting at one altitude. As little as 1 meter can mean the difference between no contrails, and a massive contrail being formed.

  2. Formation of contrails begins too soon, typically they begin around (18,000ft ~ 6,000m) in test flight.

  3. There is no upper limit of altitudes where contrails can be formed. (Tested up to 80,000ft ~ 27,000m).

  4. Contrails in-game only come in 1 variety in terms of how visibly thick they are.

These four characteristics highlight a very simple model for contrail formation currently being used in War Thunder. With the use of literature, here is how a more detailed contrail model should be implemented, but most importantly why it matters for gameplay.

Contrail’s according to literature.
A publication by T. Brauer describes the contrail formation process best.

“At altitudes above 8 km and temperatures below −40°C, the hot aircraft engine exhaust is mixed with ambient air in the wake vortex and thereby cools down within seconds. The conditions in the cooling exhaust plumes are highly water super-saturated. Water vapor condenses on any available particle surface, but most favorably onto the larger soot particles (Kleine et al., 2018; Wong & Miake-Lye, 2010). Once activated, the particles become hygroscopic, continue to grow by condensation of ambient and engine-emitted water vapor and quickly undergo heterogeneous freezing to become ice particles (Heymsfield et al., 2010; Kärcher, 2018). The conditions for contrail formation are determined by the Schmidt–Appleman criterion, which defines a threshold temperature (TSA) below which contrails can form (Schumann, 1996).”- “Airborne Measurements of Contrail Ice Properties—Dependence on Temperature and Humidity” T. Bräuer, C. Voigt, D. Sauer

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GL092166

A key take-away from this publication is that contrail formation requires very cold temperatures to freeze the water that condenses on jet exhaust particles. Their data suggests temperatures below −40°C are needed, this temperature serves as a threshold above which contrails do not readily form. Typical temperature lapse rates have this temperature occurring no lower than 25,000ft (8 km).

Another important factor is how visibly thick a contrail can be, which is heavily dependent on ambient moisture. It seems that the water content in the jet exhaust, combined with low temperatures is not enough. As evidenced by the following graph, we see that particle diameter (which I will refer to as contrail thickness) remains strongest around 30,000ft (10km) and begins tapering off at 35,000ft (11.6km) due to a decrease in ambient moisture. This drop in moisture can be accredited to the transition from the moist Troposphere where temperature decreases with altitude, and the dry Stratosphere where temperature starts increasing with altitude. This transition is called the tropopause.

image

“initial extinction was highest at the medium flight altitudes between 9.1 and 9.8 km. It was reduced at lower and therefore warmer flight altitudes below 8.2 km and smallest at the highest and therefore drier altitudes above 11.4 km.”

Summarizing this publication, what we should see in game are contrails that begin forming quickly (not instantly) at an altitude no lower than 25,000ft (8.3km), becoming thickest and longest lasting at 30,000ft (10km) and begin slowly tapering off in thickness at 35,000ft (11.6km). And I would continue that logical trend by saying eventually contrails should be very faint at and above 40,000ft (13.3km).

The next publication I wish to highlight is named “The dependence of contrail formation on the weather pattern and altitude in the North Atlantic” by E. A. Irvine, B. J. Hoskins, K. P. Shine.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL051909

To further emphasize my previous claims, this publication also shows a sweet spot for contrail formation around 30,000ft (10km) over the northern hemisphere, and a heavy drop beginning at 35,000ft (11.6km) and very light contrail formation at 40,000’ (13.3km). This graph is using pressure as a reference for altitude, 300hPa ~ 30,000ft, 250hPa ~ 34,000ft, 200 hPa ~ 38,000ft.

image

Key take-aways concerning contrail formation found in literature.

  1. Contrail formation depends on numerous factors, sufficiently cold temperatures, ambient moisture, Soot content from jet exhaust, and radioactive heating.

  2. literature agrees that the temperature threshold for contrail formation is -40C, this doesn’t happen until 25,000ft (8.3km) typically. While local atmospheric conditions can change this, those cases should be the exception, not the rule.

  3. A “sweet spot” of 30,000ft (10km) is typically observed for contrail formation. Contrails also last longer at and above this altitude, as seen by the “extinction coefficients” in the first publication I listed.

  4. We also find that ambient moisture decreases with altitude when nearing the tropopause at 35,000ft (11.6km) and consistent with data, contrails get thinner and thinner above this altitude, eventually to the point they would become un-noticeable.

  5. Contrail thickness also depends on soot content of jet exhaust. Aircraft with dirty, sooty engines would naturally produce an environment ripe for contrail formation. (Yes, I’m looking at you Mig-29)

My closing thoughts.

While at first glance, implementation of a more detailed contrail model may seem like an arduous, and fruitless endeavor, I would implore the developers to consider differently. As we move deeper into the modern BVR environment, beginning with the F-14 we currently have in game which combines long range FOX-3 missiles with a powerful radar, I believe detailed contrails could introduce a unique and immersive game mechanic.

Pilots are tempted to climb to higher altitudes, where thin air increases the range and hit probability of their missiles. Consequently, they must also be aware that the “conning altitudes” found up high might also give their position away. This would force pilots to be aware of when they start conning. This could be made to depend on local map conditions. A cold map might mean contrails begin as low as 25,000ft, or perhaps 20,000ft in extreme cases, again -40C being the benchmark. Where a hot Tunisia style map could mean they can climb to 30,000ft before giving away their position. Or pilots must climb extremely high, above 40,000ft to get above the con layer and reduce the thickness of their contrail, therefore limiting chances of being spotted.

In a BVR style shootout, detailed contrails force the pilot to think that much more about altitude. It’s no longer, “higher and faster is always better” it’s now a more detailed consideration like, “where are the conns today, do I have the necessary missile performance at an altitude right below the conns, or do I have enough time and engine power to climb above the cons, or should I risk staying in the conns”

I believe these kinds of details, that fall in line with reality, are just a small piece of what we need in War Thunder, if we are to move this game towards a more engaging and immersive, fighter pilot simulator.

Thank you for your time.

~Sky-King

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Here is a NASA Q&A saying much the same.

https://web.archive.org/web/20160408184845/http://science-edu.larc.nasa.gov/contrail-edu/faq.php

The FAA also agrees that contrails generally have a floor of 25,000ft. And although not explicitly stated, the picture presented also suggests that contrails at higher altitudes become faded compared to contrails formed down low.

https://www.faa.gov/about/office_org/headquarters_offices/apl/noise_emissions/contrails/

Not that wiki is a source, but to further the point, it too says contrails happen predominantly between 25,000ft and 40,000ft.

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I actually took the time to read your essay and was impressed. Impressed what ai support can do to a simple request for adjusting the con trail alt.

I mean the topic is not really new as there were various discussions in the old forum exactly about this and there is even “acknowledged” bug report:

https://community.gaijin.net/issues/p/warthunder/i/QeaGfZLGMnzR

I have no clue why the link doesn’t work so you have to copy it manually. In the bug report i added than you can see them also much lower…

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Contrails in the game seem to start generating with the ambient air temperature as a trigger. For example, a snowfield map has a lower starting altitude for cloud generation than a desert map. This is a phenomenon that has also been confirmed by manipulating the outside temperature with a debug tool for test flights.

My personal concern is that WT clouds are exposed to the same color of sunlight at all altitudes.
The orange color of a sunset is determined by the thickness of the atmosphere between the sun and the observer. Therefore, even if you can observe the sunset on the ground, you cannot observe the sunset because the sun angle is high at the high point at the same time.

As a result, when you look up at the sky at sunset, you can observe pure white contrails that have not yet turned orange.

WT cannot observe this impressive phenomenon.

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I would agree there seems to be an attempt from the devs to make conning change based on temp, as can clearly be seen when flying on cold maps… however, the details are pure speculation.

regardless, the bottom line is that its we have an over simplified model compared to what is found in literature, or so it seems. Pulling from my experience as an airline pilot, the literature I listed in the OP definitely lines up with my experience.

I’m pretty sure clouds change color in war thunder based on sun angle, ill have to check that.

Here’s a visual picture of what I’m proposing. edited more detailed version

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Again, nice job. With reference to my comments regarding contrails at 15.000 in the bug report - have in mind that they were not uncommon and based on very cold arctic air/weather fronts raising south towards Germany in the winter time.

Unfortunately you find only a few hints in memoirs (bomber pilots or night fighters) for this phenomenon; same as almost forgotten allied weather flights which were a major advantage as the weather above central Europe is dominated by weather fronts coming from the west.

So if you are able to predict the weather very precisely you can time your bombing raids by day so that German interceptors could not take off due to very low 10/10 clouds at very low alt (1,000 ft or lower). The combat advantage to predict contrail altitudes for the RAF BC boys are obvious and described in the bug report.