Contrails over Europe

[Klauss]

Hi, I found a tweet indicating a trial under way @ Maastricht. Wonder how that develops - less contrails ? Might be good for the environment.
https://twitter.com/ALARM_H2020/status/1352579798334373889

[FlightlessParrot]

Umm, I always thought contrails were almost entirely water. If that is so, I would have thought the only impact they would have on the environment would be to very, very slightly increase the reflectivity of the upper atmosphere, which would be benign in an epoch of warming. What have I missed?

[beardy]

Yes, water vapour is, in itself, a greenhouse gas and whilst the condensate slightly attenuates irradiation of the earth from the shorter wavelength solar radiation it also reflects back downwards the longer wavelength earth radiation somewhat more efficiently.

[wiggy]

Here you go for starters, I'm sure shorter reads are available:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL080899


Buried in the text:

Our conclusions also confirm the results of Hong et al. (2008 (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL080899#grl58313-bib-0009)), who predicted that weather patterns might overprint the contrail effect but that a small effect might still exist.

[OldLurker]

AFAIK the effect of contrails - which can persist for some time in the right conditions - is to increase the high cloud cover somewhat, increasing reflectivity as mentioned above. It doesn't make any difference to the actual greenhouse effect because cruising aircraft emit water vapour into the upper atmosphere whether or not the conditions are right for contrails.

[FlightlessParrot]

Thank you to the posters who have explained that to me. I see that the Wikipedia article cites a predominance of studies that suggest the greenhouse effects outweigh the increase of reflection.

[compressor stall]

The trial is mentioned in the German FIR NOTAMS

[EatMyShorts!]

In the aftermath of 9/11 all flights over the US were banned for 2 days (or longer?). I remember that meteorologists reported that during this time the temperature amplitude in the US was slightly higher, meaning that at night more excess energy was able to radiate out into space instead of bouncing back from those water molecules. Now, was it the reduction in water molecules or the reduction in contrails that caused this?

[ATC Watcher]

Reducing contrails over the Netherlands was always a demand from the political green party in the country, They even had a plan years ago that wanted to restrict altitudes below the condensation layer, forcing on some days everybody below 30.000ft . That was years ago in their program , maybe as new elections are coming up in NL the idea resurfaces. Public is generally for as contrails can block the sun on certain days and everyone can see it.

[the_stranger]

So to prevent contrails, which have a slight (possible) effect on cllimate change, we burn more fuel when diverting from our most efficient route?

Unless of course if this means more directs, I am all for it, but that then poses the question why directs are possible now and not before?

[Momoe]

Fine margins already for optimising fuel and therefore emissions.

Trading off visible contrails for additional emissions makes little sense imo

This gives an idea of the margins involved

[old,not bold]

On many occasions (pre-Covid) I have seen, driving eastwards in the early morning along the M4, that the contrails 0ver SE England had spread until they were covering, very, very thinly, at least 50% of what would otherwise have been a cloudless sky.

[Klauss]

Hi, found a movie about the trial on the Eurocontrol Youtube channel . Fun to watch.


https://youtu.be/vLEFA-p_L68

[EastofKoksy]

If water droplets in the atmosphere are so harmful, maybe we should consider measures to reduce cloud cover. Perhaps the greens think clouds are OK because they are not created by aircraft.

[Winemaker]

I seriously doubt that a two day window for data gathering on weather changes has any significance.

[NoelEvans]

Contrails are not 'water droplets'. At -50C or so they very, very, very rapidly become ice particles.

Water droplet clouds, lower and medium levels, are the cause of overwhelmingly the most of the 'greenhouse effect' (CO2, methane, etc. are tiny by comparison).

Ice particle clouds, cirrus, reflect more solar radiation away from the earth than any tiny, tiny (from that high altitude) 'greenhouse effect' that they can have.

Therefore, because contrails are effectively cirrus clouds (and you can often see how they linger to form clear cirrus clouds), they have an overall cooling effect on the earth.

The temperature rise over North America (tiny, but still there) was due to the grounding of all airliners and the lack of contrails.

This last year there was an increase in melting of ice in Greenland because of increased sunshine. Guess what?! There were very, very few airliners on those northerly trans-Atlantic routes that cross Greenland last summer, meaning a loss of all of those cirrus clouds (contrails) and hence more sunshine and more ice melting.

Conclusion? If you are worried about global temperature rises and ice melt, we need more airliners flying!!

[spangzilla]

NoelEvans

But they trap heat radiating from the earth's surface at night, and their net effect on radiative forcing is positive
Estimates vary on the extent of it, just google for contrails and radiative forcing

[Peter Fanelli]

Now wait a minute, doesn't the Virus and vaccination expert Bill Gates propose launching billions and billions and billions of tiny mirrors into space to reflect the sunlight back into space to reduce global warming? Seems to me that would have the same effect as contrails, but now contrails are a bad thing???
Is it any wonder people are sceptical about this global warming malarky, the story keeps changing.

[the_stranger]

The story never really changed.

Clouds and contrails do block the sun, reducing the temperature. However they also act as a blanket, trapping heat from escaping into space and therefore increasing the temperature.The net effect is still an increase in temperature. This in turn causes an increase in evaporation of surface water, further increasing cloudcover.

[FlightlessParrot]

meleagertoo

Yes, but no. The greenhouse effect is a very long established explanation for why the earth is warmer than it would otherwise be. First proposed by Fourier (who has a few big discoveries to his name) in 1824, confirmed in the middle of the 19th century. Water vapour is the major gas responsible. Wikipedia on Greenhouse Effect (https://en.wikipedia.org/wiki/Greenhouse_effect)

What all responsible people are worried about is that the large scale emission of other greenhouse gases since global industrialisation is increasing the degree of greenhouse warming, and that there are probably going to be positive feedback effects, so that the climate will change at an unprecedentedly rapid rate. This will not cause the earth to be uninhabitable by human beings, but will disrupt patterns of settlement and agriculture on a large enough scale to cause great disruption, huge movements of human beings, probably wars, and certainly great economic costs. Given the difficulty industrialised societies have with current movements of refugees from war and poverty, most people think it would be a good idea to minimise the degree of change, quite apart from any ethical considerations.

I am 76 and have no kids, so for myself, I don't give a monkey's. The contrail effect is probably small, but the way to determine whether it is worth doing anything about is by careful measurement of the effects, and the consequences of mitigation. There is obviously a need to balance efforts to limit change, and efforts to adjust to it. Since aviation is dependent on hydrocarbons, the first response must be adjustment requires such small gains as are possible, and arguing for the reduction of the use of fossil fuels wherever they are not essential; the second stage, limiting change, probably involves working on synthetic fuels which might be climate neutral.

This is no more a hoax than Covid-19. The way to consider it is by calm consideration of the best available knowledge (which may be incorrect, but which can be corrected) and by an acknowledgement of reality, not by echoing talking points put out by the owners of coal mines, or sounding like that contrarian with a loud voice who makes you find another pub.

[NoelEvans]

So much is said about 'clouds'. Please note the careful difference that I pointed out between water droplet clouds and ice particle clouds. (They are very, very different. I know, I have done research flights into them. And any turboprop-pilot-wanting-to-be-a-jet-pilot can also confirm that they are very different!!)

Water droplet clouds are low level, more extensive and have a large 'greenhouse' effect. They also reflect solar radiation, but as they are more extensive and only reflect that solar radiation during the day, the overall effect is warming. And thank goodness for that, or else our planet would mostly be uninhabitable.

Ice particle clouds are very high and not nearly as extensive as the low level water droplet clouds. Being ice and very high, the reflect a lot of solar radiation before it is able to get into the more dense lower air and warm it up. Being relatively sparse and very high they have a very small 'greenhouse' effect.

Then there's the comment about contrails (which, as has been pointed out, become 'ice particle clouds' within a second or so of being formed):But they trap heat radiating from the earth's surface at nightI can assure you that there are (in normal circumstances) many, many, many more contrails on the daytime side of the earth than on the night-time side of the earth. (Been there, seen that!) So that minimal night-time effect from any one contrail is reduced further by the fact that there are far less of them. The daytime cooling effect of the reflected solar radiation is increased further by the fact that there are many more of them.

That brings us back to:If you are worried about global temperature rises and ice melt, we need more airliners flying!!

[screwdriver]

There was a BBC Horizon programme some years ago on the subject of "Global Dimming". It examined the effect of contrails on climate change. The research was instigated post 9/11 when the effect of having no contrails was noticed.

[Imperator1300]

Putting aside all of the global warming arguments (which are important), there is also the visual impact of contrails. They can be persistent, spread out and change the appearance of the sky from the ground, in fact making it less ‘blue’, more grey/white. I have noticed the difference over the last year.

[beardy]

This last year there was an increase in melting of ice in Greenland because of increased sunshine. Guess what?! There were very, very few airliners on those northerly trans-Atlantic routes that cross Greenland last summer, meaning a loss of all of those cirrus clouds (contrails) and hence more sunshine and more ice melting.

You are confusing causation and correlation. There was less traffic AND there was an increase in ice melt, you have not shown and probably will not find a causative link. Your deductive reasoning is incomplete in that there are other factors you have not considered.

Then there's the comment about contrails (which, as has been pointed out, become 'ice particle clouds' within a second or so of being formed): That depends. Sometimes the ice crystals persist, at other times they evaporate, dumping water vapour into the atmosphere, water vapour which in itself is a potent greenhouse gas.
I can assure you that there are (in normal circumstances) many, many, many more contrails on the daytime side of the earth than on the night-time side of the earth. (Been there, seen that!) Have you never wondered where they went? And what happened to the water vapour?


If you are worried about global temperature rises and ice melt, we need more airliners flying!! ignoring the NOx, CO2 et al which add to the greenhouse effect.

[NoelEvans]

... Sometimes the ice crystals persist, at other times they evaporate, dumping water vapour into the atmosphere, water vapour which in itself is a potent greenhouse gas.
...
Let me explain how those contrails form. The aeroplane does not emit that water to forms the contrails, The aeroplane disturbs moisture that is already in the air causing it to condense and form the contrail. Watch on days where there is a very dry upper atmosphere, You can easily see on those dry days how short the contrail 'tails' on aeroplanes are. Hopefully that has helped you a bit?

Regarding the comment:I can assure you that there are (in normal circumstances) many, many, many more contrails on the daytime side of the earth than on the night-time side of the earth. (Been there, seen that!)... you say:
...
Have you never wondered where they went? And what happened to the water vapour?
...
Ummm...!! - There - are - less - contrails - at - night - ... - because - there - are - less - aeroplanes - at - night.
Just try listening to any ATC frequency at 2pm and compare with 2am.
Those contrails never 'went' anywhere, they were never created!

If you are worried about global temperature rises and ice melt, we need more airliners flying!!

[beardy]

et me explain how those contrails form. The aeroplane does not emit that water to forms the contrails, The aeroplane disturbs moisture that is already in the air causing it to condense and form the contrail. Watch on days where there is a very dry upper atmosphere, You can easily see on those dry days how short the contrail 'tails' on aeroplanes are. Hopefully that has helped you a bit?

Ah, now I see your confusion. The contrails are formed from the water produced from burning hydrocarbons in the fuel. The carbon produces CO2 and the hydrogen produces H2O. Look out the window and you will see that they form in the exhaust from the engines, not at the wing tips. Four from a four engined aircraft, two from a twin which then merge. They are not the same as the condensate produced by wing tip vortices. I take it that you have never been in trail behind a contrailing heavy. Contrail formation is an interesting topic involving partial pressures of both water and ice at the temperature and pressure at altitude. The saturation point of the different states of water are different. Look at ice formation overnight in frosty weather, water vapour sublimates directly into hoar frost (similar to contrail and ice clouds) without ever forming dew (similar to wet cloud.)

Ummm...!! - There - are - less - contrails - at - night - ... - because - there - are - less - aeroplanes - at - night.
Just try listening to any ATC frequency at 2pm and compare with 2am.
Those contrails never 'went' anywhere, they were never created! Quite correct there are fewer aircraft, especially shorthaul, at night and so fewer contrails. However contrails do form at night and do disperse with or without daylight. When they disperse they do increase the proportion of water vapour in the atmosphere.

[NoelEvans]

Ah, now I see your confusion. The contrails are formed from the water produced from burning hydrocarbons in the fuel. The carbon produces CO2 and the hydrogen produces H2O. Look out the window and you will see that they form in the exhaust from the engines, not at the wing tips. Four from a four engined aircraft, two from a twin which then merge.
...
I see ... ... ...
So I suppose those very short contrails are when the aeroplane is not using much fuel and the long contrails are when they are using a lot?

(Does a contrail 'come out of the engine', or does it forms in the disturbed air behind that high speed exhaust air? But I really do like the idea that the length of the contrails could be an indicator of the engine power settings!!)

However contrails do form at nightYes, and they look very pretty in the moonlight, especially when you see them close up.

[3FG]

Here's an open access article explaining in detail how contrails form. As a rough summary, the soot particles in the exhaust are nucleation site for ice crystals to develop from the water vapor in the exhaust. The ice crystals grow further from ambient water vapor.
www.nature@com/articles/s41467-018-04068-0
Replace @ with dot

[beardy]

Quite correct they form in the diluted exhaust efflux. And no the dispersion rate has nothing to do with power setting, I'm not sure why you would think so. The dispersion of contrails is due to evaporation of the ice crystals. It is all within the scope of A level physics (or it used to be) not that complicated really.

[beardy]

Here is an interesting piece about contrail formation and dissipation :

https://www.google.com/url?sa=t&source=web&rct=j&url=https://ir.library.oregonstate.edu/downloads/k3569587k&ved=2ahUKEwj1446xobXuAhWwRBUIHVyaCZkQFjAWegQIDxAB&usg=AOvVaw3nz9wzLFies-sQdSJhRQq_
Or perhaps in layman's terms

https://en.m.wikipedia.org/wiki/Contrail

[hoistop]

With some basic observations you can figure out that length and persistency of contrails depends pretty much on weather. I live on south side of Alps and contrail behavior is one of indicators to do my own forecast. Short, tiny contrails mean air in upper troposphere is generally dry and that is common when we have dry, cold northerly wind across the Alps, bringing sunny, dry and cooler weather. When contrails become long and in particular, when they spread into wide swaths of cirrus-like clouds and persist, that means moist air is coming, forecasting bad, (means cloudy, possible rainy) weather is getting in. It works every time. No rocket science here.

[beardy]

This last year there was an increase in melting of ice in Greenland because of increased sunshine. Guess what?! There were very, very few airliners on those northerly trans-Atlantic routes that cross Greenland last summer, meaning a loss of all of those cirrus clouds (contrails) and hence more sunshine and more ice melting.

From Carbon Brief (https://www.carbonbrief.org/guest-post-how-the-greenland-ice-sheet-fared-in-2020#:~:text=This%20means%20that%20the%20Greenland,of%20arou nd%20250bn%20per%20year.)

However, unlike in 2019, Greenland has actually had a relatively “normal” year with regard to ice changes at its surface. Yet losses via the breaking off of icebergs remain at the high end compared to the early years of the satellite record, which stretches back to the late 1970s.

The ice sheet ends the season losing about 152bn tonnes when accounting for both surface melting and discharge of icebergs. This means that the ice sheet is continuing to lose ice, though at a slower rate than seen in 2019.


No mention of greater insolation, but emphasis is given to stable high pressure and warmer air.

[jmmoric]

And it is very important that airliners stay at the "crossover altitude", so they get as fast as possible from A to B.... so we can save time before the next flight.

[Hadley Rille]

Question. Why does the aviation world refer to the deposition (of ice) as sublimation?
Sublimation is the change directly from solid to gas.

[beardy]

I apologise for having used the wrong phrase I meant desublimation for gas to solid phase transition and sublimation for evaporation from solid to gas all without the intervening liquid phase.

[Hadley Rille]

Apologies if my question came across as a personal attack, it certainly wasn't intended that way.
Sublimation is erroneously applied in many aviation publications and instructional material e.g.
https://www.skybrary.aero/index.php/Ice_Formation_on_Aircraft

Curious how something so wrong is just accepted.

[beardy]

No need to apologise, I didn't take it as a personal attack.
The term was used for phase transition both ways on my university course, the emphasis being on the missing out of the liquid phase by manipulating the temperature and/or pressure. However, I accept that the meaning may have changed over the (many) years and useage.

[NoelEvans]

I watched three contrails today (there weren't many).

The first was a long contrail that lingered for a long, long time.

Some hours later, the next was very short. It disappeared a short way behind the aeroplane.

A few hours later another one was very long, but did not linger nearly as long as the first.

All were high, on trans-Atlantic tracks (I checked the last one, it was a KLM Cargo 747 going to Chicago).

Were they all emitting water differently? Was the second one emitting very little water? How?

[beardy]

If you want to do some work finding out, here's a long answer for you:

How Well Can Persistent Contrails Be Predicted (https://www.mdpi.com/2226-4310/7/12/169/htm)
"There are two atmospheric conditions necessary for persistent contrails. First, the so called Schmidt-Appleman criterion must be fulfilled, which is a pure thermodynamic criterion that rules whether contrail formation is possible in a given situation or not. This criterion covers both short-term (lifetime seconds to a few minutes) and persistent contrails. Thus a second condition is required for persistent contrails: the ambient air must be in a state of ice supersaturation, that is, the relative humidity with respect to ice, RHi must exceed 100%"

Although there are other articles that cover the same topic

[dixi188]

Different flight levels?

[NoelEvans]

As I said a while back:

The aeroplane disturbs moisture that is already in the air causing it to condense and form the contrail. Watch on days where there is a very dry upper atmosphere, You can easily see on those dry days how short the contrail 'tails' on aeroplanes are.

So we now agree!! Relative humidity is the defining factor for length of contrails.

Yes, the aeroplane emits a wee bit of water, but not all of the water for those contrails that run from one end of the sky to the other.

(About the flight levels, being several hours apart would have been the factor, they would have been fairly similar flight levels.)

Out of interest, I have had a contrail forming behind me in a Piper Aztec! Aeroplanes at different levels (up and down) told me. I tried to 'turn back' to see it, but as I was at a level that was humid enough for the contrail to form, the 'background' patches of cloud at the same level meant that my contrail was 'camouflaged' against them and I couldn't see it. The other aeroplane said that it was absolutely clear though and my wife has just this minute confirmed that as she was in one of those aeroplanes. (Just a wee bit of weather research that we did once where we learnt a thing or two about the atmosphere first hand...)

[Australopithecus]

You are confusing two different phenomena. The classic contrails are formed when the water in the exhaust forms ice crystals and the subsequent longevity is determined by the atmosphere's relative humidity.

Contrail formation has been around since WWll...in fact predicting contrail formation level was a vital skill for wartime meteorologists.

[Australopithecus]

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/2000x1504/b34c3781_bf8b_4f54_b014_217870a001cb_987d3f8d867987ded7bf96d 3898af330974ba9cb.jpeg
Medium level trails formed by disturbance of the very high humidity air (in this case my aircraft filmed by a colleague in the sub-tropics)
https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/2000x1504/0d7513d3_db72_43fd_89f7_a46e7a4328e7_5a29976e572ffc6b3448dcf 7865db0d23ded333d.jpeg
The high level engine exhaust seeded ice trails, In this case over the western Pacific. By the way...contrails form as readily at night as seen under moonlight, its just that there are fewer of us aloft then.

[beardy]

Water droplet clouds, lower and medium levels, are the cause of overwhelmingly the most of the 'greenhouse effect' (CO2, methane, etc. are tiny by comparison).

​​​​​

British Geological survey (https://www.bgs.ac.uk/discovering-geology/climate-change/how-does-the-greenhouse-effect-work/)


Water vapour, as a gas, not droplet, has the greatest contribution to the 'greenhouse effect' and it is present throughout the whole depth of the atmosphere.


​​​​As an illustration of the relative importance of clouds compared to gasses in the terrestrial energy balance :
https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/943x456/jone_fig_1_833495169b95080560fc182397af500f8e5d6c1b.gif
Note clear sky emissions to surface compared to {319}compared to cloud {26.6}

[megan]

The aeroplane does not emit that water to forms the contrailsWhat then happens to the H2O the engine does emit?


https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/600x335/nvjte_e081b81cb36a9ff842726c4eed2b6843ef4e9c8d.gif

[Australopithecus]

I don’t know why I didn’t think of this before, but back in the 70’s I was taxiing behind a DC-8 at CYWG...temps around -40°. He was making contrails down the taxiway, and they were spreading in the almost calm air. He turned a clear morning into zero visibility in ice fog. All of that of course came from engine exhaust water.

[cats_five]

The moisture in the air could have been super-cooled.

[beardy]

Initially my thoughts too. But, at - 40°c the amount of water in the atmosphere would probably not have led to a dense fog.
I witnessed a similar event when a pair of F4s took off in clear, still air at about - 3°c from a frosty airfield, the fog formed from the exhaust plume and rolled across the airfield from the runway. It stayed all day, the pair diverted.

[megan]

Some papers on contrails, aerodynamically and engine caused.

https://www.faa.gov/about/office_org/headquarters_offices/apl/research/science_integrated_modeling/accri/media/Contrail%20Microphysics.pdf

https://journals.ametsoc.org/view/journals/atsc/72/9/jas-d-14-0362.1.xml

Re posts # 47 to 49, possibilities?

https://glossary.ametsoc.org/wiki/Steam_fog

https://glossary.ametsoc.org/wiki/Ice_fog

[tdracer]

In brief, aerodynamic 'fog' or visible vapor are caused when the local acceleration of the air around the aircraft drops the static air temperature - if the relative humidity is already high, that drop in SAT can cause moisture in the air to condense forming fog. Aerodynamic fog is visible immediately behind the aero surface that is creating it. Further, although there are exceptions, aerodynamic fog normally dissipates quite quickly - as the air settles down the SAT returns to it's original warmer value and the moisture can 'evaporate' back into the air.
Engine 'fog' - commonly known as contrails - are due to water added to the air from the engine exhaust as a product of combustion - as megan posted the combustion of a ton of jet fuel produces over a ton of water. Engine fog is generally not visible immediately behind the engine since the exhaust is hot - only becoming visible as the exhaust cools to the surrounding temperature. Since engine fog is generated by water being added to the surrounding air, in the right conditions engine fog can persist a long time - sometimes for hours.

Of course engine fog isn't just formed behind aircraft - cars and trucks can generate it (usually when it's really cold). Years ago I was driving across central Utah in the middle of winter when it was extremely cold - around -35 deg. C. At one point when I looked in the mirror, all I could see behind was a white cloud emanating from my car. I was briefly worried that something had just gone seriously wrong with the engine, but a quick check of the gauges showed everything normal - it was just the moisture in the engine exhaust turning to fog in the bitter cold. A few miles further on the ambient conditions changed enough that the fog pretty much disappeared.

[FlightlessParrot]

A sad thought occurs to me. Since water produced by combustion is important for climate change, hydrogen fuel is not such an attractive alternative to hydrocarbons. The exhaust of hydrogen combustion is only water, which on the ground is a win, but in the upper atmosphere, apparently, not so much.

[beardy]

That depends on where the hydrogen comes from. If it is part of a closed cycle then it is already part of the balance, if it is introduced into the cycle (as carbon from combustion is) then it is a problem. On balance I think that it will be a problem since the evaporation part of the current cycle is principally a surface phenomenon.

[HarryMann]

A lot of that first image looks to be forming over the wing root upper surface... i.e., aerodynamic depression fog.

[HarryMann]

tdracer

A very clear account of aerod fog (depression fog) Vs Hydrocarbon burning engine fog

[megan]

Aircraft cause snow.

https://simpleflying.com/chicago-snow-landing-planes/

[pontifex]

When I was trainng on Harvards in Canada in 1956 I recall that once, when the first few started up in the morning the airfield was effectively "blacked" out for an hour or two. When the temperature had risen to about minus 25F things became workable. I guess that settles an argument about disturbing the airmass.

[billovitch]

Down here in southern Germany contrails have been mainly conspicuous, for many months, by their absence.
Before the pandemic however, our neighbour - an ex Luftwaffe ground engineer - used to regale me with conspiracy theories about bauxite being thrown out to creat rain...
After several failed learned discussions with him I tried asking why then, after weeks of blue skies and mucho “bauxite”, it hadn’t rained - and also why, following days of floods and downpours, “bauxite” was being chucked out on the first clear day. He has since been a bit quiet on the subject.

[billovitch]

Pontoflex,
That happened at Benson (also one morning) with dew point at ambient temperature but still clear sky. An Argosy driver started his first engine and a small patch of fog formed. By the time he got all four going the field was socked in and he could shut down again... All it needs is a trigger.

[NoelEvans]

billovitch

It is not 'bauxite' that us used in attempts to create rain. In the very early days they tried soot. In the days that I was involved they were using silver iodide or dry ice (the silver iodide was from burning flares impregnated with it, the dry ice was in centimetre cubed blocks), I have heard that later trials found much more success using salts. It is all about trying to find the most effective hygroscopic nuclei, all of those mentioned were just that, but it appears that the salt was most effective. Although, none of them were really that effective, but it was all valuable research. (And quite good fun!!)