Tweaking the climate models: Researchers show emissions from forests influence very first stage of cloud formation

CloudsClouds are the largest source of uncertainty in present climate models. Much of the uncertainty surrounding clouds’ effect on climate stems from the complexity of cloud formation. New research from scientists at the CLOUD experiment at CERN, including Carnegie Mellon’s Neil Donahue, sheds light on new particle formation — the very first step of cloud formation. The findings, published in Science, closely match observations in the atmosphere and can help make climate prediction models more accurate. 

Clouds_over_Amazon

These clouds are almost certainly a result of evapotranspiration. The clouds are distributed evenly across the forest, but no clouds formed over the Amazon River and its floodplain, where there is no tall canopy of trees. While water may evaporate from the Amazon River itself, the air near the ground is too warm for clouds to form. Trees, on the other hand, release water vapor at higher levels of the atmosphere, so the water vapor more quickly reaches an altitude where the air is cool enough for clouds to form. When water vapor condenses, it releases heat into the atmosphere. (NASA image courtesy Jeff Schmaltz, MODIS Rapid Response at NASA GSFC )

In the midst of all the Climate McCarthyism on display, I almost missed this important finding. Of course, most daily forecast meteorologists that watch satellite and radar have known this for decades, but it is nice to see climate science catching up.

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Press Release: International Group of Researchers Shows Emissions From Forests Influence Very First Stage of Cloud Formation

Research from CLOUD Experiment at CERN, Which Includes Carnegie Mellon’s Neil Donahue, Contributes to Better Understanding of Connection Between Clouds and Climate

PITTSBURGH—Clouds play a critical role in Earth’s climate. Clouds also are the largest source of uncertainty in present climate models, according to the latest report of the Intergovernmental Panel on Climate Change. Much of the uncertainty surrounding clouds’ effect on climate stems from the complexity of cloud formation.

New research from scientists at the CLOUD (Cosmics Leaving OUtdoor Droplets) experiment at CERN, including Carnegie Mellon University’s Neil Donahue, sheds light on new-particle formation — the very first step of cloud formation and a critical component of climate models. The findings, published in the May 16 issue of Science, closely match observations in the atmosphere and can help make climate prediction models more accurate.

Cloud droplets form when water vapor in the atmosphere condenses onto tiny particles. These particles are emitted directly from natural sources or human activity, or they form from precursors emitted originally as gaseous pollutants. The transformation of gas molecules into clusters and then into particles, a process called nucleation, produces more than half of the particles that seed cloud formation around the world today. But the mechanisms underlying nucleation remain unclear. Although scientists have observed that the nucleation process nearly always involves sulfuric acid, sulfuric acid concentrations aren’t high enough to explain the rate of new particle formation that occurs in the atmosphere. This new study uncovers an indispensable ingredient to the long sought-after cloud formation recipe — highly oxidized organic compounds.

“Our measurements connect oxidized organics directly, and in detail, with the very first steps of new particle formation and growth,” said Donahue, professor of chemistry, chemical engineering, engineering and public policy, and director of CMU’s Steinbrenner Institute for Environmental Education and Research. “We had no idea a year ago that this chemistry was happening. There’s a whole branch of oxidation chemistry that we didn’t really understand. It’s an exciting time.”

The air we breathe is chock-full of organic compounds, tiny liquid or solid particles that come from hundreds of sources including trees, volcanoes, cars, trucks and wood fires. Once they enter the atmosphere, these so-called organics start to change. In research published in the Proceedings of the National Academy of Sciences in 2012, Donahue and colleagues showed conclusively that organic molecules given off by pine trees, called alpha-pinene, are chemically transformed multiple times in the highly oxidizing environment of the atmosphere. Additionally, other research, including from Donahue’s lab, has suggested that such oxidized organics might take part in nucleation — both in new particle formation and in their subsequent growth. Donahue and an international team of researchers with the CLOUD experiment at CERN set out to test that hypothesis.

The CLOUD project at CERN is a unique facility that allows scientists to reproduce a typical atmospheric setting inside of an essentially contaminant-free, stainless steel chamber. By performing experiments in the precisely controlled environment of the CLOUD chamber, the project’s scientists can change the concentrations of chemicals involved in nucleation and then measure the rate at which new particles are created with extreme precision.

In the current work, the team filled the chamber with sulfur dioxide and pinnanediol (an oxidation product of alpha-pinene) and then generated hydroxyl radicals (the dominant oxidant in Earth’s atmosphere). Then they watched the oxidation chemistry unfold. Using very high-resolution mass spectrometry, the scientists were able to observe particles growing from single, gaseous molecules to clusters of up to 10 molecules stuck together, as they grew molecule by molecule.

“It turns out that sulfuric acid and these oxidized organic compounds are unusually attracted to each other. This remarkably strong association may be a big part of why organics are really drawn to sulfuric acid under modern polluted conditions,” Donahue said.

After confirming that oxidized organics are involved in the formation and growth of particles under atmospheric conditions, the scientists incorporated their findings into a global particle formation model. The fine-tuned model not only predicted nucleation rates more accurately but also predicted the increases and decreases of nucleation observed in field experiments over the course of a year, especially for measurements near forests. This latter test is a strong confirmation of the fundamental role of emissions from forests in the very first stage of cloud formation, and that the new work may have succeeded in modeling that influence.

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New research from scientists at the CLOUD (Cosmics Leaving OUtdoor Droplets) experiment at CERN, including Carnegie Mellon’s Neil Donahue, is contributing to a better understanding of the connection between clouds and climate.

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Philip

Hmm … I might have put the difference down to differences in elevation between the river flood-plain and the surrounding higher ground.

stewart pid

From the article “Trees, on the other hand, release water vapor at higher levels of the atmosphere” … the trees grow way higher there than where Iive 😉

stewart pid

Dang should be … I live

milodonharlani

IMO. it’s not just that the GIGO GCMs ignore cloud effects, but that their assumptions about water vapor are false. They take into account only the radiative effect of water vapor as a GHG, without considering evaporative cooling or convection currents, while also assuming more evaporation than has been observed as a positive feedback of CO2-induced warming. The models also IMO don’t adequately consider the effect of combination & overlap of GHGs in the atmosphere, where on average water vapor is on the order of 100 times more plentiful than CO2.
The models not only lack predictive value, but also fail to hindcast the past. They can’t explain the heat of the Cretaceous, for instance. One paper suggested that the warmth & even distribution of temperature from equator to poles during that period could be explained by relative lack of clouds, due to reduction in CCNs from lower biological productivity in hot tropical seas.
GCMs forecast T rising with CO2 because that’s what they’re designed to do. The whole exercise is one massive logical fallacy, ie begging the question or assuming beforehand what you set out to demonstrate, dressed up in the fancy sciencey garb of computer modeling.

Former Senior Minister at my Church (www.colonialchurch.org)…rounded up almost a thousand folks “of means” during the first 1/2 of the ’80s. Dragged them over to Ethiopia. They planted 2 MILLION trees. They helped dig 3000 wells (plenty of ground water, about 50 to 100 ft down in most of the country.)
The RAINS came back by the mid ’80s to the end of the 80’s. Been regular ever since. And the locals know the trees are “sacred”. THANK YOU Rev. Dr. Arthur Rouner for this good work!
PS: Sent him a copy of this photo at his http://www.pilgrimcenter.org site. You’ve undoubtedly put the biggest smile on the face of an 84 year old Saint, that you ever could.

Latitude

BREAKING….
Climate Scientists look outside…..discover clouds for the first time
film at 11…………..

richard

last night, haven’t watched it all, a bbc 4 programme about the monsoon in India, they talked about warming from the tilt of the earth , many things but so far nothing about man made global warming.

They keep trying to make it predictable. Problem: Constants aren’t and variables won’t, and that plays hobbes with computer models. Clouds are part of daily life and any computer model that doesn’t take them into account is flawed because of it.

“These clouds are almost certainly a result of evapotranspiration.”
Well…I’d say the moisture content of the boundary layer air is fairly uniform. The convection, IMHU, is being suppressed over the water because the air over the water is slightly cooler than the air over the tree canopy (remember, land heats faster than water). That is why you see the little ‘popcorn’ shallow cumulus over the forest area & none in the immediate area of the water. However, because there is only shallow convection, it looks like there is a capping inversion preventing any deep convection from firing so, in a situation like this, you may get scattered clouds, a real humid & hot boundary layer, but no precip until there is sufficient heating to break through the inversion or, a convergence boundary comes along to force lift through the inversion or, upper-level dynamics (divergence) arrives to lift the parcels through the inversion…just like it is all through the tropics!
Jeff

Remember the Acid rain and lake acid issue years ago. The E=GREEN went after coal plants for sulphur smoke only to find out that the acid was coming from the forests. Ie: natural events.

“The convection, IMHU,”
Oooops…the should be “The convection, IMHO,”
(fat fingers…)
Jeff

Makarieva?

re: Philip says May 16, 2014 at 10:08 am
Hmm … I might have put the difference down to differences in elevation’ ..
‘Thermals’ don’t appear over water … ask any one who pilots a glider …
. …

Nik

If CO2 fertilisation leads to more transpiring plant area, then what does that mean for future cloud formation?

deforestation is more relevant than co2 imo. co2 is earths way of saying plants to grow more.

Dave Wendt

Very much OT, but from the hashtag #Zombieboondoggleapocalypse
http://hotair.com/archives/2014/05/16/congress-once-again-circling-back-to-the-wind-production-tax-credit-that-refuses-to-quit/
Call your representatives!

richardscourtney

Friends:
Slowly but surely examination of the Svensmark Hypothesis is happening. And if that hypothesis is determined to be true then it would dramatically alter climate models and not merely “tweek” them.
Last year WUWT ran a thread titled
Svensmark’s cosmic ray theory of clouds and global warming looks to be confirmed.
On September 4, 2013 at 9:53 am in that thread I wrote at here a post that said

Elevation of the Svensmark Hypothesis to be the Svensmark Theory would open up entire new fields of climate physics. That elevation requires that the experimental observation can be understood theoretically.

That theoretical understanding required more knowledge of the chemistry of cloud nucleation and the above article reports

New research from scientists at the CLOUD (Cosmics Leaving OUtdoor Droplets) experiment at CERN, including Carnegie Mellon University’s Neil Donahue, sheds light on new-particle formation — the very first step of cloud formation and a critical component of climate models. The findings, published in the May 16 issue of Science, closely match observations in the atmosphere and can help make climate prediction models more accurate.

We are almost there!
Richard
PS I also draw attention to a WUWT article from 2011 titled
BREAKING NEWS – CERN Experiment Confirms Cosmic Rays Influence Cloud Seeds
The associated thread of that article includes this comment from me which explains the importance of the matter.

cnxtim

Smartest bloke i ever knew MJO said ” it rains more on the green bits than the brown bits”

Mac the Knife

Really fascinating work!!!
I’d like to see a ‘lighting strike’ simulation added to the chamber atmosphere(s) described above, both before significant nucleation and after it is well developed. How would this effect the organic compounds (+/- alpha pinene isomers, sulphur compound, and free radicals -OH and -NO3), their resulting reaction products and reaction rates?
Then repeat the same series of experiments, with a magnetic field applied to simulate atmospheric charge build up to just below the lightning discharge level and re-evaluate reaction products and rates..
Run each group of experiments at sea level, 3000 feet, and 30,000 feet equivalent atmospheric pressures to simulate effects at ‘lightning ground strike’, typical cumulonimbus cloud base, and near cumulonimbus cloud top levels for the average summer afternoon ‘thundershower’.
This is very, very interesting work, opening up many new avenues to better understanding of our ‘settled science’ (/sarc….) dynamic planetary atmosphere!
There are some other interesting attributes to alpha-pinene mentioned here : http://en.wikipedia.org/wiki/Alpha-Pinene
It is purported to have both anti-inflammatory and a broad spectrum antibiotic properties. It is also reported to have a ‘bronchodilator effect’ on humans at low levels. I wonder if that is part of the reason we feel like we can ‘breathe better and get more oxygen’, when we take a stroll in a forest on a hot summer day?
Mac

joe

“Of course, most daily forecast meteorologists that watch satellite and radar have known this for decades, but it is nice to see climate science catching up.”
Likewise, fisherman have known about the ocean oscillations for centuries, and climate scientists have known about them for a least a couple of decades, yet they still don’t acknowledge them in the climate models nor do they attribute any of the warming from the 80’s/90’s to the ocean oscillations or the current pause in the warming. Nor do the attribute any of the prior warming and cooling/pause cycles to the ocean oscillations. Seems the climate scientists are much smarter than us mere mortals.

Mac the Knife

Nik says:
May 16, 2014 at 11:08 am
If CO2 fertilisation leads to more transpiring plant area, then what does that mean for future cloud formation?
Nik,
Good thought experiment!
But what is/are the reaction limiting input species (alpha pinenes, sulphur compounds, preferred organic/inorganic dust nucleation sites, free radicals, etc)? And how do other typical atmospheric effects (pressure, temperature, humidity, clear skies vs cloudy, night vs day solar radiation effects) alter the nucleation events, organic reactions, and final products?
Thanks for ‘the next step’ thought!
Mac

Walt The Physicist

This publication is a great example of current ridiculous state of science resulting from “organizing” science into labs, institutes and “research” universities and from strangeness of the public funding system feeding this organized science. An article with more than 50 authors reporting study of water condensation “under well-controlled laboratory conditions” using multimillion dollar equipment originally designed for nuclear research?! This is when current theory of evaporation is completely inadequate and actually contradicts to the current theory of condensed matter?! And look, they had to invent this sexy name, Cosmics Leaving Outdoor Droplets”, to attract funding, as plainly stating that they will study formation of water droplets from water vapor in air would be too “boring”. Fellas like this with their shysterism and shenanigans make life of real scientists very difficult. These Nature, Science, Cell, Scientific Merican publish mostly garbage.

Walt The Physicist

@Mac the Knife says:
May 16, 2014 at 12:22 pm
You must be kidding! You want your taxes double?

BioBob

> cnxtim says: ” it rains more on the green bits than the brown bits”
but perhaps it should be said as “more rain equals the green bits and less the brown bits and then it rains more on the green bits than the brown bits”
could be that it’s just not that simple…

Doug Proctor

Okay. We already know that clearing the base of Kilamanjaro was responsible for the loss of ice on the ice mass at the summit (if it ain’t flowing, it ain’t a glacier, it’s a stagnant ice mass). So when the Northern Hemisphere was clearcut by various human groups, and then regrew during slow times in planetary destruction, do we see rainfall patterns changing?
I’d thought the oceans were primary sources of nuclei, but perhaps they are not a few miles inland: any living along the west US coast knows the fogs don’t go far inland. Perhaps the nuclei, though in theory loft high above, don’t either.
A theory will multiple areas and times for testing.

Gamecock

“The findings, published in the May 16 issue of Science, closely match observations in the atmosphere and can help make climate prediction models more accurate.”
More accurate than WHAT ?!?!
Past models weren’t accurate? How decadent Man has become that we can spend a trillion dollars on climate nonsense.

Gores Chakra

I would like to suggest an approach to unsettle this “settled science” BS. Immediately defund all “settle science” research. After all, it’s settled – why spend anymore on it?

LT

Joe
In the field of climate science, oceans are only allowed to enforce negative feedbacks, lol..

“We had no idea a year ago that this chemistry was happening. There’s a whole branch of oxidation chemistry that we didn’t really understand. It’s an exciting time.”
But NOW we kow everything.

Bruce Cobb

“The findings, published in Science, closely match observations in the atmosphere and can help make climate prediction models more accurate.”
Doubtful, but miracles sometimes occur. One thing is certain; it would be impossible for them to make the GCMs any less accurate.

Jeff

“Doug Proctor says:
May 16, 2014 at 12:56 pm”
Being an ancient Californian, I remember the weather in the Central Valley and California in general changed when I-5 (major north-south interstate freeway/motorway) was built/expanded way back when. At least it SEEMED to change….
Somehow I wonder if all the changes we make somehow end up cancelling each other out… just hope that the water situation in CA gets straightenend out (I know, I know….)…

NikFromNYC

This means that the literal smell of pine forests is forming clouds as the highly volatile pinene and related terpene molecules raise into the atmosphere, and are oxidized by photochemically-generated hydroxy radicals to make poly carboxylic acids that can then form complexes with sulfate ions act as templates for water condensation into cloud water droplets. Here is a quick graphic of the chemistry:
http://oi61.tinypic.com/2cg0nz7.jpg
They ran mass spectroscopy to show how specific small complexes, likely hydrogen bonded, between a specific carboxylic acid laden molecule and some sulfates inside of their big CLOUD machine that simulates the atmosphere.

NikFromNYC

…in other news, scientists discover the urban heat island (UHI) effect on thermometers within a climate model:
“Excess heat from air conditioners causes higher nighttime temperatures”
http://phys.org/news/2014-05-excess-air-conditioners-higher-nighttime.html

Stephen Skinner

“Cloud droplets form when water vapor in the atmosphere condenses onto tiny particles. These particles are emitted directly from natural sources or human activity, or they form from precursors emitted originally as gaseous pollutants. The transformation of gas molecules into clusters and then into particles, a process called nucleation, produces more than half of the particles that seed cloud formation around the world today. But the mechanisms underlying nucleation remain unclear.”
There is no mention of Dew Point or Condensation Level or how saturated air is? I think this explanation seems very complex and I cannot tell if any references have been made to long standing knowledge on cloud formation or that old profession of Meteorology. Here in the UK we will get clouds regularly and the dew point / condensation level is distinctive as shown by the flat underside of the clouds. The amount of clouds depends on moisture in the ground and/or air and having unstable air (cold air mass passing over warm ground) so that thermals form. On the rare days that we get thermals and no clouds it would appear the air is too dry.

D.J. Hawkins

Mac the Knife says:
May 16, 2014 at 12:40 pm
Nik says:
May 16, 2014 at 11:08 am
If CO2 fertilisation leads to more transpiring plant area, then what does that mean for future cloud formation?
Nik,
Good thought experiment!
But what is/are the reaction limiting input species (alpha pinenes, sulphur compounds, preferred organic/inorganic dust nucleation sites, free radicals, etc)? And how do other typical atmospheric effects (pressure, temperature, humidity, clear skies vs cloudy, night vs day solar radiation effects) alter the nucleation events, organic reactions, and final products?
Thanks for ‘the next step’ thought!
Mac
Let’s not forget that one of the reasons for the “greening” effect of increased CO2 is the reduction of stoma density in leaf structures which reduces evapotranspiration. In a general region “R” with plant coverage of “A0” at “t0”, after greening to “A1” at “t1”, what is the net change in evapotranspiration over region “R”?

D.J. Hawkins

Uh oh, lost a / on the blockquote. Mods, help!

TrueNorthist

It just occurred to me; trees and plants are causing an awful lot of climate change. Burn them!

Tom O

I lived in Maine for the better part of 40 years. Lots of forests there, or at least was when I lived there. Strange thing. I can remember going out hiking in and around the forests many, many times, but I don’t recall looking up and seeing clouds forming over the forests. I do recall see the clouds forming on the rising side of hills and mountains – okay, what we called mountains would be called hills in some places – but I don’t recall seeing them forming over just the forests and no place else. So I have to ask, is this “affect” in the pictures over the forests in the Amazon just something that happens in the tropics? does anyone know if there usually are clouds forming over the Redwood forests? Something in this article just doesn’t quite make sense to me, though there are people, I suppose, that would say “so what’s different about this article as nothing seems to make much sense to you.”

Stephen Skinner

“These clouds are almost certainly a result of evapotranspiration. The clouds are distributed evenly across the forest, but no clouds formed over the Amazon River and its floodplain, where there is no tall canopy of trees. While water may evaporate from the Amazon River itself, the air near the ground is too warm for clouds to form. Trees, on the other hand, release water vapor at higher levels of the atmosphere, so the water vapor more quickly reaches an altitude where the air is cool enough for clouds to form.”
Someone needs to do a spot of gliding and read some books on meteorology. “The air near the ground is too warm for clouds to form”? What?. Air that is hot will expand and become buoyant and at some point break away from the ground as a thermal. If the air is very hot and very dry then either no clouds will form or they will form very high. In this case air over a river, particularly one in the Amazon will not be dry so there is plenty of moisture for clouds! But thermals tend not to form over water as too much heat is lost to evaporation and the air above the water never warms sufficiently. IMHO

Stephen Skinner

“Trees, on the other hand, release water vapor at higher levels of the atmosphere, so the water vapor more quickly reaches an altitude where the air is cool enough for clouds to form.”
Is this for real?

Mac the Knife

Walt The Physicist says:
May 16, 2014 at 12:42 pm
@Mac the Knife says:
May 16, 2014 at 12:22 pm
You must be kidding! You want your taxes double?
Walt,
I’m not kidding at all! It doesn’t require new taxes, unless you accept the Progressive agenda on taxes of “We Want MORE!”
It requires redirection of existing taxes that are spent wastefully. No more ‘Cash for Clunkers’ programs or Pigford Settlement ‘farmer’ payments to people that had never farmed. No more subsidies for solar/wind energies, cow and termite flatulence ‘studies’,….. or ‘free Obama phones’. No more government forcing banks to provide home loans to folks that can barely make the first payment with ‘no money down’, and then ‘bailing them out’ after the crash and holding government run inquisitions after the fact to progressively pretend they weren’t the root cause of the whole damn fiasco.
Stop pissing taxpayer money away on things that create greater human dependency on government. Stop pissing money away on developing the next ‘big scare’ program designed to progressively herd citizens in the ‘settled science and consensus’ social engineering direction!
Just Stop Pi$$ing Money Away On Bull$hit. We’ll have plenty of funds available then for real ‘hypothesize and experiment to confirm or deny’ hard science.
Mac

Stephen Skinner

Jeff Krob says:
May 16, 2014 at 10:39 am
“Well…I’d say the moisture content of the boundary layer air is fairly uniform. The convection,…”
I should have read your comment before writing mine.

Mac the Knife

D.J. Hawkins says:
May 16, 2014 at 2:17 pm
Let’s not forget that one of the reasons for the “greening” effect of increased CO2 is the reduction of stoma density in leaf structures which reduces evapotranspiration. In a general region “R” with plant coverage of “A0″ at “t0″, after greening to “A1″ at “t1″, what is the net change in evapotranspiration over region “R”?
D.J.,
That is why I included the following line in my comment to Nik, to address the very large and multiple source variable ‘humidity’….
And how do other typical atmospheric effects (pressure, temperature, humidity, clear skies vs cloudy, night vs day solar radiation effects) alter the nucleation events, organic reactions, and final products?
Mac

taxed

This effect is real, as l have seen it myself.
Where l live we have woodland cover on top of a low ridge of hills. Once in a while on misty mornings when the air is still and damp. l have seen the only clouds in the sky sitting these wooded hills. The clouds where very low and small only 500 to 1000 feet above the wood, but it was only above the woods where these clouds formed.

taxed

Should have posted “sitting above these wooded hills”.

Stephen Skinner says:
May 16, 2014 at 3:24 pm
Jeff Krob says:
May 16, 2014 at 10:39 am
“Well…I’d say the moisture content of the boundary layer air is fairly uniform. The convection,…”
I should have read your comment before writing mine.
Hey, no problem – glad there was someone to back up my observation 🙂

Walt The Physicist

@Mac the Knife says:
May 16, 2014 at 3:19 pm
Yes we must stop wasting our money to support shysters. However, how to set up an efficient science funding system? Current system works as described in Leo Szillard’s “Mark Gable Foundation” story – retarding science and looks pretty corrupt. What’s instead? I would suggest pre-WWII system – private companies do fundamental science for own sake and wealthy private individuals support scientific research in the Universities. This should go with lower taxes. Get rid of NSF, NIH, DITRA, etc.

ROM

Jim says @ 11.06am
re: Philip says May 16, 2014 at 10:08 am
Hmm … I might have put the difference down to differences in elevation’ ..
‘Thermals’ don’t appear over water … ask any one who pilots a glider …
_________________________________
With some 3000 hours in gliders over some 50 years I say that “Jim” has that about right.
Trees as in a collective forest or scrublands, trap heat below their canopies through the day.
That heat will be released as the air temperature cools towards evening [ “evening thermals” which can continue well after dark sometimes ] or if the temperature contrasts with the tree free or low tree concentrations areas or the air temperatures above the tree canopy change during the day or reach the critical contrasts in temperature that triggers thermal activity as seen from the speckled cloud array in the photo.
All Clouds of course from the bottom of the atmosphere to the high cirrus type clouds seen when frontal type systems are approaching or forming, are just an outcome of the moisture in rising and therefore cooling air, condensing out into cloud droplets and ice crystals at altitude, when the temperature at altitude gets down to the water vapour condensation temperatures.
The speckled cloud array in the photo are the clouds created by and sitting on top of the warm air thermals and thermal sources originating from the warm air pools in the forest canopy below.
Anybody who has ridden in the back of an open topped vehicle on a calm hot day will have experienced the rapid changes of air temperatures of a couple of degrees over a few tens of metres whilst the vehlcle is travelling only to have it change back again another couple of hundreds of metres further along.
These are the very localised differences in air temperatures that act as triggers for thermals which are nothing more than pools of warner and therefore slightly lighter air masses trying to rise through pools of slightly cooler and therefore denser heavier air masses.
Where air goes up, air also has to come down.
So when you have a a whole forest area covered with closely arrayed thermal sources, there will be some heavy sink holes in glider pilot parlance, ie; quickly sinking areas of air where there are very low contrasts in temperature and terrain and therefore low numbers of thermal triggering ie contrasting local air temperature sources of cooler and warmer patches of air which act to trigger thermals and therefore clouds.
The rivers that are clear of clouds in the photo have a very low contrast in temperatures across their expanse and therefore have little in the way of very local contrasting temperatures that can act as thermal trigger sources. So as in this case, they are the areas of sinking air which then flows back under the tree canopies to replace the air rising in those cloud topped thermal columns.
Of course these rising air and sinking air areas don’t need to be strictly local as I have experienced very heavy sustained sink everywhere in totally clear air, not a cloud anywhere, when I was launched from our local field in half a dozen successive very short flights over a couple of hours. Whilst some 100 kms away, there was a long line of massive thunderstorms with their storm cells of very fast rising air.
The total air mass over some hundreds of square kilometres in area was rising that 100 kms away whilst it was sinking over some hundreds of square kilometres where I was trying to fly
There is no doubt that trees as in a forest or scrubland do trigger rain.
Where there is definite and sharply defined edge between the large cleared farming areas in low rainfall [ 14″ / 350 mms annual rainfall or less ] areas and SE Australian scrublands I know of anecdotal evidence where the local farmers along the edge of the cleared scrubline can point to within metres where the showers come out of the scrub into the vast open cleared paddocks and then just stop usually some tens of metres out side of the scrub line in the cleared paddock.
And that phenomena has been consistent over decades.
It is on this very simple system of slightly different heating and cooling rates and the consequent slightly contrasting in temperatures between adjacent air masses and their consequent changes in density relative to one another on every scale which leads the rising and descending of those air masses as they mix and interact with one another that our entire global weather systems of every type and form are created from

BioBob says:
but perhaps it should be said as “more rain equals the green bits and less the brown bits and then it rains more on the green bits than the brown bits”
I kind of like the idea that plants bring their own water with them as this suggests:
http://curryja.files.wordpress.com/2014/04/water_cycle_on_land.gif
From the post: ‘Forest climate and condensation’