The Rainmakers

Guest Post by Willis Eschenbach

For more than a decade now I’ve been saying something without getting much agreement, which was:

“When you cut down the trees, you cut down the clouds”.

I based my saying on my own experience, first growing up in a ponderosa pine and fir forest, and later living in a redwood forest for half of the last thirty years. My theory was that the trees created the rain in several ways.

First, they “transpire”, meaning that they release water into the atmosphere. And not just a little water. Lots.

Next, they absorb sunlight and use it to drive chemical processes through photosynthesis. This means that the sunlight is NOT turned into heat, which leaves the area cooler.

Next, they shade the ground, again cooling the surface and the local area. This allows the surface to stay moist, increasing the amount of water available for the plants to transpire.

Finally, some of the plants rake the fog out of the air, collecting it on their surface. From there it drops to the ground, watering the forest.

However, I’ve never had a scrap of evidence to support my theory that if you cut down the trees, you cut down the clouds. So I was very happy to find the following article in Science magazine:

amazon clouds

Clouds over the Amazon.

Trees in the Amazon make their own rain

By Ilima LoomisAug. 4, 2017 , 2:45 PM

The Amazon rainforest is home to strange weather. One peculiarity is that rains begin 2 to 3 months before seasonal winds start to bring in moist air from the ocean. Now, researchers say they have finally figured out where this early moisture comes from: the trees themselves.

The study provides concrete data for something scientists had theorized for a long time, says Michael Keller, a forest ecologist and research scientist for the U.S. Forest Service based in Pasadena, California, who was not involved with the work. The evidence the team provides, he says, is “the smoking gun.”

Previous research showed early accumulation of moisture in the atmosphere over the Amazon, but scientists weren’t sure why. “All you can see is the water vapor, but you don’t know where it comes from,” says Rong Fu, a climate scientist at the University of California, Los Angeles. Satellite data showed that the increase coincided with a “greening” of the rainforest, or an increase in fresh leaves, leading researchers to suspect the moisture might be water vapor released during photosynthesis. In a process called transpiration, plants release water vapor from small pores on the underside of their leaves.

Fu thought it was possible that plants were releasing enough moisture to build low-level clouds over the Amazon. But she needed to explicitly connect the moisture to the tropical forest.

So Fu and her colleagues observed water vapor over the Amazon with NASA’s Aura satellite, a spacecraft dedicated to studying the chemistry of Earth’s atmosphere. Moisture that evaporates from the ocean tends to be lighter than water vapor released into the atmosphere by plants. That’s because during evaporation, water molecules containing deuterium, a heavy isotope of hydrogen made of one proton and one neutron, get left behind in the ocean. By contrast, in transpiration, plants simply suck water out of the soil and push it into the air without changing its isotopic composition.

Aura found that the early moisture accumulating over the rainforest was high in deuterium—“too high to be explained by water vapor from the ocean,” Fu says. What’s more, the deuterium content was highest at the end of the Amazon’s dry season, during the “greening” period when photosynthesis was strongest.

In looking around while writing this post, I also find this very interesting file from the National Academies, which says in part:

Certain trees are highly adapted to harvest fog water—some, like the Californian redwood trees, satisfy the majority of their water needs in this way. The trees create a physical barrier that intercepts and precipitates fog that would otherwise rise and dissipate in the warm air. In doing so, the trees create a localized water cycle: The fog water collected on leaves drips down and nourishes grasses, shrubs, and other plants that in turn trap their own water. All this dripping water sinks into the ground, filling wells and giving rise to small streams that people can use.

In addition, the redwoods provide the lovely sound of dripping rain, that marvelous music of watery wealth, even when there is no rain falling … what’s not to like? Here’s a photo I just took of the source of our liquid musical accompaniment, with my house in the foreground …

redwoods plus umbrella

Finally, that same article that discusses the redwood trees is mostly about collecting water from fog nets. This is something I’ve discussed before in the context of the “no-regrets” option for responding to possible future climate change. See my post “Harvesting Fog: The No-Regrets Option” for one example.

My point was that IF someone wants to fight the eeevil CO2, they should do things that bring us value whether or not CO2 is the culprit. In that post, I said that if you are concerned for example about future droughts, do something now to fix the effects of present droughts. That way, you’ve taken a step that will help regardless of the future. It obeys the doctor’s maxim, “First, do no harm” …

Overall? It’s a lovely summer’s day, I’m back from the gold mines, we’re winning the climate battle, and life is good.

Best to each of you, sunshine and following winds …


My Usual Request: Misunderstandings start easily and can last forever. I politely request that commenters QUOTE THE EXACT WORDS YOU DISAGREE WITH, so we can all understand your objection.


126 thoughts on “The Rainmakers

  1. Exhibit A:

    Ice cap on Kilimanjaro, shrunken not because of CO2 or temperature change, but from deforestation downslope.

    Algore’s bad. Again.

  2. The 19th century settled science directing political, economic, and immigration policy was that “The Rain Follows the Plow”. Send people unwanted in the settled East (U.S.) to bust sod in the wide open prairie (A.K.A. Great American Desert) and the tilling of the soil would release some sort of catalyst that would bring rains.

    We really believed that.

    MAYBE the idea was that the sod-busters also planted decorative and wind-breaking trees around the prairie homestead and a few decades later, the rains would follow. But that’s not what we based immigration policy upon.

    Anyhow, I had the impression it was settled science that forests offered “transportation” to mid-altitude moisture from the coasts over the mountains, but I can’t find the decade-old references at the moment. I’m sure the idea is not wholly novel. But it does bear publicity.

    • Same in South Australia at around the same time. And it worked for a few years too. Then it turned out that it was just a period of above average rainfall. The government then commissioned research resulting in the Goyder line North of which agriculture was deemed unsustainable. That line, actually a curve, proved remarkably accurate and durable.

  3. Coastal forests are part of what is called the Biotic pump which is well written about. The trees by attracting coastal moisture and also releasing it allows the moisture to be progressively “pumped” inland.
    The need to harvest timber for the building of houses etc, for populations concentrated along coastlines, and then for boat building resulted in coastal forests firstly becoming depleted and then gradually extending the cutting down further and further inland disrupting the Biotic pump and changing the climates on a regional scale. All this began before the Industrial Age particularly when people started to take to the oceans in large numbers in larger and larger vessels, once the fear of falling off the edge of the world proved to be a nonsense. For some reason the alarmists and their tipping point keeps reminding of that time in history.

    • Falling off the edge of the earth was not a concern from at least AD 600 in the Christian world, if not earlier, and from c. 600 BC in the pagan world. Some Early Church Fathers, following the Bible, did insist that the earth is flat, but long before 1492, the Church adopted spherical pagan Greek and Roman cosmology. And mariners could see that Earth is curved.

      • This actually is for “Marv August 8, 2017 at 6:14 pm” I liked the article you linked to that told of the amphoras in the Brazilian bay/harbor, it is a good well-I’ll-be, but if I’m understanding correctly, the hypothesized chain of events is, a Roman ship is blown off course and far westward, the Roman vessel makes landfall in the area of Rio, the occupants of the ship excavate the reef enough to bring the ship into the harbor, the ship then sinks in the harbor for whatever reason. If this chain of events is correct, then the knowledge of the New World was not conveyed back to Europe in any way, shape or fashion. It’s not like they could pick up a phone and let relatives know what they found, or even radio in the news. So as far as European history is concerned, this is sort of a non-event. Kind of like if we ever find the remains of Amelia Earhart, it would be interesting news, but Ms. Earhart would still be dead. And if any Roman did return to Europe with news of a land overseas, would anyone in that time and place have cared?

      • @ Gloateus August 8, 2017 at 2:57 pm

        Falling off the edge of the earth was not a concern from at least AD 600 in the Christian world, if not earlier, and from c. 600 BC in the pagan world.

        Probably not a concern of the “heathens” as far back as c. 1800 BC, …… and/or for sure, as far back as c. 1200 BC.

        Hipparchus (190-120 BC), was a Greek astronomer, geographer, and mathematician of the Hellenistic period. ……. Hipparchus is considered the greatest ancient astronomical observer. He was the first whose quantitative and accurate models for the motion of the Sun and Moon survive. He developed trigonometry and constructed trigonometric tables, and he solved several problems of spherical trigonometry. With his solar and lunar theories and his trigonometry, he may have been the first to develop a reliable method to predict solar eclipses

        And Hipparchus learned from the historical records of the Mesopotamians and Babylonians, to wit:

        According to Asger Aaboe, the origins of Western astronomy can be found in Mesopotamia, and all Western efforts in the exact sciences are descendants in direct line from the work of the late Babylonian astronomers (c. 1800 BC), continuing through the Middle Babylonian and Middle Assyrian periods (c. 1200 BC).[

    • In his 1267 “Greater Work”, Roger Bacon suggested that the distance from Spain to India wasn’t great. He, like Columbus, underestimated the size of the earth and overestimated the eastward extent of Asia.

      In 1291, Vivaldi brothers of Genoa tried to reach the East by sailing (in galleys) West, but disappeared somewhere on the African coast, discouraging further attempts for some time.

      But the Turks’ conquest of Byzantium forced Italian and Iberian merchant adventurers to revive the idea.

      Portuguese and Spanish scholars opposed Columbus’ proposal not because of the shape of earth, which they knew to be spherical, but because of its size, which they also correctly knew to be larger than assumed by Columbus. What they didn’t know was that the Americas lie about where Columbus hoped to find Asia.

      Columbus had probably heard of land within sailing distance to the west while in Iceland and the Azores. Basque cod fishermen had also possibly been secretly visiting the Grand Banks of Newfoundland in the late 15th century. Columbus’ pilot was Basque.

      • The Vivaldis were accompanied by two Franciscan friars, who well may have read their fellow Franciscan Bacon’s book.

      • Thank you for the reminder of the Basque fishermen. When the Pilgrims landed in 1620 or so, the local Indians were well acquainted with white people thanks to those fishermen, I have read. The local Indians were happy to allow the whites to set up a trading colony because they planned on monopolizing the trade with the Europeans. This system worked for some decades until the whites became more numerous and starting buying up more land with money that the Indians needed for European goods. The only thing the Indians had to sell was animal pelts, and after a while this resource ran out. It is very hard to go back to clay pots and stone knives after having iron pots and steel knives.This led to King Philips War. A very interesting story.

      • Joel,

        The only issue with Basque fishermen is whether they were on the Grand Banks before Columbus. Plenty of historical and archaeological evidence shows them there after 1492, but had they secretly already discovered the cod riches of the Grand Banks before that date?

        The Spanish of course visited “Virginia” frequently during the Anglo-Spanish War of 1585-1604. The Jamestown colony included a Spanish spy. Massachusetts Indian Squanto, so important to the Pilgrims, was captured by English fishermen in 1614 to be sold in Spain as a slave, but somehow escaped and lived with an associate of John Smith. When he finally got back to his village, its population had been wiped out by the epidemic which had ravaged coastal New England tribes.

      • Phoenician traders included Ireland in their routes between 800 & 475BCE, founded Carthage & Cadiz as bases. (Claim to have founded Sidon & Tyre c. 2800BCE or before.) Some claim Joseph of Arimathea included the British Isles in his trade routes c. 30CE.

        From what I read, the flat earth theory was much more limited in currency than some educationists claim.

      • Yes, I was thinking it is doubly tragic that great swathes of old forest in Europe and the US are being fed into power stations in Yorkshire (shipped over in heavy-oil burning container ships) in the name of ‘saving the planet’. I think there needs to be an addendum to the saying ‘never attribute to malice what can be explained by incompetence’: ‘..but never attribute to incompetence what can be explained by greed’.

  4. And that’s not to mention terpenes:
    “Pine: The New Smell of Rain
    Terpenes are abundant in nature, produced by a wide variety of plants for an even wider variety of purposes. They’re a bit like LEGO structures, comprised of building blocks: a five-carbon molecule called isoprene gets linked to other isoprene units by the plant, resulting in countless different shapes with an enormous variety of properties. The compound studied in the Nature article was alpha-pinene, so named because it’s largely responsible for the pleasing odor of pine trees. More exotic terpenes are responsible for citrus scents as well as the smell of green apple, hops, and even some floral aromas.
    In the study, the researchers found that ozone—which is found in the upper atmosphere thanks to the ionizing action of cosmic rays—sticks to molecules of alpha-pinene, disrupting their otherwise-stable chemical bonds and transforming them into a more reactive structure. These highly-ozonated molecules can then react with one another and stick together to form heavier particles, which can serve as the aerosol base for a raindrop.”

  5. I don’t know where my comment went.
    I just wanted to mention terpenes produced by pine trees:
    “In the study, the researchers found that ozone—which is found in the upper atmosphere thanks to the ionizing action of cosmic rays—sticks to molecules of alpha-pinene, disrupting their otherwise-stable chemical bonds and transforming them into a more reactive structure. These highly-ozonated molecules can then react with one another and stick together to form heavier particles, which can serve as the aerosol base for a raindrop.”

    • Here’s another link linking tree produced aerosols with climate change.

      It seems that global greening results in more clouds, one way or another, and that should result in a cooler planet.

      If the greenies manage to drive CO2 down to below preindustrial levels, all the trees will die and the planet will get really hot.

      • Gloateus August 8, 2017 at 3:43 pm

        All trees are C3 plants.

        Okay … most plants are C3. link Here’s a link to a column by Matt Ridley on the benefits of CO2 and global greening.

      • Commie,

        My point is that thus trees benefit from CO2 enhancement more than the many C4 crop plants. It’s C4 and CAM plants which don’t need higher CO2.

        The most important C4 crop is corn, followed by sugar cane, sorghum and millet. Many weeds are also C4 plants, so the argument that more CO2 will fertilize weeds as well as crops is largely bogus.

        I guess I should have spelled that out.

  6. YES! Evapotranspiration is a HUGE part of surface temperature regulation AND sub mesoscale convection. Here in the formerly baked environ of Central Texas, I notice the congregation of carrion vultures and decided to conduct an experiment in evapotranspiration induced convection using the subjective measure of attracting vultures into the produced lift.
    I waited for a day where winds were slack and solar heating a maximum. Inducing simulated evapotranspiration was the next chore, I selected a 4 gallon per minute 4000 PSI pressure washer as the equipment as it can atomize water fully in a 100 F air temperature with a dew point below 45 F.

    Sure enough, pointing the point nozzle overhead resulted in an evaporation of about 34 pounds of water per minute with minimal horizontal dispersion. This induced an updraft of sufficient strength to intice said vultures to the area. These birds rely on convection for altitude gain to preserve glycogen levels. Efficiency is a competetive advantage especially in a primarily ketogenic species (compared to the metabolism of a hummingbird where conversion of carbohydrate to glycogen is much more efficient.
    I know vultures are also keen sensors of noise, a large explosion can attract their presence. The noise of the engine driving the pressure washer pump could have been a factor but as the engine noise is similar to most outdoor power equipment, it should be a minimal effect.

    I have repeated this experiment several times, each to the same effect. Evaporating water IS a significant inducer of low level convection and this convection is a great moderator of surface temperatures.
    This makes low level lapse rate deviate from the assumed standards in cells far smaller than those used in GCMs for many areas (1 km^2). While rain forests are expansive, they do not represent the typical oak/juniper motts common to the area of my residence. The ground water demands of the Ashe Juniper and Honey Bean Mesquite are well known, they survive the typical summer droughts by deep tap roots drawing on unconfined aquifers and as such, reduce stream flow and choke out grass species.

    • Amigo, on my planet that is awesome for two reasons.

      First, rather than theorizing, you DID THE EXPERIMENT. Whether the experiment is in the real world or in the mathematical world, you didn’t just say “here’s how it is”. You backed it up by an actual demonstration of how to do it.

      I get so frustrated with folks telling me I’m doing something wrong … but when I invite them to show us all how to do it right, they are afflicted by a curious shyness and sometimes even fade out like the Cheshire Cat. Mosh and 1sky1 are professionals in that regard, but they are far from the only offenders.

      Second, one of the least appreciated aspects of thunderstorms is that they are dual-fuel, plus they generate their own fuel. They can run on either temperature (hot air is less dense and rises) or moisture (as your experiment beautifully demonstrates, moist air is less dense and rises). Now, evaporation rises linearly with wind speed. So by generating winds around the base, evaporation goes through the roof, and that moist air drives the thunderstorm.

      This gives thunderstorms a unique quality. They are able to cool the surface to BELOW the temperature required for their initiation. This overshoot is what allows them to regulate the tropical temperatures.

      But I digress … thanks for the fascinating description of an excellent experiment.


      • This give me reason to revive/repair my 10 year-old pressure washer that’s been taking up space in the garden shed. :-)

      • … moist air is less dense and rises …

        It’s true but it feels wrong. Humidity feels heavy and oppressive to me. Cold dry air feels light and wonderful. WUWT?

  7. I recall my first realization that human activities were probably affecting regional climates.
    Flying at 33,000 feet over the landscape, there were manicured fields of crops from horizon to horizon. Hardly a copse of trees left.
    Broadscale land clearing has had much more influence on regional climates than CO2, imo.

    • That’s exactly what Roger Pielke Sr. has been saying for a long time. link Sadly for him, that’s not a message the CAGW alarmists can tolerate.

    • there are two towns called Horizon ? and they both have airports ? now thats a fanastic real life co-incidence.
      of course , unless you mean….
      there is really a double horizon which is probably due to refraction through water vapour.
      of course unless you mean…

    • It’s the unintended consequences that getcha.

      Ironically, fully half of American corn is used to make ethanol, to avoid fossil fuel use, and thereby supposedly to “fight” climate change. That’s about 50 million acres, which is more than the land area of the nine smallest American States, combined: Maryland, Vermont, New Hampshire, Massachusetts, New Jersey, Hawaii, Connecticut, Delaware and Rhode Island.

      BTW, in my experience, when yeast ferments carbohydrates to make alcohol, it also makes a lot of CO2. Does anyone know how much CO2 is released as a byproduct of making ethanol from corn?

  8. If you burn less petroleum the price goes down and somewhere someone else burns more because it is cheaper. If you plant a tree someone somewhere else in the world doesn’t cut one down. Therefore the best option for ameliorating a warming world is to plant trees. And if you are wrong and the world gets cold you have more firewood.

      • I believe your theory also goes a long way towards explaining why in Roman times Libya was the greatest grain producing region they controlled, but today is mostly Sahara. If trees and vegetation can create their own rain, it seems plausible that an expanding desert could create its own self-perpetuating drought.

  9. I was surprised by the technique used to distinguish between moisture evaporated versus transpired by plants. I’d had this “How did they do that” question when I saw the synopsis, and a wonderful WOW moment when saw the fundamental property on which it was based. Nerd heaven.

  10. Willis, beautiful post. We have a cabin in the Chattahoochee National Forest in north Georgia. Just below the Great Smokies National Park, so we also get summer ‘smoke’ at our ~3000 foot plus minus 500 altitude. So I researched it, and can add a bit of science to your post.
    All coniferous forests (boreal, the pines of the Smokeys) produce organic terpene aerosols (related to turpentine). All deciduous (oaks, maples, tulip poplar in our part of the Smokey’s) and tropical forests (Amazon) produce organic isoprene aerosols. Both are strong fog/cloud nucleators. Hence the summer Smokey smoke, which is really just forest self generated fog from the humid air. Your redwoods are creating and then using their own fog.
    And, just to round this observation out, the oceans do the same. The organic nucleator molecule is dimethylsulfide, produced by phytoplankton. So the more there is oceanic CO2 fertilization, the more the oceans produce their own albedo from cloud cover. Neat self regulation.
    The world is a grand, fascinating, self regulating place. Else we would not be here to blog about it.
    Highest regards

    • One explanation for Cretaceous heat and equanimity equator to poles is that its oceans were too hot for plankton over the tropics, thus reducing CCNs.

      • Those very high temperatures are rather dubious to put it mildly. They require ignoring all modern TEX86 data from the Red Sea (which are probably the best ones for very warm water). If one includes the Red Sea in the calibration paleotemperatures drops about 8 degrees C.
        Also there doesn’t seem to be any evidence of “plankton-free” Cretaceous seas.

    • Observation. I have seen a patch of the Atlantic several hundred miles long which was smoothed to the extent that wave breaking was suppressed. The wind must have been about Force 4 judging from the action in unsmoothed areas. No breaking waves. Less stratocu. Assumption: fewer salt/organic aerosols are generated by smoothed ocean areas. Inference: ‘smoothing’ pollution will lower ocean albedo.

      From the literature. Forest areas generate aerosols by various mechanisms, by trees, bacteria and fungi. Assumption: aerosols seed low level cloud, and without the forest the amount of low level cloud will decrease. Inference: reduced tree cover will lower land albedo.

      These effects will both contribute to global warming. I am prepared, if given sufficiently large grants, to investigate this matter. Please send money.


    • In USA’s arm-pit, subjective observations over more than a decade support the impression that tree canopy, and even heavy under-growth, hold moisture. Have even experienced it raining under the canopy but not in breaks…and, no, am not talking about breeze dislodging droplets of rain from earlier showers off of leaves. It is rare, but sometimes it actually rains under there as the air becomes super-saturated.

    • This is my 1st post. Great to see my Smoky Mountains as an example of large forested areas producing their own rain. Over 2 million acres between the National Park, Forest Service lands, and conservation easements alone still provide the vegetative trigger to fuel convection (not to forget neighboring states’ lands and vast areas in Forest Service management).
      Living near some of the wettest temperate areas, the western NC/east TN areas provide a great classroom of elevation impacts, vegetation, local winds, patterns, etc, especially for weather geeks, like myself.
      One of the greatest local weather forecasters (I share this with some jest), would have a daily summer forecast for Asheville, NC of 83F and a 30% chance of storms -almost everyday! And, most summer days he was not too far from reality. While he had little meteorological training, he spent enough time outdoors to understand the big picture.

  11. Singapore was very dry; when they stopped widespread clear-cutting and replanted new forests, the rains came. Or so I was told during a visit…

  12. “… whether or not CO2 is the culprit …”.
    “Culprit” implies causing some problem or defect.

  13. The ‘plant the trees, the water will come’ phenomena is also demonstrated someplace in Africa where there is an oasis in the middle of the desert – lot of trees, river, water, etc – in the center is a religious center – they have been slowly planting more trees on the outer edges of their borders and the trees are successfully living. The surrounding area once had trees that had been harvested long ago. Unfortunately I do not recall the specific location. We learned of it during a Rotary presentation.

    • I’ve been planting oak and apple trees in dry sand dunes, basically desert conditions, the problem has been dunes getting bone dry in top top 3 feet when the tree roots arent yet deep enough to hit groundwater. I found if I ring the tree seedling with flat rocks angled towards the center, it gets enough water if it rains once every few weeks. It doesn’t work if I angle sand towards the base of the seedling, because when light rain hits sand it doesn’t penetrate very far, then it evaporates because sand gets so hot during the day, and rain never gets to the roots. There probably is something conical on the market already that does the same thing more efficiently, I would think.

  14. from the Loomis piece “That’s because during evaporation, water molecules containing deuterium, a heavy isotope of hydrogen made of one proton and one neutron, get left behind in the ocean.” How much gets left behind? All of it, most of it, some of it? All the time, most of the time, some of the time?
    While it may be true, the reasons why are not obvious. Sure the simple fact that a water molecule with Deuterium instead of Hydrogen atom is very very slightly heavier is not a good enough reason. The energy to evaporate (heat of vaporization) cannot be that different. Water evaporates because the vibrations at the surface caused by increasing temperature eventually reach an amplitude causing the bond holding the water molecules together is break. The relative mass of the Duterium water molecule is not that much greater than an ordinary water molecule – percentage wise.

    • Water that evaporates from tropical oceans under goes millions of evaporation/condensation cycles before it finally reaches the poles. Each of those cycles fractionates a very small fraction of the isotopes that add up to a significant measurable difference. That measurable difference should be a function of the number of cycles (latitude dependent).

  15. I have long wondered if intentionally planting deep rooted trees would be useful in terms of freeing water sequestered underground. Planting lawns only accesses water in the top layer of the topsoil, so a lot of water that had previously been in the water cycle becomes trapped in the soil.

    • I am surrounded by forest. During some sunny days in the summer the creek will dry up as the trees suck up groundwater, and will be flowing again in the morning.

  16. Willis, it would have been nice to include a link –

    I am taking exception to a statement in the article, “during evaporation, water molecules containing deuterium, a heavy isotope of hydrogen made of one proton and one neutron, get left behind in the ocean. By contrast, in transpiration, plants simply suck water out of the soil and push it into the air without changing its isotopic composition… the early moisture accumulating over the rainforest was high in deuterium” Possible. But all water in the Amazon basin comes from rain, which is supposed to be poor in deuterium. Where do trees get their deuterium from? Does it just circulate over Amazon and never disssipate? Do trees have a secret source of deuterium?

    I speculate – neither Aura nor deuterium-water spectra being my areas of interest – that the satellite readings may have been misinterpreted. There is a difference between H2O and HDO infrared spectrum, but spectral lines are broadened by Doppler effect and therefore they depend on temperature. A wrong assumption about temperature may influence the result. I did find a methodology for Envisat – “The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument on Envisat measures high resolution infrared spectra from which HDO and H2 O concentrations can be retrieved from cloud–free scenes.” Note the “cloud-free” qualification; that explains “the early moisture” caveat in the first quote. It would be interesting to know if authors also measured the early moisture over oceans. Now a crazy speculation – the early moisture is mostly the water vapor left in the atmosphere overnight – maybe HDO condenses at a lower rate than H2O?

    My source was

  17. Over 5 decades ago, we were told that deforestation in Africa led to decreased precipitation there. I am little surprised that this theory is just get scientifically confirmed now.

    • I’m with you on this one….this new scientific discovery….is just common sense…..figured everyone knew already

  18. Trees belch out vast amounts of volatile molecules derived from isoprene. The organic photochemistry of conjugated poly-enes is rich and varied.

    I’d bet dimes for dollars that some such molecules, polarized by interaction with UV in sunlight, help in the nucleation and growth of pre-raindrop aerosols in air saturated with moisture. It’s just waiting for the grant money before someone can be bothered to “discover” it.

    • From memory, the idea that terpenes et al. serve as cloud nuclei is well established … hang on …

      Hey, check this out, an article on the VOC (volatile organic compounds) and rainfall in … wait for it … the Amazon Basin …


      • Thanks, Willis.
        Searching forward in time, Google Scholar lists only one newer citing article,
        “Volatile organic compounds and their importance in the environmental context”
        While the main article is in Spanish, I do speak chemical, and unfortunately it doesn’t seem to add much more. I would guess that studying aerosol development is not easy due to the cost of large equipment.

  19. Trees that tall around my house make me nervous, so I cut all of mine down. Doesn’t the fire hazard that close to your house bother you?

    • Yes, it’s a fire risk, but we’re in a redwood forest. They’re pretty fire-resistant, plus as noted they tend to stay wet. We get lots of fog here, if I’d taken the picture above two hours earlier all you’d have seen is gray.

      As to the height, they’re downwind of us. Around here the trees almost always blow down to the NE, which is the direction the picture was taken. Plus it’s rare for a redwood to fall. Pines and oaks, sure, but redwoons only very occasionally.

      Beyond that, well, I’m reminded of what a 70-year-old Aikido sensei once told me. He said, “If you are on the Golden Gate Bridge when the earthquake strikes, what use are your strength and flexibility? The best aikido is that when the fight starts on Third Street … you’re walking down Fifth Street”.

      My best to you,


    • I have the same concern. I have cut down a few trees that were close to an extension I am planning off the back of my house, but I did plant six more a little further out.

    • Thanks, Hans. It puts me to thinking about the global effect of all of the deforestation. Something around half of the prehistoric world’s forests were cut down by modern times, although some of that has regrown. This would assuredly reduce the amount of rainfall. However, wherever we see reduced rainfall we’re told it is the result of CO2 …

      Some day, alarmist climate scientists will stop being CO2 junkies. But for now, it seems they’ve got to have their CO2 fix in every explanation and every phenomenon …


  20. There is a historical path through a 1900s era white pine lumbering area by my cabin in Upper Michigan, and placards on the path said there used to be plenty of large 3 pound brook trout in the creek. There is no way such trout could live in the creek today, the water flow just isn’t high enough year to year (there are still small brook trout upstream) and the creek gets too warm in summer. I wonder if the creek flow was much higher before the great white pine forests were cut down, and this allowed the trout to flourish.

  21. Dubai is the classic example, as they made Dubai greener it got wetter. The drive across the mountains to Fujairah, has got greener and greener over the years.

    Those in California who plant desert gardens have got it seriously wrong

  22. Here in SE Australia in the west of the state of Victoria, the Mallee scrub lands were cleared for agriculture back at the beginning of the 20th century.
    Some large areas of scrub land ie “Big Desert” of some 1500 sq kilometres, the Sunset Country of NW Victoria and the Little Desert were left as the soils under those scrublands were too poor for even those early pioneers to clear and farm.

    Those farmers living and farming along the cleared farmlands that butt up against the edges of the Desert vegetation are quite adamant that there is a substantial rain fall drop off when they go more than a few hundred metres away from the desert vegatioin edge into the cleared farmlands.
    A couple have told me that in some places on their farms nearest the Desert edge they can quite literally walk along the leading edges of showers of rain coming out of the Desert and not get wet as the shower just stops abruptly at a very close and constant edge / line a few tens of metres out side of the desert vegetation edges and into the hundred year old cleared farm land.

    • The effect is probably particularly strong in Australia since Eucalypts are essentially natural CCN (Cloud Condensation Nuclei) makers.

  23. Rain-making cloud seeding experiments of the 20th century were just trying to mimic nature. But usually with the wrong CCN.

  24. I saw an example of the same process reducing available water in central Montana. In surveying most of a township (36 sq. mi) I used original notes from the late 1800s that noted water courses and timbered areas. I also visited with all the old timers I could find to discus their memories of boundary markers. About 4 years before my survey a large fire burned several thousand acres in the area. When I got there the streams mentioned in the old notes were active with new growth starting along them. The old timers said they had never seen water in most of the stream beds except in the spring. My theory is the pines matured throughout the early 1900s and eventually transpired most of the ground water. When the fires killed them there was enough to get to the surface again. The area was high rolling semi-arid (about 12 in./yr) mostly plains with patches of timber.

  25. Hi Willis: I expected your article to be at least in part based on your experience with islands in the South Pacific. This is only anecdotal, but the island of Kahoolawe, just off Maui in the Hawaiian islands and readily seen from my Maui home, was at one time covered with trees. But WWII occured and the island was used as a practice bombing range until the 1980s, completely denuding it. Bombing no longer occurs, but the island now gets very little, if any, of the daily island, tropical rains. But I listened to a talk from a person who lived on Maui, across from the readily seen Kahoolawe prior to WWII. She said the island used to be covered with trees, and clouds formed regularly over Kahoolawe with resulting rain, and she lamented that since the trees were destroyed such rain clouds no longer form. And it is true that rain clouds now seldom form over Kahoolowe. Additionally, the inhabitants of the neighboring Island of Lanai also attribute some of their rain to the trees covering the top of that island. Again, sorry, only anecdotal. Both Kahoolawe and Lanai are otherwise below the normal 4,000 foot cloud ceiling at this location.

    • Do trees make it rain?


      My experience in the Galapagos Islands, as everywhere else I’ve been, lived and worked, say yes, as well.

      In the Galapagos, there is essentially no topsoil. The “trees” are giant dandelions. But islands of the right geological age to sport such forests enjoy sufficient rainfall to support large animals. Those too young or too old, don’t.

      Rain follows the trees, not the plough.

  26. “Next, they absorb sunlight and use it to drive chemical processes through photosynthesis. This means that the sunlight is NOT turned into heat, which leaves the area cooler.”

    If true, this would be a powerful negative feedback on rising temperatures from CO2, since that added CO2 would stimulate plant growth and diminish the amount of radiative energy used to warm the surface that would have been exposed but for the new plant growth,

    • The idiotic, ignorant assumption of “climate science” charlatans and morons is that net feedbacks are positive. This is almost never the case on our homeostatic planet. Net feedbacks from more CO2 are almost certainly negative on this self-regulating world.

      Hence, the lab value of 1.2 degrees C temperature increase per doubling of CO2 is almost certainly less than that after net feedback effects. Possibly if not probably net cooling.

  27. The Greening of Ascension Island
    Two hundred years ago, Ascension Island was a barren volcanic island. Today, its peaks are covered by a lush tropical forest.
    Thanks to the experiment of Charles Darwin and his buddy Joseph Hooker. If it weren’t for the plantations of trees from Kew Gardens, the present population and permanent military base would be unsustainable because of the lack of water.

  28. CERN in Switzerland, through their CLOUD experiment, has shown that biogenic vapours emitted by trees and oxidised in the atmosphere have a significant impact on the formation of clouds, thus helping to cool the planet. These biogenic aerosols are what give forests seen from afar their characteristic blue haze. The CLOUD study shows that the oxidised biogenic vapours bind with sulphuric acid to form embryonic particles which can then grow to become the seeds on which cloud droplets can form. This result follows previous measurements from CLOUD showing that sulphuric acid alone could not form new particles in the atmosphere as had been previously assumed.


    • I was going to chime in pointing out terpene chemistry is important in cloud condensation nucleii formation. I have observed in Oz there is a very strong smell of Eucalptus oil after rainfall breaks a dry spell. Hard to say if that is because the trees emit more VOCs after rain, or rain washes out the oxidants that remove VOCs. I always thought it would be an interesting area of study, not without its challenges, but important given it could be a climate feedback, and therefore play a role in explaining rainfall pattern changes over the last 500 years of extensive deforestation. Silly me, its all due to dirty coal though isnt it.

  29. You’re not the first to observe this effect Willis. In 1776 the Main Ridge forest reserve on Tobago was created:

    “for the purpose of attracting frequent showers of rain upon which the fertility of lands in these climates doth entirely depend.”

    And in 1791 the Kings Hill Enclosure act was put into effect on St Vincent “to preserve the trees growing thereon in order to attract clouds and rain”.

    As far as I know those were the two first forest reserves in the Western Hemisphere (a centurey before Yellowstone). At least the Main Ridge reserve is still in effect (been there).

    And I have been told by locals in Guatemala that logging of the mountain forests there decreases the amount of rain.

  30. Trees make more of what they like. Rain. The produce both the moisture and the nucleus to form upon to enable more forrest thereby making the habitat they like spreading themselves.

  31. few years ago I was in the rice terraces in Southern China and was talking with a local about the water supply to the terraces – streams from higher up in the hills. He said that in the Mao era, they were forced to cut all of the trees on the upper slopes to provide charcoal for local iron-making furnaces. As a result, the streams stopped and rice crops failed as a result. They have since reforested the area and bingo, the stream flow recovered. Much reforestation happening in China, for the combination of water supply, slope stability and wood supply – they plant in terrain rows and harvest the same way, with rows of usually 3 ages.
    I expect there is a lot on forests and water supply in Chinese academic literature.

  32. This is a really great discussion and I want to raise a possible explanation for why N.Africa became a huge desert in the last ten thousand years. Since we know that what is now largely the Sahara desert was a rich grassland with water inhabited by rich game and people, it would seem reasonable to follow on with the reason for desertification being a fall in global CO2 levels leading to tree loss rather than people chopping trees down or drying up from wind pattern changes. Actually if the distance in time back to the end of the last ice age is greater than the age of the Sahara desert, CO2 depletion is a better explanation since globally there was more moisture in the air.

    The Egyptians may well have circumnavigated Africa according to one text of one of their voyages.
    But I love the image of Coke snorting pharaoh, unfortunately the mummies don’t seem to have eroded septums. Maybe they just mixed it in evening cocktails.

    • My thoughts are that it’s more to do with animal husbandry. I blame mostly reduction in trees in a marginal place, goats are by far the worst offenders of human livestock. They make deserts of near anything. Many islands in the Mediterranean Sea have also lost all their trees and behold become mostly deserts. The updrafts of humidity break up the air and make thundershowers. A good study would be to remove the goats and replant a few islands and see what happens.

      • Mydrrin August 9, 2017 at 8:58 am

        My thoughts are that it’s more to do with animal husbandry. I blame mostly reduction in trees in a marginal place, goats are by far the worst offenders of human livestock. They make deserts of near anything. Many islands in the Mediterranean Sea have also lost all their trees and behold become mostly deserts. The updrafts of humidity break up the air and make thundershowers. A good study would be to remove the goats and replant a few islands and see what happens.

        When I lived in Africa, I used to say that Africa would soon be a paradise if every day, every African killed one goat and planted one tree … people thought I was kidding.


    • Once upon a time i saw the following explanation (hypothesis) for the desertification of Sahara:
      Sometime between 11 an 13 thousand year ago some event (possibly a big asteroid collision ) the caused significant change in the orbit around and the mean distance of the moon from the Earth. This in turn lead to at change in the declination of the Earth’s rotational axis big enough to change the the hydrology of Africa and turn a previously lush green area into a desert within a couple of millennia. Do not remember on what the basis of data and facts for this was , but i remember it looked to me it might very possible the hypothesis was correct on the face of it.

  33. It would seem unnecessary to use such sophisticated methods to prove that the clouds/rain in the Amazon is mostly recirculated locally. The Amazon basin is unique in the world in having a very wet climate deep in the center of a continent. Ultimately the water comes from the ocean and returns there but it quite obviously is recycled several times on the way.

    The Amazon basin is unique in several ways. Originally it opened to the west and became wider in that direction as river basins mostly do. Then the Andes rose and blocked the original mouth and created a vast inland sea draining through a narrow gap to the east. This sea silted up and turned into a huge swampy dead-flat plain. Now the Amazon collects rain falling in the surrounding highlands all the way from the Caribbean to Mato Grosso which slowly meanders across that plain and finally makes it way to the Atlantic. But it has evaporated and rained back several times on the way.

    One only has to look at a satellite image like this (from EUMETSAT) to see how it works:

    Just “drag” the time on the right and watch how the thunderstorms build up during the day and rain out and collapse during the night. Every day, all the year, though the center shifts from the northern part of the basin in summer to the southern part in winter.

    By the way EUMETSAT happens to be located over the only other area on Earth with a somewhat similar geography and climate, but on a much smaller scale, i. e. the Congo basin.

  34. Funny. I had a college professor that simply stated as fact that trees make clouds (about 15 years ago).

    Next, they’ll realize that the millions are acres of corn in the Midwest cause thunderstorms.

    Either way, the total volume of water vapor isn’t the only factor. Many plants release cloud condensation nuclei.

    When we look at ocean iron fertilization (growing forests in the ocean) they make more clouds, too. Phytoplankton release cloud condensation nuclei.

    In theory, the perfect ocean iron fertilization program could have a huge return on investment. They restore fish populations, can make clouds to reduce droughts, and transport some amount of CO2 from the atmosphere to the bottom of the ocean.

  35. Photosynthesis does consume energy, however, from what I have read photosynthesis is only about 2 to 3% efficient.
    The color of the leaf compared to the color of the ground it is shading will have a much greater impact. If the leaf is darker than the ground, then the lower albedo will have a much greater impact than the tiny amount of energy converted by photosynthesis.

  36. Willis,
    The study of trees and evaporation has a long history in hydrology and this question is of great interest to water engineers. For example in this 1979 paper:-

    Results from the Institute of Hydrology’s hydrometeorological and hydrological studies on water use by forest and grassland confirm earlier predictions of a reduction in water yields following afforestation. This reduction is due primarily to the increased interception losses from forests. This paper shows how the water yield from uplands is related to the relative proportions of land under forest and hill farming, and estimates how water yields will change if a greater proportion of hill land is afforested.

    Calder, I.R. and Newson, M.D., 1979. LAND‐USE AND UPLAND WATER RESOURCES IN BRITAIN‐A STRATEGIC LOOK. JAWRA Journal of the American Water Resources Association, 15(6), pp.1628-1639.

    In the 1970s I and my fellow undergraduates visited the experimental forest lysimeter in the Stocks Reservoir catchment in the Forest of Bowland, Lancashire. Here we saw the first such experimental work in a representative British upland area devised by the late Frank Law, Engineer to the Fylde Water Board, which showed that the plantation fir trees “used” more water than the sedge sheep pasture of the hill farms on the surrounding fells.

    I cannot now be exactly certain of the tree species in the lysimeter, but I think that they were they were spruce (either Norway or Sitka). The explanation given to us for the fact that the land under the tree canopy produced less runoff into the reservoir was that the trees intercepted the rain and the water re-evaporated directly from the dense needles, rather than increased evapotranspiration. I later remembered this result, with its confirmation of the simple experience of any upland hill walker in the British Isles that if you want shelter from the rain then a dense spruce plantation is a good bet, when I saw the effect that a group of Wych Elms had on a sea mist on the east coast of England. In contrast with the dry floor of the spruce plantation on the Bowland Fells, the hairy twigs of the elms were literally stripping the moisture out of the mist so quickly that the water dripped to the ground almost as if it was raining.

    So the direct physical presence of the trees changes the moisture interaction of the air and the ground in different ways depending on the species and the climate circumstances. Dense needle firs can intercept the falling rain, return the water to the low level air and reduce soil moisture below the canopy, whereas other species, like the Wych Elm, can directly harvest moisture from a fog and so increase the water in the ground below the trees, as is observed in cloud forests.

  37. I’m with you Willis. I can’t believe you didn’t reference “The Man Who Planted Trees” to set the mood for your essay. GK

  38. On a wooded hill, the trees will slow the wind somewhat and thus increase the chance of rain or condensation. The cooler area in the shade below the trees might help. I wouldn’t know the name of such phenomena, absolutely not my area.

  39. So your saying we shouldn’t cut down forests for biofuel crops?

    I dunno how to begin calculating, but one would imagine that this can impact local/regional temperatures.

    Let’s look after the forests. Its just a smart thing to do.

  40. This seems to be related:

    “A small belt of two to three metre high saxaul trees (Haloxylon persicum) found inland to the south of Abu Dhabi has been dubbed the ‘dew forest’ on account of its foliage dripping condensation. The plant essentially waters itself, as any dew that so forms will drip onto the ground around the plant before it evaporates. This fog moisture is clearly of great importance to the saxaul because it has a highly restricted distribution locally, covering an area of something under 2000 square kilometres (less than 1 per cent of the total area of the UAE). ”

    So if you do visit the UAE, go out into the desert early in the morning, and marvel at the Abu Dhabi dew.

  41. Very informative article. I have a slightly different perspective on some aspects:

    “Next, [trees] shade the ground, again cooling the surface and the local area. This allows the surface to stay moist, increasing the amount of water available for the plants to transpire.”

    Does it make any difference if SWR is absorbed by trees or the ground below? Both transfer the energy they absorb (except for photosynthesis) into heat which they radiate, convect or conduct to the atmosphere. Some surfaces – like sand on a beach on a sunny day – can get much hotter than trees do and therefore radiate away more heat. Trees obviously have more surface area for conduction.

    Corn is supposed to be one of the most efficient plants are converting absorbed photons into chemical energy. It is about 1% efficient. So the other 99% of absorbed SWR becomes thermal energy.

    However, trees can reflect more SWR (which we perceive as their green color) than the ground. This cooling effect depends on the albedo of the ground beneath the trees.

    Once water has soaked into the ground, it is very hard for it to reach the surface again to evaporate. Tree roots take up ground water and move to leaves where it can evaporate. In the Amazon, tree roots reach 10 m below the surface into the water table.

    The roots from even grasses bring water up from a foot below the surface. Ancedotal sources suggest that the AOGCMs find two stable states for the Sahara desert: today’s desert or grasslands that return precipitation to the atmosphere as water vapor that can fall again rather than sink underground. As you probably know, about 5,000 years ago, the Sahara was green.

  42. When a river is dammed I’ve heard it told that the reservoir area receives less rainfall, frustrating engineers. This makes perfect sense, as a lush river bottom full of trees makes clouds above it, but a large area of water evaporates less …. counterintuitive as it seems like lakes evaporate more but they don’t … and in fact bodies of water create downdrafts and clear out clouds above the body of water. There are satellite photos of cumulous clouds over the Amazon and Great Lakes other areas and it always amazes me that the puffy cumulous clouds form above trees but above a river or lake there will be clear skies.

  43. “Now, researchers say they have finally figured out where this early moisture comes from: the trees themselves.” Duh. It didn’t take Einstein to figure that one out. This happens between Texas and Wisconsin dozens of times a year.

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