Smoke is Cooler than Clouds

Not long after arriving at NASA’s Goddard Space Flight Center, Ilan Koren reached a dead end in his first research project, or so it seemed. He was trying to find satellite images over the Amazon rainforest showing heavy smoke from burning vegetation mingling with low-level cumulus clouds. Once he found such scenes, Koren’s second objective was to use the satellite data to observe and measure how the smoke particles altered the clouds. The problem was that after spending days combing through NASA’s Terra and Aqua data sets, Koren could not find enough examples of smoke mingling with low-level cumulus clouds.

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  Photograph of smoke layer over Brazil

It was November 2002, and Koren had arrived at NASA only 3 months earlier, having just received his doctorate degree from the University of Tel Aviv, where he studied clouds. In school, Koren loved delving into the mysteries of how and why clouds form and change over time. On one level, he appreciated clouds for their beauty. On a deeper level, he recognized their power—clouds regulate the flow of radiant energy into and out of Earth’s climate system. “They are nature’s way of drawing in the sky the physics of what exactly is going on in the air,” he said. “If you can reduce their shapes and patterns to numbers and plug them into mathematical formulas, perhaps you can predict the weather, or even forecast changes in the climate.”


Smoke, interspersed with a few clouds, obscures the Brazilian landscape in this photograph. Ground-breaking NASA research is revealing new connections between smoke and the formation of clouds over the Amazon rainforest. These interactions alter the amount of sunlight reflected by the Earth, and may influence climate on a global scale. (Photograph courtesy USDA Forest Service Brazilfire)

  Photograph of Clouds over the Amazon from the International Space Station

This insight led Koren to focus his doctorate research on clouds. If he could help solve the physics problems of how and why clouds form and change over time, he could give the science community a key missing piece of the larger climate change puzzle: how much radiant energy do clouds trap within Earth’s atmosphere and how much do they reflect back out into space? Because there are so many different types of clouds, and because they form, move, change, and dissipate so rapidly over time and space, it is a difficult question to answer. NASA satellite technology has helped to carry cloud research forward in great leaps over the last several decades, and has revealed that as environmental conditions change, so too do clouds and their roles within the climate system. If scientists can’t get the cloud physics right in their models, they cannot predict how cloud changes will affect Earth’s total energy budget, and therefore they cannot accurately predict future climate change.

So, filled with enthusiasm, Koren came to NASA to join a team of physical scientists, led by Yoram Kaufman and Lorraine Remer, studying one particular way in which cloud changes influence climate. When tiny particles of pollution in the air (called “aerosols”) mingle with certain kinds of clouds they change the clouds’ properties, making them whiter, more reflective, and longer lasting, which enhances their ability to shade and cool the surface below them. Dubbed the “indirect effect of aerosols,” Kaufman and Remer wanted to find out just how much brightening and prolonging clouds enhanced their ability to reflect sunlight back to space.

They were using a new instrument called the Moderate Resolution Imaging Spectroradiometer (MODIS) that NASA had launched aboard both its Terra and Aqua satellites. Because it sees almost the entire surface of Earth every day, across a wide portion of the color spectrum, MODIS is an ideal tool for studying clouds and aerosols. Koren was very excited to work at NASA on such an interesting and important scientific problem, using some of the most advanced satellite technology in the world. Only, now that he had the opportunity, after days of searching he could not find enough examples in the data showing low-level cumulus clouds mingling with smoke plumes.


Clouds help regulate the Earth’s climate by reflecting sunlight into space, thus cooling the surface. When cloud patterns change, they modify the Earth’s energy balance in turn, and temperatures on the Earth’s surface. (NASA Astronaut Photograph ISS009-E-8627)

  Satellite Image of Smoke in the Amazon

“Specifically, I was looking for a ‘well-behaved’ cloud field,” Koren recalled. “The clouds had to sit on the same layer of atmosphere at roughly the same height. Also, they had to be the same in their convective properties. The clouds all had to be the same type, same size, same distance between them, and the same in their meteorological properties, both inside and outside the smoke.”


Satellite images of the Amazon rainforest rarely show smoke and cumulus clouds together. Smoke, mainly from agricultural fires, displaces the cumulus clouds that normally form above the forest each afternoon. (NASA image by Jesse Allen and Robert Simmon)

  Satellite Image of Clouds in the Amazon

The Amazon Basin was the perfect place to look. Every day, the skies over the rainforest produce widely scattered cumulus clouds sitting at the top of the boundary layer—the layer of atmosphere from the surface to anywhere from 1 to 3 kilometers high. These cumulus clouds are all about the same size, they are distributed in a very uniform way with good spacing in between, and they cover most of the Amazon jungle during the dry season in the afternoon. Moreover, during the dry season there from August through October, there are typically hundreds of intense fires burning across the region, producing thick plumes of smoke traveling for hundreds of kilometers. These were perfect conditions for studying how smoke affects clouds.

“Amazonia was almost too good to be true,” Koren said. “All I had to do was find areas where there was heavy smoke and these clouds. And it was puzzling that I could not find smoke and clouds together.”


A uniform layer of scattered cumulus clouds is typically present, along with some thunderstorms, over the Amazon rainforest. Compare this image of a day with little smoke, with the image above. Both images were acquired by the Moderate Resolution Imaging Spectroradiometer aboard NASA’s Aqua satellite, on August 11 (top) and November 15 (bottom), 2002. (NASA image by Jesse Allen and Robert Simmon)


Completely unexpected, Koren’s observation forced the team to step back from their assumptions and shift their attention to the bigger picture. Interestingly, the pattern of where there were no clouds often seemed to match the pattern of where there was smoke. In other words, where there was heavy smoke, the cumulus cloud cover went from an average of about 40 percent to zero!

Could the smoke somehow be suppressing the formation of clouds? Koren suspected the smoke was to blame. They noted that the disappearance of the clouds in the presence of the smoke reminded them of observations made by V. Ramanathan thousands of miles away in the Indian Ocean. During an experiment there, Ramanathan noticed that as he flew above the smoke he did not see many clouds. But in the south, beyond the reach of the smoke, clouds did exist. Was this cloud pattern due to differences in the meteorology of the region or due to the effect of smoke? At this point Koren changed the focus of his research to seek answers to these questions. He began by turning to the scientific literature to see if any other scientists had observed the same phenomenon and provided an explanation.


The data used in this study are available in one or more of NASA's Earth Science Data Centers.


Aerosols: Particles of Many Personalities


Koren’s search through the scientific literature yielded two leads. In a 1997 paper titled “The Missing Climate Forcing,” James Hansen, of NASA’s Goddard Institute for Space Studies, and co-authors described their efforts to construct a global climate model in which they accounted for various components in the Earth system that exert warming and cooling influences. The authors stated that, on a global scale, aerosols’ overall effect is to cool the planet. Even more substantial than aerosols’ indirect cooling effect (making clouds more reflective) is the way in which the tiny particles directly scatter and reflect incoming sunlight back to space. Called the “direct effect,” aerosols also cool by reducing the amount of sunlight reaching the surface. (See sidebar at right.)

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Graph of stratospheric aerosol optical 
depth and temperature

Aerosols in the stratosphere increase the Earth’s reflectivity, causing cooling. The eruptions of the El Chichon (1982) and Mount Pinatubo (1991) volcanoes placed sulfate aerosols in the stratosphere (the blue line represents stratospheric aerosol concentration). It took several years for the aerosols to be flushed out of the atmosphere, and during that time the Earth experienced slight cooling (the red line represents average global surface temperature). (Graph by Robert Simmon, based on data from the Goddard Institute for Space Studies)

In addition to the indirect and direct cooling effects of aerosols, Hansen and his co-authors also predicted a significant third way in which aerosols can influence climate. They called it the “semi-direct aerosol effect on clouds”—the process by which dark-colored aerosol particles (i.e., the soot in smoke) absorb incoming sunlight and warm the atmosphere relative to the temperature of the surface. The net effect of this atmospheric warming, they hypothesized, would be to reduce the upward movement of moisture and, in turn, reduce cloud cover. The authors’ theory seemed to match what Koren saw in the MODIS images over the Amazon, and what Ramanathan observed flying over the Indian Ocean.

Koren’s second lead was another paper published in 2000, entitled “Reduction of Tropical Cloudiness by Soot.” In that study Andy Ackerman, of NASA’s Ames Research Center, and co-authors used a computer model to demonstrate that energy-absorbing aerosols can have a semi-direct affect on cumulus clouds over the ocean. At the time he wrote his paper, Ackerman was unaware of Hansen’s paper and so he wasn’t familiar with Hansen’s term “semi-direct effect.” Instead, Ackerman described it as the “cloud-burning effect of soot.” But both groups of scientists described the basic underlying physics of the process in pretty much the same way: as the top of the boundary layer becomes filled with dark-colored particles (like soot), the aerosols absorb sunlight and warm the temperature of the air relative to the temperature of the surface. According to Ackerman, this heating at the top of the boundary layer burns away clouds in two ways: (1) by accelerating the process of evaporation of existing clouds, and (2) by suppressing the upward flow of moisture from the surface needed to form new clouds.

Comparison of atmospheric conditions with and without smoke

“Up to that point, most people were thinking about how aerosols [through both their direct and indirect effects] oppose the warming due to greenhouse gas forcing,” Ackerman recalled. “But we found aerosols can also amplify the greenhouse forcing. Fewer clouds means more solar energy enters the Earth system, which amplifies the warming effect of carbon dioxide.”

The papers confirmed Koren’s suspicions about what he observed in the Aqua satellite data acquired over the Amazon Basin—the smoke was suppressing the formation of low-level cumulus clouds. The next obvious question was just how strong an influence do aerosols exert on Earth’s radiant energy budget through their semi-direct effect? Pursuing the answer to this question put Koren on the path toward a new goal: measure the semi-direct effect of smoke aerosols on the Earth’s energy budget.


The presence of smoke over the Amazon alters the atmosphere in a number of ways that reduce cloud cover. In clear conditions (top) temperature (red line) gradually decreases with altitude, which in turn increases relative humidity (blue line). At the “boundary layer” relative humidity reaches 100 percent, and moisture condenses out of the air, forming clouds. Additionally, the trees transpire (during photosynthesis), releasing water vapor. Sunlight also warms the surface, creating warm air that rises, then cools and falls back to the surface in the process of convection.

When smoke is present, the dark soot particles absorb sunlight, heating the atmosphere and cooling the surface. This heating reduces relative humidity in the smoke layer, inhibiting the formation of clouds. Furthermore, plants reduce transpiration in response to the smoke, which lowers the amount of water vapor in the air. Because the smoke warms the air above the surface, and simultaneously cools the surface, the difference in temperature between the upper layer of air and air near the surface is reduced, limiting convection, and further discouraging cloud formation. (Graphs and Illustrations by Robert Simmon)


When Smoke Kills Clouds


Koren paced, tapping his chin thoughtfully with his fingertips as he searched for the right words to convey the challenges he faced in the next phase of his research. “It is one thing to see with your eyes that smoke kills clouds,” he began. “It’s another thing to measure it.”

First, the team had to make sure that the absence of clouds in the smoky areas was not due to differences in meteorology. They accessed weather data—temperature, humidity, wind speed and direction at various barometric pressure levels, etc.—from the National Centers for Environmental Prediction (NCEP). Once they determined what the average weather conditions were like over the Amazon Basin during the dry season in 2002, they then looked for days in which there were anomalous conditions, such as frontal systems passing through or other shifts in meteorology that might have influenced the cloud patterns. They did not analyze Aqua data on any of the days in which they saw anomalous weather. On most of the days, the NCEP weather data proved that the meteorological conditions were exactly the same in the smoky regions as they were in the cloudy regions. So they knew the smoke had to be the reason why average cumulus cloud cover dropped from forty percent to zero in the presence of heavy smoke.

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Graph of reduction in cloud cover due to smoke

“During the dry season,” Koren explained, “you can see clouds form in the morning in the east and by early afternoon the whole Amazon Basin—from the east to the Andes Mountains—is covered by cumulus clouds.” The main sources of humidity for forming these clouds are: windblown moisture carried by strong sea breezes from the Atlantic, and the upward transport of moisture from the surface. The steady flows of moisture into the air over the rainforest make it a very cloudy place, even during the dry season.


Satellite data for the western Amazon show a reduction in cloud cover with increasing smoke optical depth. Cloud cover is about 40 percent with no smoke, and is reduced near zero at a smoke optical depth of 1.2. (Optical depth is a measurement of the amount of smoke. An optical depth of 0.1 indicates a relatively clear day with visibility more than 40 km. An optical depth of 0.5 is quite hazy, and visibility is only about 10 km. An optical depth of 1.0 and above would virtually obscure the sky from view.)(Graph redrawn by Robert Simmon from Koren, 2004)

  Morning and afternoon satellite images showing cloud formation

Those clouds Koren had so loved back in college, however, became a major challenge when he returned his attention to the Aqua imagery. In order to determine which pixels in a given scene contained aerosols, he had to determine their “aerosol optical thickness,” which is a measure of how much sunlight the smoke prevented from traveling down through the column of atmosphere. This was nothing new; scientists have been accurately estimating aerosol optical thickness using satellite data for decades.

“But when measuring aerosol optical thickness, you want to make sure you don’t have cloud-contaminated pixels,” Koren said. “Normally, you’re willing to throw away pixels just to be sure you’re not analyzing aerosol data contaminated by clouds. But in this study, we had to be able to measure aerosol optical thickness in the presence of clouds, and so we had to make sure we were detecting the clouds in a very precise way in order to determine which pixels were cloudy, which were smoke, and which clear-air pixels were actually clear and not confused with shadow from the clouds.”

To help them better identify cloud-free pixels, and measure aerosol optical thickness in only those pixels, the team turned to Vanderlei Martins, a physical scientist at NASA visiting from Brazil. Martins wrote a new algorithm for using Aqua MODIS data to detect clouds especially for this investigation. Martins’ algorithm trained the computer to clearly distinguish between cloudy and smoky pixels, allowing the team to move forward in their investigation.

“For this project, we had to stretch our ability to estimate the smoke’s optical thickness even in cloudy skies.” Koren said. “We found that in fully cloudy conditions, the average aerosol optical thickness was very small.” In other words, there was very little or no smoke in the cloudy patches.

Once they knew which pixels contained cumulus clouds and which contained smoke, they could calculate how much of the area was covered by each. They fed these aerosol optical thickness and cloud coverage data into a computer model to determine how the changing atmospheric conditions affected the region’s radiant energy budget. The team was surprised by what the model showed.

Given a square patch of Amazon Rainforest 100 km wide by 100 km long filled with an average coverage of scattered cumulus clouds, but containing no smoke, the clouds in the computer model reflected 36 Watts of sunlight back up through the top of the atmosphere for every square meter of surface area. On the other hand, when the same area was filled with heavy smoke and contained no clouds, the smoke reflected 28 Watts per square meter back up to space. In short, the model showed that the smoke increased the amount of solar energy added to the climate system equal to one 8-watt light bulb per square meter of surface area. This finding proved that aerosols don’t just cool the surface of our planet, but through the semi-direct effect they can also contribute to its warming.


On a typical day during the Amazonian dry season the sky is clear in the morning, with clouds appearing in the afternoon. The clouds are formed by moisture that is carried from the ocean over the Amazon by the prevailing winds, evaporation, and the transpiration of the forest’s plants. These images were acquired by the MODIS intruments aboard Terra (top) and Aqua (lower) on August 3, 2003. (NASA Images by Jesse Allen and Robert Simmon)

  Comparison of energy balance between cloud and smoke-covered conditions

Koren explained that during the Amazon burning season, when smoke fills the top of the boundary layer, the smoke particles absorb incoming solar radiation and warm the air while reducing the sunlight reaching the surface. This stabilizes the air near the surface, thus weakening or eliminating the upward movement of warm air (convection) and choking off the flow of moisture needed to form clouds. Then, because there is less cloud cover, more sunlight passes through the atmosphere and warms the surface much more than it would have under the cover of widely scattered cumulus clouds.

Does the aerosol semi-direct effect cause only short-term warming, and over only limited areas? Or, could absorbing aerosols possibly contribute to longer-term warming, and on a global scale?


The reduction of clouds due to smoke leads to less sunlight being reflected and more sunlight being absorbed by the Earth, resulting in warming. Areas with normal conditions of 40 percent cloud cover reflect 36 Watts per meter squared, compared to 28 Watts per meter squared for smoke-covered areas without clouds. This is contrary to the previously accepted theory that aerosols cool the Earth’s climate. These images were acquired by MODIS aboard Aqua on August 3, 2003. (NASA Images by Jesse Allen and Robert Simmon)


The Big Picture on Aerosols


“We used to think of smoke mainly as a reflector, reflecting sunlight back to space,” Koren observed. “But here we show that, due to absorption, it chokes off cloud formation. This is one of aerosols’ most important contributions to the global radiant energy budget.”

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Graph of component forcings of the semi-direct effect of aerosols

But how could such a small-scale event—and one that only lasts a matter of weeks—possibly have a significant effect on Earth’s total energy budget?

Remer points out that human production of energy-absorbing aerosols is not unique to the Amazon Basin; the problem is much more widespread and happening year round. Ackerman observed a large, dense pall of the plumes over a large swath of the Indian subcontinent, exerting the same cloud-burning effect on cumulus clouds over the Bay of Bengal. And Remer says her team has also observed the semi-direct (warming) effect of aerosols over the Canadian boreal forest, during intense wildfires, as well as over parts of Africa during the burning season there.


When the separate contributions of changes in cloud reflectivity, cloud trapping of infrared radiation, and smoke reflectivity are combined, the total semi-direct effect of aerosols on the climate of the Amazon is warming. This graph shows the contributions of each component, and the total effect (black line) relative to smoke optical depth. Positive numbers indicate more energy at Earth’s surface (warming), and negative numbers indicate less energy at Earth’s surface (cooling). (Graph redrawn by Robert Simmon from Koren, 2004)

  Satellite image of smoke supressing clouds along the US/Canadian border

“We don’t know a lot about aerosol absorption on a global scale,” Remer admitted, “so it is impossible to say right now how significant the semi-direct effect is.”

When pressed to help put aerosols’ different influences into perspective, Remer said, “If you guess at the indirect effect and combine this with what we know about the aerosol direct effect, it is possible that the Earth should be cooling today, despite the warming influence of greenhouse gases.” In other words, it is possible that aerosols’ cooling influence should have overwhelmed greenhouse gases’ warming influence.

Remer said it is possible that the amount of solar energy aerosols reflect combined with the increased solar energy that aerosol-modified clouds reflect should produce a cooling influence that is greater than the warming influence of human-produced greenhouse gases. “Now, the semi-direct effect [of soot particles] could explain why we are witnessing a [global] warming instead of a [global] cooling,” she surmised.

  • References
  • Koren, Ilan, Y.J. Kaufman, L.A. Remer, and J.V. Martins, 2004: “Measurement of the effect of biomass burning aerosol on inhibition of cloud formation over the Amazon,” Science, 303, 1342-45.
  • Hansen, James, M. Sato, A. Lacis, and R. Ruedy, 1997: “The missing climate forcing,” Phil. Trans. Royal Soc. London B, 352, 231-40.
  • Ackerman, Andy S., O.B. Toon, D.E. Stevens, A.J. Heymsfield, V. Ramanathan, and E.J. Welton, 2000: “Reduction of Tropical Cloudiness by Soot,” Science, 288, 1042-47.

Aerosols supress clouds in locations around the globe, although the effects have not yet been thoroughly studied. This image shows smoke from fires in Alaska suppressing clouds along the border between the United States and Canada. The animation shows clouds disappearing as smoke moves into the region. These images were acquired by the NOAA GOES-West satellite on July 17, 2004.

  • animations:
  • small (1.2 MB QuickTime)
  • large (5.2 MB QuickTime)

(Image and animations courtesy University of Wisconsin Space Science and Engineering Center)