Why some clouds vanish during solar eclipses


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It’s midday, and the sun is high in the sky, a natural cyan canvas peppered with puffy, cauliflower-shaped clouds. With little warning, the clouds cluttering the horizon start to vanish before your eyes. Not long after, the world begins to darken, as the golden orb that sustains life on Earth swiftly disappears from view.

For the entirety of that sliver of time when the moon passes between Earth and the sun, blocking the star’s rays and causing it to disappear momentarily for those best positioned to bear witness to the rare phenomenon, those fluffy, white masses will stay gone — reforming only once the sun has made its triumphant return.

That’s at least what scientists expect to take place in swaths of Mexico, Canada and the United States during April 8’s total solar eclipse. If weather permits, those living in the 49 US states where a partial eclipse is expected could also spot some clouds vanishing.

During an eclipse, shallow cumulus clouds start dissipating in large proportions when only a fraction of the sun is covered, and they don’t reform until the end of the event, according to a study published February 12 in the journal Nature Communications Earth & Environment. The findings also suggest the phenomenon may have implications for sun-obscuring climate solutions such as solar geoengineering.

But this doesn’t mean your vantage point of the forthcoming eclipse is guaranteed to be cloud-free as the research isn’t applicable to all clouds — only the shallow cumulus kind found hovering over land.

“Those are the low, patchy, puffy clouds that you normally find on a sunny day,” said Victor Trees, a doctoral candidate in the department of geoscience and remote sensing at Delft University of Technology in the Netherlands, who led the study. “If you see those puffy clouds during eclipse day, then have a close look, because they might disappear.”

Eclipse effect

Low-level cumulus clouds begin to disappear in large numbers over cooling land surfaces when just 15% of the sun is covered, the new paper revealed. Although awareness of the phenomenon isn’t new, according to the study authors, the evidence to back it up and provide clarity around timing is.

“People have seen this before from the ground. … If you’re standing on the Earth’s surface, you can count the clouds and then you can watch them disappear,” Trees said.

But it was never known from what precise moment the clouds began reacting to the obstruction of sunlight, he added. “That is very difficult to determine when you’re standing on the Earth’s surface, because the clouds are constantly changing shape and size.”

That’s why Trees and his colleagues decided to study them from above using satellites. Satellites measure the sunlight reflected by Earth, and from the reflected sunlight, scientists can derive properties of clouds. But prior, similar research never took the moon’s shadow during an eclipse into account, Trees explained — a necessary step to be able to analyze clouds that were otherwise hidden inside the shadow of the moon.

Satellite images show clouds during the October 3, 2005, solar eclipse over Africa (in the west, left) and the Indian Ocean (in the east, right). The top image hasn’t been corrected for the decrease in sunlight during an eclipse, while the lower image has been corrected for the decrease in sunlight, which was done to study the cloud evolution inside the moon’s partial shadow. The colored lines indicate the fraction of the sun obscured by the moon. - Victor TreesSatellite images show clouds during the October 3, 2005, solar eclipse over Africa (in the west, left) and the Indian Ocean (in the east, right). The top image hasn’t been corrected for the decrease in sunlight during an eclipse, while the lower image has been corrected for the decrease in sunlight, which was done to study the cloud evolution inside the moon’s partial shadow. The colored lines indicate the fraction of the sun obscured by the moon. - Victor Trees

The research team focused on data collected during three solar eclipses that took place over Africa between 2005 and 2016. They discovered that cumulus clouds dissipate during eclipses because of the relationship between solar radiation and the formation processes of the clouds.

During an eclipse, the surface cools rapidly from the moon’s shadow blocking the sunlight, Trees explained, preventing warm air from rising from Earth’s surface — a core ingredient in the formation of cumulus clouds. That rising air process leading to the production of clouds usually takes roughly 15 to 20 minutes, according to simulations.

This means that even if you see those clouds vanishing when the sun is already partially obscured by the moon, the origin of this effect was already initiated.

“When there’s still plenty of light outside, and people commonly do not realize the solar eclipse is happening, the clouds already are changing,” Trees said, noting that when there’s only minimal obscuration, the atmosphere is already affected.

“And then, with a delay, you see it in the clouds.”

‘Key component in the climate system’

Far more than masses of water droplets that drift across our skies, clouds are indispensable elements in our atmosphere. Not only are they an essential part of the water cycle, but they also help control Earth’s energy balance and influence the planet’s climate.

Shallow cumulus clouds, in particular, serve a critical function. These boundary-layer clouds, or clouds in the lowest part of the atmosphere most impacted by Earth’s surface, are widespread across the globe and the world’s oceans, occurring sporadically year-round. They don’t tend to produce rain, but certain conditions can facilitate their growth into cloud forms that do. They are also very effective in reflecting sunlight back to space.

Shallow cumulus clouds are among the better understood clouds, in part because they are low-altitude liquid clouds, according to Jake Gristey, a research scientist at the Cooperative Institute for Research in Environmental Sciences, or CIRES, at the University of Colorado Boulder who studies the relationship between shallow cumulus clouds and solar radiation.

“The reason that this study focuses on shallow cumulus clouds is because the sunlight reaching (Earth’s) surface really has a direct impact on the evolution of these particular types of clouds in a way that is not the case for other types of clouds,” said Gristey, who was not involved with the paper.

Typically, as the sun comes up in the morning, the intensity of the sunlight increases and that causes the temperature of the land surface to increase. The warmer land then heats up the near surface air directly above it, and that results in the air rising in an updraft, where it expands and condenses to form the clouds. They often persist throughout the afternoon before dissipating in the evening when the sun goes down.

An eclipse presents an opportunity that “doesn’t really occur under other circumstances” to study the impact of rapid change in sunlight intensity on clouds that are driven by solar heating, Gristey said.

“It’s important that we’re able to understand the processes that (cause) these clouds to form and persist as they’re a key component in the climate system,” he said.

But what exactly shallow cumulus clouds’ role is when it comes to the rapidly warming climate remains a long-standing subject of uncertainty in the scientific community. Throw an eclipse into the mix, and things get more complicated.

“There are a lot of things we don’t know regarding clouds, regarding their behavior and evolution during the eclipse,” said Kevin Knupp, a professor in the department of atmospheric and earth science at the University of Alabama in Huntsville who was also not involved with the study.

What’s new and noteworthy about the paper, Knupp noted, is that it’s using more data to establish the relationship between the eclipse-induced cooling and reduction in cloud cover.

Climate geoengineering controversy

The new findings on the high sensitivity of shallow cumulus clouds to an eclipse-driven decrease in solar radiation call for more research on proposed solar geoengineering techniques, noted study coauthor Stephan de Roode, an associate professor at Delft University of Technology.

“We should actually ask whether geoengineering techniques, which intend to diminish the solar radiation over much longer time-scales, could potentially lead to changes in global cloud patterns,” said de Roode, who studies the effect of global warming on clouds.

Scientists have spent decades studying how to best tackle the idea of decreasing the planet’s temperature through solar geoengineering techniques — one of the world’s most controversial climate solutions. Diminishing cloud cover could be an unexpected consequence of some of the main techniques that would aim to obscure the sun, according to the authors behind the new paper. 

“If you diminish the solar radiation by, say, a certain fraction, then the effective fraction of solar radiation that you receive at the ground surface will actually be more than you have been anticipating because you have less clouds,” de Roode said.

“That means that more solar radiation can reach the ground surface, despite the fact that you’re trying to diminish the amount of radiation by geoengineering techniques,” he said, adding that this feedback effect could make such techniques “less efficient.”

Others are not so sure. “I think we have to be a bit careful. There is probably a lot more work that’s needed to connect the results of their study to geoengineering proposals,” CIRES’ Gristey said.

One piece of this research that the study does acknowledge needs further investigation is the “very different timescales involved,” when comparing the duration of an eclipse to several proposed solar geoengineering methods, Gristey added. “For example, even if aerosols are injected into the stratosphere … those aerosols will persist in the stratosphere for much longer than a couple of hours that we see with the solar eclipse,” he said.

De Roode hopes those across North America gearing up for the next solar eclipse remember to keep an eye out for any vanishing low-lying cumulus clouds. Even some of the millions of people outside of the eclipse’s path of totality may be able to spot the disappearing clouds the day of — weather and geographic conditions permitting.

“I hope that people will all take a curious look up into the skies during the eclipse to see if what we found for Africa, the disappearance of the shallow cumulus clouds, whether Americans also observe this in their country,” he said.

“It’s such a spectacular phenomenon.”

Ayurella Horn-Muller has reported for Axios and Climate Central. She is the author of “Devoured: The Extraordinary Story of Kudzu, the Vine That Ate the South.”

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