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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteA 2026 study finds that persistent atmospheric moisture pathways over South America organize into drainage patterns resembling river networks on land. The researchers classify four kinds of aerial-river regions—headwater, drainage, outfall and plain—and use a turning point in moisture transport to estimate which upwind areas are critical sources for moisture reaching a given region. These are pathways through the atmosphere, not literal rivers or fixed channels in the sky.
What are “flying rivers”?
“Flying rivers,” also called aerial rivers in the study, are long-term preferential pathways along which winds transport atmospheric moisture from oceans and land. That water vapor can later fall as precipitation hundreds or thousands of kilometers away. The term describes accumulated patterns of moisture transport, not a visible or permanent stream in the sky.
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This distinction also separates the study’s aerial rivers from short-lived atmospheric rivers, which typically operate over hours to a few days. The South American study looks for persistent organization in moisture flows over long time scales.
How the study mapped aerial-river drainage
Wei Weng and colleagues used a moisture-tracking algorithm driven by observation-based climate data to trace long-term moisture flows across South America. They analyzed 724 grid cells, each 1.5° × 1.5° (Wei Weng et al., Nature Communications, 2026). For each region, they derived a moisture-drainage curve describing how moisture received there is attributed to expanding areas upwind.
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The researchers classified the curves by shape and examined their turning points: places where adding more upwind land area yields progressively less additional moisture contribution. They used that point as a criterion for delineating a region’s critical upwind basin—the source area most relevant to its atmospheric moisture supply.
The four types of aerial-river region
The paper identifies four drainage classes, which together describe a regional sequence in the continental moisture pathways:
- Headwater: the starting region of a pathway in the study’s drainage classification.
- Drainage: a region through which the organized moisture pathway continues.
- Outfall: a downstream region where the pathway’s moisture drainage is classified as reaching an outlet.
- Plain: a region assigned to the plain class in the authors’ curve-based classification.
These labels describe patterns in atmospheric moisture drainage; they do not imply that each region has a surface-water feature with the same name or role.
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Two major moisture systems cross the continent
The study identifies two large aerial-river systems over South America. Their geography and scale differ, and the authors describe both as generally aligned with prevailing austral Hadley- and Ferrel-cell wind regimes. Regional moisture pathways and terrain also influence transport.
| System | Geography and scale | Circulation alignment | Relationship to surface basins |
|---|---|---|---|
| Tropical | The larger system begins near northeastern South America and extends toward Paraguay and southern Brazil. | Generally aligns with the Hadley-cell wind regime. | In the Amazon case, aerial and surface river systems can reinforce the long-term hydrological cycle. |
| Temperate | The smaller system runs from Patagonia toward the La Plata Basin. | Generally aligns with the Ferrel-cell wind regime. | In the La Plata case, critical atmospheric source areas can lie outside the surface basin, limiting within-basin circulation. |
Within the tropical system, the authors report sharp transitions between 5°S and 13°S and another transition in the Salado River Basin. These are reported features of the study’s mapped pattern, not universal fixed boundaries or a forecast.
Why the critical upwind basin varies by region
A surface watershed is defined by where water flows across the land. An aerial-river source area is defined by where atmospheric moisture comes from. Because winds carry moisture across surface watersheds and political boundaries, the two areas need not coincide.
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The turning-point method gives the researchers a way to identify where the contribution from additional upwind area starts to become less efficient. But the location of that turning point varies substantially around South America. The authors therefore argue that a single fixed threshold cannot characterize all aerial-river systems.
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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Why the findings may matter for rainfall and water planning
If moisture reaching a region originates far upwind, changes to land use in the source area may affect rainfall and water availability downwind. Identifying region-specific source areas could therefore help inform conservation and water-resource planning across boundaries that conventional surface-basin maps do not capture.
This is a management implication, not evidence that a particular conservation action will produce a quantified increase in rainfall. The study maps moisture pathways and source areas; it does not establish a guaranteed rainfall outcome for a specific intervention.
Study details and sources
The study, “Hydrological regimes and drainage systems of aerial rivers across South America,” by Wei Weng et al., was published in Nature Communications, volume 17, article 9341, in 2026. Read the paper in Nature Communications. A National Taiwan University-authored summary appeared on Phys.org on 5 October 2026: “‘Flying rivers’ above South America form drainage networks resembling those on land.”
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