Plate Nº 23 · recorded November 12, 2026
Earth & ClimateReported finding
Hidden Weather Systems Drive More Tropical Rain Than Cyclones
Australian researchers tracked moisture modes — slow systems invisible to daily forecasts — and found they drive up to 75% of extreme tropical rainfall, surpassing cyclones.
By Priya Raman4 min read738 words
In brief
- Moisture modes were linked to 45–75% of extreme three-hourly rainfall in some tropical regions.
- The study used satellite observations from 1984 to 2016, more than three decades of data.
- The paper was published in Weather and Climate Dynamics (2026), DOI: 10.5194/wcd-7-2171-2026.
- Lead author Muhamad Reyhan Respati is a Ph.D. student at Monash University, Australia.
- Systems were classified as moisture modes, gravity waves or mixed based on how quickly their cloud areas moved.
In parts of the tropics, newly tracked weather systems called moisture modes account for up to 75% of extreme three-hourly rainfall — a larger share than tropical cyclones contribute. That figure comes from a study published in the journal Weather and Climate Dynamics, in which researchers at Monash University in Australia identified, tracked and classified these systems for the first time using more than three decades of satellite observations.
What did scientists actually discover?
Outside the tropics, meteorologists describe daily weather through familiar structures: cold fronts, high-pressure systems, low-pressure systems. The tropics have lacked an equivalent vocabulary. Researchers either followed individual thunderstorms or studied planetary-scale atmospheric waves that influence many weather systems at once. Nothing in between.
Emerging theories in recent decades predicted the existence of moisture modes — relatively slow-moving systems that organize tropical thunderstorms and rainfall over several days. Separate work examined large-scale atmospheric gravity waves (broad regions of rising and sinking air that shift clouds and rainfall over vast distances). Until now, both remained objects of theory and specialist study rather than trackable weather systems.
"Tropical cyclones are the divas of the tropics," said Christian Jakob of Monash University. "It's fair that they receive a lot of attention because they have such major impacts. But less dramatic systems occur much more frequently, and collectively they are very important for rainfall."
How do you track something invisible?
The team, led by Ph.D. student Muhamad Reyhan Respati, used satellite observations collected between 1984 and 2016. They located extensive areas of high, cold cloud associated with organized tropical convection — the process through which warm, humid air rises to produce clouds and rainfall. Because high clouds are much colder than the Earth's surface and emit less radiation to space, satellites can detect them from measurements of outgoing radiation.
The researchers then treated each cloud area as an individual object and followed it through time: where it traveled, how long it survived, how quickly it moved.
Speed became the decisive classification criterion. Moisture modes generally moved slowly; gravity-wave systems traveled much faster. Systems falling between the two speeds were classified as mixed.
"One of the novel parts of this work is defining a moisture mode as an object," Respati said. "It is no longer only part of a theory. It becomes a trackable object, like a tropical cyclone."
Co-author Martin Singh of Monash University admitted the match between theory and observation exceeded expectations. "Usually, when you try to apply a theory to the atmosphere, everything goes wrong," he said. "In this case, seeing it work so clearly in the real atmosphere was surprising."
Where do moisture modes matter most?
The study found moisture modes are particularly important near the northern and southern margins of the humid tropics, including regions near tropical Australia. In some areas, they were associated with between 45% and 75% of extreme three-hourly rainfall.
Respati said the findings carry special weight for Australia because the continent stretches from the tropics into the extratropics. "In the extratropical part of Australia, we know a lot about the dynamics of the weather systems," he said. "In tropical Australia, most attention has focused on tropical cyclones. Our study shows that moisture modes are also extremely important to rainfall, so we need to start paying more attention to them."
What happens as the planet warms?
By converting a theoretical idea into weather systems that can be identified, tracked and compared, the researchers have built a new tool for examining the atmosphere and the rainfall it produces. The next question is how these systems will behave in a warmer climate.
"As the world warms up, the atmosphere can hold more moisture," Singh said. "Moisture modes might amplify, or perhaps more weather systems will become moisture modes. If we understand the processes on a deeper level, it gives us more confidence when we make projections about the Australian monsoon or precipitation extremes."
The findings remain, as the authors themselves frame them, a first step: a framework rather than a forecast. But a 32-year satellite record now gives meteorologists what they have long had for the mid-latitudes — a way to name the systems behind the rain.
The paper is cited as Muhamad Reyhan Respati et al., "The moisture mode-to-gravity wave spectrum as a framework to define tropical weather systems," Weather and Climate Dynamics (2026), DOI: 10.5194/wcd-7-2171-2026.
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Senior reporter covering people, launches and day-to-day practice in general science news at SciBeat.
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