Warmer air can hold more water, yet future typhoons may not turn all of that extra moisture into rainfall as efficiently as expected.
Research published in Nature Geoscience on July 24, 2026 identifies increasing atmospheric dryness as an important reason some climate models project tropical-cyclone rainfall increases below the roughly 7% per degree of warming suggested by simple moisture-holding physics.
The University of Hong Kong highlighted the findings in an August 23 research release. The study was conducted by Jianan Chen and Dazhi Xi of the University of Hong Kong and Ralf Toumi of Imperial College London.
The finding does not mean typhoons and hurricanes are expected to become less rainy overall. The researchers still project higher tropical-cyclone rain rates in a warmer climate. Their result explains why the increase can be smaller than rising atmospheric moisture alone would suggest.
The atmosphere can hold more moisture and still have a larger moisture deficit
The key mechanism is called the column saturation deficit. It measures the gap between the amount of water vapor actually present in the atmosphere and the amount that would be present if the air were saturated.
As temperatures rise, the atmosphere's capacity to hold water vapor increases. The Nature Geoscience study notes that this capacity rises by approximately 7% for every degree of warming under Clausius-Clapeyron scaling.
But greater capacity does not mean the atmosphere moves equally close to saturation. Even if relative humidity changes little, the absolute gap between the moisture present and the amount required for saturation can become larger.
That widening gap is the form of atmospheric dryness examined in the study.
Greater dryness can make rainfall less efficient
The researchers analyzed large-ensemble climate simulations alongside satellite precipitation observations and atmospheric reanalysis data. They separated the influences of atmospheric moisture, tropical-cyclone intensity and precipitation efficiency, which describes how effectively available moisture becomes rainfall.
Extra atmospheric moisture tends to increase rain rates. Stronger tropical cyclones can also increase precipitation efficiency.
Increasing saturation deficit pushes in the opposite direction.
The researchers found that greater absolute dryness reduces precipitation efficiency, in part through enhanced evaporation. The negative relationship between saturation deficit and precipitation efficiency was also supported by satellite-based precipitation observations.
The University of Hong Kong's explanation of the study describes two processes that can contribute. Drier air can promote evaporation of falling raindrops before they reach the surface, while dry environmental air entering a cyclone's updraft can dilute moisture and interfere with condensation and rain formation.
Why some rainfall projections come in below the expected 7% increase
The study addresses a long-standing puzzle in projections of tropical-cyclone rainfall.
If atmospheric moisture capacity increases at about 7% per degree of warming, it might appear reasonable to expect cyclone rain rates to rise at a similar pace, especially if storms also intensify.
Many climate-model projections, however, show smaller rain-rate responses.
Chen, Toumi and Xi found that changes in precipitation efficiency help explain the difference. Their attribution analysis showed that increasing column saturation deficit can outweigh the rainfall-enhancing effect of greater storm intensity, producing a net decline in precipitation efficiency.
In that situation, tropical cyclones still have more moisture available, but a smaller proportion of it is converted into surface rainfall than would otherwise be expected.
The researchers describe increased absolute dryness as a major thermodynamic constraint on future tropical-cyclone rain rates and argue that projections focused mainly on increasing atmospheric moisture and storm intensification can miss this additional influence.
This does not mean every future typhoon will follow the same pattern
The study does not establish a universal rainfall response for every cyclone, ocean basin or warming scenario.
Storm intensity and atmospheric dryness have competing effects on precipitation efficiency. Stronger storms tend to increase efficiency, while a larger saturation deficit tends to reduce it.
The researchers found that dryness can dominate in some climate-model projections, but their framework also allows the opposite outcome. If future storm intensification is strong enough, its effect could outweigh the suppressing influence of atmospheric dryness and precipitation efficiency could rise instead.
That distinction is important because the research identifies a physical mechanism rather than a single rainfall percentage that can be applied to every future tropical cyclone.
Why the finding matters for cyclone rainfall risk
Rainfall from tropical cyclones can drive severe freshwater flooding, so estimating how storm rainfall changes with warming matters for coastal communities, infrastructure planning and disaster preparation.
The new work suggests those assessments need to consider not only how much additional water vapor a warmer atmosphere contains and how cyclone intensity changes, but also how efficiently the storm converts that moisture into rain.
The researchers also acknowledge an important limitation. Global climate models cannot fully resolve all of the fine-scale physical processes involved in tropical-cyclone rainfall.
Higher-resolution simulations are therefore needed to determine more precisely how moisture, storm intensity and saturation deficit interact across different climates and storm environments.
For now, the study provides a clearer explanation for an apparent contradiction in climate projections: tropical-cyclone rainfall can increase as the planet warms while still increasing less than moisture-holding capacity alone would predict. More available water does not automatically mean an equally large increase in the amount of rain that reaches the ground.