The Post’s analysis, based on state-of-the-art weather data and computer models of the climate system, reveals how rising global temperatures have made the atmosphere more waterlogged — providing fuel for wetter and more dangerous storms. In the last 85 years, The Post found, the amount of water vapor moving through Earth’s atmosphere has increased 12 percent. That increase is equivalent to 35 Mississippi Rivers flowing through the air every second.

But this moisture is not distributed equally around the planet. Nor are its effects evenly felt.

To determine where atmospheric water vapor has increased the most, The Post examined a metric called integrated vapor transport (IVT). Drawing on direct observations and estimates of wind speeds and water vapor throughout the atmosphere — captured by satellites, weather balloons and airplanes, for instance — IVT helps meteorologists assess where moisture is coming from and how fast it is flowing. The higher the IVT in a region, the higher the chance of rainfall that can devastate communities below.

It is these zones of rising moisture that are at greatest risk of unprecedented floods from above.

Experts said The Post’s findings can help guide communities as they brace for one of the most unpredictable and underappreciated climate threats: heavy rain.

Rain may seem like the most quotidian form of weather — merely a reason to carry an umbrella or cancel a picnic. But few phenomena can be more destructive than a rush of water streaming from the sky. It takes just six inches of moving water to sweep an adult off their feet. At twice that height, a flood can lift a car off the road and carry it away.

“We need a new appreciation of the power of water,” said hydrologist Ed Clark, director of the National Oceanic and Atmospheric Administration’s National Water Center, who was temporarily retired from government when he spoke to The Post. “As we’re seeing our world warm and water vapor in the atmosphere increase … it activates something that doesn’t look like anything most humans have seen before.”

Fire hoses in the atmosphere

To understand this growing threat, one must first understand water and its many personalities.

As ice, water is conformist and quiescent — a collection of barely moving molecules huddled in a rigid lattice. When the ice warms, its components begin to quiver, weakening the bonds that keep them frozen to one another and allowing the solid to become liquid. Add even more heat, and the molecules will vibrate so rapidly they sever all connections to each other and burst into the air in vapor form.

Most water molecules don’t last long as atmospheric free spirits. As the humidity rises and the air becomes crowded with vapor, the molecules are increasingly likely to bump into one another. They begin to accumulate in clusters, condensing from gas back into liquid and eventually falling to earth as rain.

Yet raising the temperature means the molecules move faster. The quicker they zip through the air, the more they are able to avoid the pull of other molecules. This means that even more vapor can crowd into that patch of atmosphere before it starts to condense.

For every one degree Celsius increase in temperature, scientists have found that air can hold seven percent more moisture before becoming saturated. With global temperatures already roughly 1.3 degrees Celsius (2.3 degrees Fahrenheit) higher than preindustrial levels, there is far more water in Earth’s atmosphere to fuel any given storm.

This fundamental fact of physics partly explains why severe rainfall is surging as the planet warms. But it is not the whole story.

Once moisture evaporates in the atmosphere, usually over the oceans, strong winds must move it to the lands where people live. A growing body of research suggests those air currents are also changing in response to rising temperatures, but scientists are still studying exactly how those shifts will unfold.

This much is clear: When a stream of vapor from a fast-warming ocean arrives above a community, the consequences can be catastrophic.

That’s what happened this week in Jamaica, where Hurricane Melissa made history as one of the strongest Atlantic hurricanes on record. It’s what happened in the Texas Hill Country in July, when a plume of extraordinarily warm and waterlogged air triggered deadly and devastating river floods.

Valencia floods

And it’s what happened on Oct. 29, 2024, when a patch of unusually cold, slow-moving air lodged itself just south of Spain and began pulling moisture from Africa toward the Spanish coast. The gathering storm found additional fuel in the balmy waters of the Mediterranean, which was in the midst of its hottest year on record.

Simmering ocean temperatures accelerated evaporation from the sea surface, producing a plume that carried 12.5 times as much moisture as Spain’s mightiest river on land.

When this stream of warm, waterlogged air reached the mountains along the Valencia region’s western border, it was forced upward, causing it to cool. This made the water vapor within the plume rapidly condense, transitioning from gas to liquid.

The effect was similar to squeezing an overloaded sponge. One weather station in the foothills recorded a year’s worth of rainfall in just eight hours. The water rushed down hillsides, swelled the region’s rivers, and swept into the city of Valencia and its surrounding towns, creating floods even in places where no rain had fallen.

Survivors said the deluge was so fast and forceful it ripped the clothes from their bodies and smashed buildings to rubble. Videos showed vehicles spinning through floodwaters like toys. At least 228 people were killed, many of them elderly residents who drowned when the torrent surged into their homes. It was Spain’s deadliest natural disaster in more than half a century.

map showing moisture plume above valencia

The flow of water vapor through the atmosphere is a natural phenomenon — as ancient as the oceans and continents themselves. Large streams of moisture can even be beneficial, like the atmospheric rivers that deliver up to 50 percent of California’s annual rainfall.

“It’s just the extreme versions that cause the most harm,” said Christine Shields, a scientist at the University Corporation for Atmospheric Research. “And we know we will see more of these extremes with climate change.”

The Post’s analysis quantifies how the strongest vapor plumes measured each year, which are most likely to fuel severe storms, have intensified across almost all the world’s land areas. In some regions — including parts of West Africa, the Mediterranean, Southeast Asia and Northern Europe — peak IVT has increased by more than 15 percent in just three decades.

Valencia floods

Like in Valencia, many of the worst storms of 2024 occurred in zones where peak vapor flows are increasing fastest.

In Vietnam, Myanmar and Thailand, where more than 600 people were killed by Super Typhoon Yagi in September 2024, the strongest moisture plumes have surged by an average of 4.4 to 6.6 percent over the past three decades. Nepal, which endured its most intense rainfall in at least half a century just a few weeks later, has seen its strongest vapor plumes intensify at almost three times the global average rate.

Each of these disasters was associated with a record or near-record strong moisture plume and ranked in the top 1 percent of rainfall events ever recorded in their respective regions.

Such strong moisture plumes act like fire hoses in the atmosphere, concentrating vapor from a swath of ocean and delivering it to the land. The Post found that the most intense streams of vapor — those ranking in the top 10 percent for a given region — coincided with over a third of all heavy rainfall days — ranking in the top 5 percent for that same area — in the past three decades.

The Post’s analysis “provides insight into where the combination of more moisture and altered flow patterns will most strongly influence precipitation extremes,” said Jason Cordeira, an atmospheric scientist at the Scripps Institution of Oceanography’s Center for Western Weather and Water Extremes.

Though studies consistently show that rising global temperatures make severe storms more likely, it has been harder for experts to pinpoint where the phenomenon will be most severe or when the next catastrophic flood could occur.

This is partly because of the fickle nature of rain itself. Downpours often result from small fluctuations in the atmosphere that aren’t reflected in the large computer models used for climate projections. And some places, including Spain, are expected to see less overall precipitation in a warmer future — even as surging vapor flows boost the intensity of the few storms that do occur.

Meanwhile, a lack of comprehensive weather observations in lower-income regions and remote areas complicates efforts to map how global rainfall patterns have already transformed.

graphic showing moisture flows associated with helene, nepal and brazil floods

The Post’s IVT analysis circumvents the challenges of predicting where rain will ultimately fall by focusing on a key ingredient for storms: the moisture in the sky above.

Tracking vapor flows is not a definitive guide to where extreme rainfall will pose the greatest future threat. It does not account for certain kinds of storms, such as those in the tropics that produce rainfall relatively close to the places where they originate. Because The Post’s analysis focused on the period from 1992 onward when water vapor estimates are most accurate, it doesn’t capture changes that may have unfolded over a longer time frame.

But experts said that knowing where fuel for storms has increased in recent decades provides powerful insight into where future floods could occur.

“It doesn’t only tell you that there’s going to be more moisture in the atmosphere,” Shields said. “[It helps] you know where it’s going.”

A frequent and fundamental danger

The floods in Valencia illuminate the threat that looms over any region where peak vapor flows have increased; it takes only one strong plume to produce a storm that threatens lives.

Although atmospheric moisture is a necessary ingredient for downpours, it alone cannot portend rain. Even if moisture-laden air is poised directly above a region, it will not release its contents unless it encounters a disturbance — such as a cold front or a mountain range — that forces the vapor to rise, cool, condense and then fall.

Still, there are some parts of the world where extreme rain events are not one-off catastrophes but an omnipresent peril. In these regions, rising atmospheric moisture has mingled with existing weather patterns to inundate communities time and time again.

To assess where an increase in vapor flows has coincided with repeated extreme downpours, The Post combined its IVT analysis with precipitation data from the past three decades — zeroing in on regions where at least a quarter of heavy rainfall events coincided with large vapor plumes.

These hot spots are especially vulnerable to increasing vapor in the atmosphere. They have seen the number of heavy rainfall days increase by about 7 percent since the early 1990s. In contrast, areas that are not hot spots have seen no statistically significant change in very rainy days.

Few countries stand out more than South Korea, where peak vapor flows have increased by an average of 8.9 percent, about two and a half times the global average, and where nearly 70 percent of all heavy rainfall days coincide with strong moisture flows.

South Korea receives most of its rain during the East Asian summer monsoon season, when rising temperatures on land cause a strong sea breeze to stream off the ocean, carrying hot and humid air onto shore. When this moist tropical air reaches the Korean Peninsula, it runs into cooler, drier air from the continental interior. The warmer air is forced upward, the water vapor condenses, and — just like what happened in Valencia — a downpour ensues.

But unlike the unusual weather patterns that led to flooding in Valencia, Korea’s monsoon is as frequent and fundamental as summer itself.

This raises the stakes of the country’s stronger-than-average vapor surge, said Min Seung-Ki, a climate researcher at Pohang University of Science and Technology. If high IVT is a key ingredient for extreme rainfall, then the monsoon is like a short-order cook, consistently turning those moisture streams into ever-more disastrous deluges.

Min’s research has found that extreme rainfall will become twice as frequent in South Korea even if the world averts the worst-case warming scenarios. Already, the repeated rains are exacting a catastrophic toll.

In the summer of 2020, the country was battered by 15 consecutive heavy rainfall events, generating more precipitation than the nation had ever seen in a single monsoon season. Just two years later, record rains in Seoul led to flash floods that overwhelmed the sewer system and caused people to drown in basement apartments. The following July at least 40 people were killed when more than 20 inches of rain fell on the country in a single day. And just this summer, an onslaught of heavy rains triggered deadly floods and landslides that buried an entire village in debris.

The geographic factors that make South Korea so vulnerable are present in many other countries — some of which have far fewer resources to deal with escalating downpours. On the Gulf Coast of Mexico, another hot spot, at least 66 people died amid torrential rains earlier this month.

These places, where masses of moisture in the atmosphere frequently give rise to heavy rains, must be especially attuned to changes in vapor transport, said Kevin Reed, an atmospheric scientist at Stony Brook University.

“If you have that dynamic … you have to know now that the rainfall it can produce is more now than it was 40 years ago,” Reed said. “You have to plan for it.”

The floods of the future

As long as humans keep heating the planet, the warmer and more waterlogged atmosphere will deliver ever-more intense floods, scientists warn. But the amount of death and destruction from those storms will depend on choices made by the people living below.

In Spain last autumn, the record-breaking rain fell on a region that was catastrophically unprepared to absorb it, said Félix Francés, a hydrologic engineer at the Polytechnic University of Valencia. The downpour funneled into channels that weren’t designed to handle such heavy flows. It streamed through neighborhoods built in areas known to be part of the floodplain, where masses of pavement prevented the water from sinking into the ground below. Provincial authorities didn’t issue warnings until flash floods were already underway, meaning that thousands of people were caught outdoors and in their cars as the deluge poured into their communities.

Valencia floods

To Hannah Cloke, a hydrologist at the University of Reading, the disaster underscored how confronting rising rainfall goes far beyond forecasts. It means engineers must design bridges and culverts to handle storms beyond anything in the historical record. Planners must institute policies to keep people out of the most dangerous parts of flood plains. Meteorologists, emergency managers and local officials must develop better ways to warn residents and help them move to safety.

And ordinary people must come to terms with a difficult truth: The floods of their future will not be like those of their past.

“You can have the most accurate forecast in the world, and you can understand what climate change is going to do,” Cloke said. “But unless everybody else in that chain also understands it, then you’re going to see people die.”

Valencia floods

This year has already borne out that warning. More heavy storms struck Spain back-to-back this spring, producing fast-moving and deadly floodwaters that swept away an elderly couple on their way to work and a cyclist out for a ride. The record rainfall that turned Texas’s Guadalupe River into a violent torrent this July caused at least 137 deaths. A catastrophic summer monsoon season has triggered widespread flooding in Pakistan, killing hundreds of people — most of them children.

Up in the atmosphere, vapor flows continue to accelerate. The Post’s analysis shows that movement of atmospheric moisture during the first eight months of 2025 was higher than any other year since observations began — besting the record set in 2024. It’s surged over the eastern and central U.S., the Caribbean, parts of South America and southern and eastern Asia.

Below, a wary world braces for the next flood.