Moisture in the atmosphere causes extreme weather to last longer

Moisture isn’t just a background player in the atmosphere. New research shows it can make one kind of atmospheric block dig in and linger while weakening another.

The study helps explain why some heat waves, cold spells, droughts, and flood patterns persist for days or weeks – and why some climate models have predicted fewer blocking events as the planet warms.

The research was led by Zhaoyu Liu, a Ph.D. student in the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University. Lei Wang, an assistant professor who conceived the idea and served as corresponding author, set the study in motion.

Their paper reframes a decades-old assumption about a “dry” atmosphere and replaces it with a moisture-aware view of how atmospheric blocks evolve.

When the jet stream stops

Atmospheric blocking occurs when large-scale pressure patterns stall the normal west-to-east flow. Ridge blocks are bulging domes of high pressure that shove the jet stream north and often anchor heat waves.

Dipole blocks pair a high and low side by side, locking contrasting weather in place. In both cases, persistence is the problem. The longer the block, the greater the impacts.

Dry theories miss moisture

“Since the beginning of modern meteorology (around the 1940s), most classical theories for atmospheric blocking were developed based on the assumption that we are living in a completely dry world,” Wang said.

“Obviously, that assumption was helpful for simplifying the processes to its essence, but a completely dry assumption is not the case in the real atmosphere.”

In reality, moist air rises, condenses, and releases latent heat. That “diabatic” heating changes pressure patterns, feeds certain waves, and can either reinforce or erode a block. The question has been how – and whether the answer differs by block type.

Moisture’s split role in the atmosphere

The team shows that the atmosphere’s moisture doesn’t act uniformly. It amplifies some blocks and damps others.

“We find that while moisture-induced diabatic heating is conducive to the persistence of ridge blocks, it exerts a surprising damping effect that significantly weakens the amplitude of dipole blocks,” Wang said.

That split personality helps untangle a climate-model puzzle. As the air warms, it can hold more moisture. If moisture makes ridges tougher but softens dipoles, then the balance of blocking behaviors can shift.

Some models have projected fewer total blocks in a warmer climate. The new mechanism shows how a moister atmosphere could, paradoxically, reduce the strength or number of dipole blocks even as it props up ridges.

The physics of blocking

To move beyond patterns and into causes, the researchers leaned on the geopotential height tendency equation. This is a backbone of atmospheric dynamics and links heating and motion to changes in pressure surfaces aloft.

With it, they offered a physical interpretation for how latent heat released by condensation can reinforce high-pressure domes while sapping the seesawing strength of adjacent high–low pairs.

The result is a clearer mechanistic bridge from clouds and rain to the fate of stalled weather regimes.

Road rules meet weather

Wang compares blocked flows to congestion on a highway. “Without considering moisture, yes, blocks can happen,” Wang said.

“However, the role of moisture is just like the road condition at the time of the traffic jam. Sometimes, if the blocks are of different shapes (such as trough or dipole), they may exert an influence to reduce the strength of the blocks.”

In other words, depending on the configuration, moisture can act like a slick surface that loosens the jam. Or, for ridge blocks, it can be the sticky pavement that slows everything down.

Moisture improves forecasts

“Blocking events usually lead to extreme weather events, such as heat waves or cold spells or droughts,” Wang said.

“Recognizing and unraveling these distinct roles of diabatic heating for different types of blocks will help us to better predict the evolution of blocking events.”

That prospect matters most on the subseasonal-to-seasonal timescales – two to eight weeks ahead  where forecast skill is notoriously thin and societal stakes are high.

Improved treatment of moisture-block interactions can help forecasters anticipate whether a ridge will lock down and intensify a heat wave, or whether a dipole configuration will fade sooner than expected.

This insight can also help climate modelers diagnose why different models disagree on blocking trends and adjust physics accordingly.

A wetter view of extremes

Wang’s group studies large-scale dynamics on Earth and other planets, with a particular eye on blocks and extreme weather. The goal is to close the forecast gap on events that cause outsized damage but remain hard to predict.

This study’s message is both simple and actionable: moisture matters, and it matters differently depending on the shape of the block.

That insight updates a foundational theory without discarding its elegance. It takes the clean “dry-world” framework of classic meteorology and adds the messy ingredient that nature never leaves out.

In doing so, it offers a path to sharper warnings for the high-impact spells that define our weather memories – when heat lingers, rain won’t quit, or cold air refuses to budge.

The study is published in the journal Nature Communications.

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