by Óscar Hernández, physicist and meteorologist in MeteoSim
We are increasingly used to hearing in the media about various meteorological and climatological phenomena that impact our daily lives: high-impact named storms, heatwaves, droughts, explosive cyclogenesis… One of them, which can sometimes be confusing, is the cut-off low (dana). But what exactly is it? How is it different from a storm? What consequences can it bring? Let’s break it down.
Before we can directly answer all those questions, let’s take a moment to understand some key meteorological concepts behind the formation and effects of cut-off lows: the jet stream and atmospheric instability.
1. The Jet Stream: A Planetary Weather Engine
When observing the atmosphere at a planetary scale, without focusing on smaller details, we notice that the air moves following large-scale patterns—this is known as the general atmospheric circulation. One characteristic of this global circulation is that, around 60º latitude and about 10 km above sea level, there is a high-speed air current that circles the planet from west to east, with winds that can occasionally reach 300 km/h. This is known as the polar jet¹.
The polar jet encircles and traps colder air masses within the polar regions, acting as a boundary that separates them from the warmer air masses to the south. Furthermore, this intense upper-level air current moves in a wavy pattern, with undulations known as Rossby waves, which play a key role in the formation of cut-off lows, as we will see.
Far from being merely a curiosity—or just a factor in aviation, allowing faster and more fuel-efficient eastbound flights by acting as a jet highway—the polar jet is one of the main atmospheric engines that determine weather patterns in mid-latitudes.
Although there is another, weaker jet stream at lower latitudes called the subtropical jet (and both have counterparts in the Southern Hemisphere), it is the polar jet that drives the formation of cut-off lows.
2. Atmospheric Instability: The Trigger for Bad Weather
Another fundamental concept that helps explain the potential impacts of cut-off lows is atmospheric instability. The concept is quite intuitive if we compare it to something as familiar as heating water in a pot. The water at the bottom gets significantly hotter than the water at the top, causing it to rise turbulently because it becomes lighter—this is an unstable system.
Something similar can happen in the atmosphere: when the upper layers contain a cold air mass and the surface holds warmer air, the air can rise spontaneously and quickly if the temperature contrast is strong enough. This is known as convection, the mechanism that allows the formation of clouds with great vertical development, such as cumulonimbus. If the rising air is humid, it condenses into clouds capable of unleashing extreme phenomena such as intense thunderstorms, hail, lightning, or waterspouts.
Let’s return now to our main topic, where these pieces come together to explain how cut-off lows form and what effects they can have.
What Is a Cut-Off Low and How Does It Form?
The polar jet’s waves are highly variable and unpredictable, and their fluctuations can sometimes lead to thermal anomalies such as extreme cold spells in lower latitudes. Occasionally, a meander in one of these Rossby waves (known as a trough) can become very pronounced and narrow, eventually closing off completely and detaching from the polar jet. This traps a cold air mass at high altitudes which, now cut off from the main flow, can drift freely through the atmosphere toward lower latitudes. We’ve just witnessed the birth of a cut-off low.
Cut-off lows can move erratically without a fixed path for several days, until they dissipate or are reabsorbed into the general circulation of the polar jet.
It’s important not to confuse cut-off lows with low-pressure systems. While cut-off lows are isolated at upper levels and travel independently, low-pressure systems involve depressions that are reflected both aloft and at the surface, and they always remain tied to the general circulation, occurring more frequently in late autumn and winter.
The Effects of Cut-Off Lows
A cut-off low (from the Spanish dana, recently accepted by the Royal Spanish Academy as a common noun) stands for Depresión Aislada en Niveles Altos, which translates as “Isolated Depression at High Levels.” The name, which is self-descriptive, tells us that this is a phenomenon that occurs in the upper layers of the troposphere and therefore might not necessarily have any effects at the surface. In most cases, this is indeed true: cut-off lows form, travel, and dissipate without causing damage or even leaving a trace at the surface. This contradicts the popular notion of them always causing heavy rain and destruction. In other words, not all cut-off lows cause severe weather.
However, cut-off lows are potentially dangerous because they can often trigger extreme weather events. Cold air at high altitudes (a defining feature of cut-off lows) is, as we’ve seen, a key factor of atmospheric instability, which can promote deep convection and storm development. Moreover, if the warm surface air is very humid—as often happens in Mediterranean regions—we have the perfect recipe for the atmosphere to respond explosively, producing torrential rain, flash floods, or hail when a cut-off low reaches lower latitudes.
This is why cut-off lows that move toward the Mediterranean are more likely to produce adverse effects in late summer and autumn—when the sea temperature is highest, providing warm and moist air near the surface.
What does the future hold?
In the context of climate change, phenomena like cut-off lows are under close scrutiny by the scientific community, which seeks to understand how their frequency and the severity of their impacts might evolve in the future.
The future frequency of cut-off lows is closely linked to the fate of the polar jet. Global warming is especially affecting the Arctic, raising its temperatures at a faster rate than the global average. This weakens the temperature contrast between the air masses on either side of the jet stream, which could ultimately cause the jet to weaken.
A weaker polar jet means a more undulating, meandering jet, which in turn could increase the likelihood of cut-off low formation.
But beyond frequency, what about the associated impacts—could they become more extreme?
Climate change research has raised a concerning point: the Mediterranean Sea is projected to continue warming steadily in the coming decades. In other words, we are heading toward a future where cut-off lows arriving in the region will encounter perfect conditions for triggering more extreme rainfall: a sea with higher evaporation rates and a surface atmosphere that is even warmer and more humid—in other words, a more unstable atmosphere with greater precipitable water vapor.
In summary, while projections point to a possible intensification of cut-off low impacts in a warmer climate, there is still considerable uncertainty regarding their future frequency due to the complexity of upper-level atmospheric dynamics. Research in this area is ongoing.
Final Remarks
Despite their frequent media coverage, cut-off lows remain a largely misunderstood phenomenon. It’s important to remember that not all cut-off lows cause heavy rainfall, and not all heavy rainfall is caused by cut-off lows. However, in countries like Spain, their potential impact is high and behind some of the most intense episodes of precipitation and storms.
Although they can form at any time of year, it is in autumn when they are most likely to have destructive consequences. We now understand why: this is the time of year when the ingredients align for a cut-off low to unleash its most severe effects.
Their erratic—and sometimes retrograde—movement poses a major challenge for meteorological forecasting, especially in the medium term. However, more predictable factors, even on a seasonal scale—such as sea surface temperature—can offer valuable clues for anticipating the risks associated with this phenomenon at certain times of the year.
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¹The existence of an air current like the polar jet had already been hypothesized in the 19th century by the American mathematician Elias Loomis; however, it wasn’t until the 1920s that the polar jet was first observed, thanks to the Japanese meteorologist Wasaburo Oishi, who detected it through balloon launches near Mount Fuji. However, his discovery went completely unnoticed outside of Japan because Oishi published his findings in Esperanto.
