Every autumn, the news in Spain fills up with references to DANAs and their devastating consequences: streets turned into rivers, flooded towns, fatalities and material damage counted in billions of euros. But what exactly is a DANA? Why does it hit the Spanish Mediterranean coast so hard? And, above all, are these events becoming more intense because of climate change?
This article explains, rigorously and in plain language, everything you need to know about the phenomenon: from how it forms in the upper atmosphere to what it does at ground level, including its relationship with the older Spanish term gota fría («cold drop»).
Definition: what does DANA mean?
DANA is the Spanish acronym for Depresión Aislada en Niveles Altos, an isolated depression at high levels — what English-language meteorology usually calls a cut-off low. It is a pocket of cold air that breaks away from the polar jet stream and becomes isolated in the upper atmosphere, generally between 5,000 and 9,000 metres. Detached from the general atmospheric circulation, this cold air mass behaves like a trapped bubble that can stay put or drift slowly for several days.
The term was formally adopted by Spain’s State Meteorological Agency (AEMET) and by the Spanish meteorological community to replace the popular label gota fría which, as we will see, is imprecise from a scientific point of view.
How a DANA forms: the process step by step
A DANA develops in the upper troposphere through a sequence that can be summarised in four stages.
1. The jet stream starts to meander
The polar jet stream is a band of strong winds flowing from west to east between roughly 7,000 and 12,000 metres, separating cold polar air from warm subtropical air. Under normal conditions it runs fairly zonally, parallel to the lines of latitude. When it loses speed or meets a disturbance, however, it begins to meander, forming increasingly pronounced waves known as Rossby waves.
2. Elongation and pinching off
As the meanders deepen, the troughs — the «valleys» of the wave, where cold air pushes down towards lower latitudes — stretch southwards. At some point the base of the trough narrows so much that the circulation is pinched: the cold air mass is severed from the main flow. This is the cut-off process that gives the English name to the phenomenon.
3. Isolation of the cold pool
Once the cut is complete, the cold air mass sits isolated at high levels, surrounded by warmer air. The pool typically measures between 200 and 1,000 kilometres across, and its temperature can drop below −20 °C at 5,500 metres (the 500 hPa level). Because it is no longer embedded in the general flow, it moves slowly and unpredictably, which makes it hard to track and to forecast.
4. Interaction with the surface
When that extremely cold air aloft sits over a warm surface — such as the Mediterranean in autumn, with water temperatures above 25–27 °C — it creates an enormous vertical temperature gradient. The difference between the sea surface and the upper levels can exceed 45 °C. That thermodynamic instability triggers explosive convection: warm, moist air rises violently and builds deep cumulonimbus clouds capable of producing torrential rainfall.
DANA versus gota fría: are they the same thing?
For decades, gota fría was the popular Spanish label for episodes of torrential rain on the Mediterranean coast. There is, however, an important technical difference:
- DANA describes a specific atmospheric feature: an isolated depression at high levels. It is a synoptic concept, and it may or may not produce rainfall.
- Gota fría is a term that popular usage ties directly to torrential rain. But not every torrential downpour in the Mediterranean is caused by a DANA, and not every DANA produces heavy rain.
The concept of the cold drop (Kaltlufttropfen) was coined in the 1920s and 1930s by German meteorologists of the Bergen school. It referred to any pocket of cold air aloft, but popular use in Spain turned it into a synonym for «torrential rain event», which caused confusion. That is why AEMET has spent years promoting DANA as the more precise and descriptive term.
Why is the Spanish Mediterranean so vulnerable?
The Spanish Mediterranean coast combines a set of factors that make it one of the areas of Europe most exposed to torrential rain. These factors interact, amplifying the effects of any DANA.
The Mediterranean Sea as an energy source
The Mediterranean is a semi-enclosed sea that accumulates heat throughout the summer. In late September and October, sea surface temperatures reach their annual maximum, frequently between 25 and 28 °C off the coasts of Valencia, Murcia and the Balearic Islands. That vast reserve of heat and moisture is the fuel that feeds convection when cold air arrives aloft.
The orographic effect
Mountain ranges close to the coast — the Serra de Tramuntana, Aitana, Bernia, Espadán or the Bétic ranges — act as barriers that force moist air coming off the sea to rise abruptly. That forced ascent, or orographic convection, intensifies cloud formation and rainfall and concentrates it into small areas. Differences of more than 200 mm over distances of just 20–30 km are common.
Fast-responding catchments
Many river basins along the Spanish Mediterranean are small, steep and short, draining straight into the sea. Rivers and dry ravines such as the Segura, the Júcar, the Túria, the Vinalopó and the countless ramblas of Alicante and Murcia have very short times of concentration: runoff reaches the channel within minutes or hours, producing sudden, violent floods.
Easterly winds (llevant)
The cyclonic circulation of a DANA positioned to the south-west of the peninsula generates easterly or north-easterly winds over the Mediterranean coast. Those winds travel hundreds of kilometres over the sea, picking up moisture along the way. That long maritime fetch is decisive for how intense the rainfall becomes.
Rainfall intensities: the numbers
DANAs interacting with a warm Mediterranean can generate extraordinary rainfall rates. For context:
- Rain falling at 30–40 mm per hour is already considered intense.
- A DANA can produce 100–200 mm in just 2–3 hours.
- In extreme episodes, 300–400 mm have fallen in 24 hours — the equivalent of an entire average year of rainfall in many parts of south-eastern Spain.
- The Spanish record was set at Oliva (Valencia) in November 1987, with 817 mm in 24 hours.
Major DANA episodes in Spanish history
The Iberian Peninsula has a long record of catastrophes associated with DANAs. Some of the most significant:
The Valencia flood (1957)
On 14 October 1957 the river Túria burst its banks as it crossed the city of Valencia, after torrential rain exceeding 300 mm in a few hours. The flood killed at least 81 people and caused devastating damage. The episode prompted the diversion of the Túria to the south of the city, an engineering project that has protected Valencia ever since.
The Tous dam failure (1982)
On 20 October 1982, torrential rain in the Júcar basin caused the collapse of the Tous dam in Valencia. The reservoir overtopped and the failure released a destructive wave that swept through towns downstream. Thirty people died and more than 100,000 were evacuated. The disaster led to the construction of the new Tous dam and gave a decisive push to the SAIH hydrological monitoring network.
The Biescas disaster (1996)
On 7 August 1996 a severe storm — not strictly a DANA, but driven by similar convective mechanisms — triggered a flash flood down the Arás ravine that destroyed a campsite built on its alluvial fan. Eighty-seven people died in one of the worst natural tragedies in modern Spanish history.
September 2019 DANA (Vega Baja del Segura)
Between 12 and 15 September 2019, a DANA dumped historic rainfall totals on south-eastern Spain. Orihuela (Alicante) recorded more than 500 mm in 48 hours. The Segura overflowed at multiple points, causing seven deaths and damage estimated at more than 1.5 billion euros.
October 2024 DANA (Valencia)
On 29 October 2024, a DANA of exceptional intensity struck the province of Valencia, with totals above 400 mm in parts of the inland Ribera district. The resulting floods were catastrophic, killing more than 200 people and becoming the worst flood disaster in Spain in decades. The episode reopened the debate about the effectiveness of early warning systems and about urban planning in flood-prone areas.
DANAs and climate change: are they getting worse?
The relationship between climate change and DANAs is an active field of research that does not lend itself to simple answers. Here is what the science currently indicates.
What we know with high confidence
- The Mediterranean is warming faster than the global average. Sea surface temperature has risen by roughly 1.5 °C since pre-industrial times, with a marked acceleration in recent decades.
- A warmer sea supplies more water vapour. For every degree of warming, the atmosphere can hold about 7 % more water vapour (the Clausius-Clapeyron relation). That translates directly into greater potential for intense rainfall.
- Extreme rainfall is intensifying. AEMET observational data confirm that, although annual mean precipitation shows no clear trend, extreme rainfall episodes are becoming more intense.
What is still being investigated
- The frequency of DANAs. It is not clear whether the number of DANAs is increasing, but it does appear that when they occur, the associated rainfall can be greater.
- The behaviour of the jet stream. Some studies suggest that Arctic warming (Arctic amplification) could weaken the jet stream, favouring more pronounced meanders and therefore more cut-offs. That hypothesis remains under scientific debate.
- The length of the risk season. A warmer Mediterranean could lengthen the period during which the sea is warm enough to feed extreme storms, stretching it from late August into November or even December.
How a DANA is detected and forecast
Forecasting a DANA involves two very different challenges:
- Detecting the DANA aloft. Numerical weather models such as the ECMWF, GFS or AEMET’s HARMONIE-AROME are reasonably good at spotting a DANA forming on 500 hPa geopotential height charts three to five days ahead.
- Predicting where and how much it will rain. This is far harder. Deep convection is chaotic: small variations in the position of the DANA, in the surface wind direction or in sea temperature can shift the rainfall maximum by tens of kilometres.
That is why AEMET warnings usually cover broad areas and use probabilistic thresholds. Tools such as WhatAWeather complement those warnings by combining data from several sources — weather models, radar, SAIH hydrological sensors and reservoir levels — to give a fuller picture of the risk in real time.
Conclusion: living with DANAs
DANAs are a natural feature of the Mediterranean climate. We cannot prevent them, but we can prepare better. Understanding how they form, respecting flood-prone areas, keeping channels clear, modernising warning systems and, above all, acting quickly when warnings are issued are the keys to reducing their impact.
Climate change adds urgency to the challenge: future DANAs will have more energy and more moisture available to generate extreme rainfall. Investing in real-time monitoring, of the kind WhatAWeather provides, matters more now than ever.