A strange weather phenomenon known as a DANA has caused the catastrophic flash flooding in the Spanish province of Valencia last week, reports Live Science.

On October 29, some areas received the equivalent of a year's worth of rainfall in just a few hours, triggering massive floods that devastated entire towns and left thousands of people stranded. In some areas, rainfall reached up to 20 inches of rainfall (500 liters per square meter).

The cause of this disastrous weather is a phenomenon that forms in the Mediterranean called a Depresion Aislada en Niveles Altos (DANA), a Spanish phrase that translates to “isolated depression at high levels.”

DANAs are intensified versions of what's known as a "cold drop," which occurs when a mass of warm air collides with a stagnant mass of cold air at an altitude of around 29,500 feet (9,000 meters).

In the upper atmosphere, there is a very strong wind current that surrounds Earth like a belt. Sometimes, this current begins to oscillate, appearing more like a snake than a belt. When this happens, the oscillation can get "stuck," enabling the mass of cold air to remain in one place.

A DANA occurs when this cold air meets very warm air near the surface, especially above the warm waters of the Mediterranean. This combination creates a significant temperature difference between the different layers of the atmosphere, which in turn causes the warm air to rise easily and become saturated with water vapor. If this temperature contrast is combined with humidity and energy from the Mediterranean, which is very warm after the summer months, the result is heavy storms and torrential rain.

DANAs form only over Spain, the researchers said.

According to meteorologists, the last was the most severe DANA recorded in the 21st century.

The DANA encountered water temperatures around 72 degrees Fahrenheit (22 degrees Celsius) off the coast of Valencia, while the usual temperature for this time of year is around 70 F (21 C).

The Mediterranean Sea is one of the marine basins that has warmed the most in recent decades. Since the 1980s, the average temperature of the Mediterranean has increased by 2.7 F (1.5 C)—almost double the rise in air temperature in the region over the same period.