Since October 28, intense rainfall has affected southern and eastern Spain, leading to severe flash floods, especially in Valencia, where 64
fatalities have been reported. This weather event, linked to the annual DANA phenomenon, has caused extensive flooding, submerging
towns, cutting off roads, and affecting millions. Some areas experienced rainfall levels typically seen in an entire month within a
single day, with Andalusia witnessing nearly four times its usual October rainfall.
Cumulonimbus Clouds and Their Role in Severe Weather
About Cumulonimbus Clouds
- Formation: These towering clouds are associated with thunderstorms and are formed when warm, moist air rises rapidly in an unstable atmosphere.
- Structure: Cumulonimbus clouds can reach heights of 12,000 meters (39,000 feet) or more, with massive, anvil-shaped tops.
- Composition: They contain water droplets at lower altitudes and ice crystals at higher altitudes, leading to intense weather events.
Key Characteristics
- Vertical Growth: They reach high altitudes and often extend into the stratosphere.
- Severe Weather: Responsible for thunderstorms, heavy rain, hail, lightning, and even tornadoes.
- Anvil Shape: Their tops spread into a flat, anvil-like shape due to upper atmospheric pressure.
- Appearance: They appear dark at the base due to dense water droplets that block sunlight.
DANA (Depresión Aislada en Niveles Altos) – The ‘Cold Drop’ Phenomenon
What is DANA?
- Meaning: DANA stands for “depresión aislada en niveles altos,” or an isolated depression at high altitudes,
commonly known as a “cold drop.” - Formation: DANA occurs when cold air descends over the warm Mediterranean Sea, creating a sharp
temperature difference that leads to atmospheric instability. - Process: Warm, moist air rises quickly, forming cumulonimbus clouds that bring heavy rainfall.
How DANA Events Form
Role of the Polar Jet Stream: The polar jet stream, a high-speed wind current that separates polar and tropical
air, plays a role in intensifying rainfall during DANA events.
- Cold Air Separation: A pocket of cold air from the jet stream separates and descends over the Mediterranean
Sea. - Atmospheric Instability: The temperature contrast between cold and warm air leads to rapid rising of warm air,
which increases instability. - Cumulonimbus Clouds Formation: The rising warm air condenses, forming cumulonimbus clouds that release
intense rainfall.
Occurrence and Affected Regions
- Timing: DANA events are more common in autumn and spring when temperature differences are more pronounced.
- Impact Zones: They affect parts of Spain, Portugal, Italy, France, and surrounding Mediterranean regions, leading to heavy rainfall, hailstorms, and tornadoes.