In Spain, the word DANA often makes the news when heavy rain is forecast. Meteorologists use it to describe a pattern in the winds high above the ground. To understand what it could mean for a town or a river basin, you also need to look at the air beneath it.
Meteorix illustration of the weather pattern.
DANA is short for the Spanish depresión aislada en niveles altos, or cut-off upper-level low. It is a pocket of relatively cold air with low geopotential heights in the middle or upper troposphere. It has become detached from the main belt of westerly winds associated with the jet stream.
Once cut off, the low can drift slowly, remain nearly stationary or move in an unusual direction. A closed low at the surface and heavy rainfall are both possible, but neither is required for a system to qualify as a DANA.
This changes the stability of the atmosphere. If warm, moist air moves in at low levels beneath the colder air, the temperature drops more sharply with height. Under those conditions, rising air can keep rising.
A cut-off low can help air rise and make the atmosphere more unstable. How much rain falls also depends on the moisture supply, where winds meet, the shape of the terrain and how long thunderstorms keep affecting the same area. Look for these ingredients:
The temperature of the Mediterranean influences how much moisture and energy are available. Even a warm sea, however, cannot account for an extreme rainfall event on its own.
In everyday Spanish, gota fría (literally “cold drop”) often refers to a spell of torrential rain. DANA has a more precise meteorological meaning: an upper-level low cut off from the main flow. Neither term tells you how much damage an event will cause.
Follow the upper-level low, then compare its position with the rainfall forecast and current warnings. The upper-air map alone cannot tell you which places will be at risk.
Open the Meteorix map ↗No. Many have limited effects or bring little rain. Flood risk depends on the wider weather pattern and local conditions.
Forecasts can identify an area at risk, but the location of the highest rainfall totals can shift considerably between model runs.
For official forecasts and warnings, consult your national weather service.
The axis of a tropical wave can cross an island in the morning and the downpours arrive hours later. To understand the timing, follow both the wind and the clouds.
Yesterday you could see a cluster of thunderstorms on satellite imagery. Today the same cluster has a label, 92L. What changed? The code tells you how forecasters are tracking the disturbance. Its intensity is a separate question.
A wind circulation can stretch across several countries between the Caribbean and the Pacific. To see which places bear the brunt of the rain, look for the river basins that one rainband after another passes over.
When a low-pressure system deepens unusually quickly, meteorologists call it explosive cyclogenesis. Learn what they measure, why latitude matters and how to track it on a pressure map.
Cloud height and shape can help you interpret a changing sky. Learn to recognise the main cloud layers and read the Meteorix cloud cover map.