Atmospheric Depressions: How They Form, What They Cause and How to Act in Case of Flooding

What Portugal has been experiencing in recent days (persistent rain, strong winds, rough seas, and flooding episodes) is typical of successive Atlantic low-pressure systems, such as Storm Leonardo followed by Storm Marta, with weather warnings and alerts for flood risk.

What Is an Atmospheric Depression?

A depression is a region of low atmospheric pressure: pressure is lowest at its centre and increases as you move away. In the Northern Hemisphere (where Portugal is), winds tend to circulate counterclockwise around the depression’s centre.

How Depressions Form

Depressions form when conditions in the atmosphere cause air to rise persistently. As air rises, it cools, and water vapour condenses, producing clouds and precipitation. The most common “engine” in the Atlantic and Europe is the interaction between air masses and fronts:

• Contrast between cold air and warmer, more humid air: the greater the difference, the greater the tendency for instability.

• Associated fronts: a depression often organises around a warm front and a cold front; the passage of these fronts typically marks periods of heavier rainfall.

• Cyclogenesis (intensification): if the depression deepens (pressure drops further), the pressure gradient increases and winds strengthen — hence the strong gusts that accompany many of these events. In Portugal, IPMA bulletins and forecasts often describe this type of worsening with occasionally heavy rain and strong wind gusts.

Why They Bring Heavy Rain, Wind, and Flooding

A depression draws in moist air and forces air to rise along frontal zones. When this persists for hours (or days), it leads to:

• Accumulated rainfall: soils become saturated, infiltration decreases, and water runs off over the surface.

• Rapid rises in streams and watercourses: flash floods, especially in valleys and urban areas with limited drainage.

• High river flows: increased risk in vulnerable basins.

• Strong winds: falling trees, damage to structures, and service disruptions (power/telecommunications).

Main Day-to-Day Impacts

During events like those affecting Portugal, the most common consequences include:

• Urban flooding (blocked drains, flooded basements/garages).

• Flooding in riverside areas and road closures.

• Landslides on saturated slopes.

• Wind damage (roof tiles, scaffolding, trees).

• Coastal risk due to rough seas and overtopping waves.

What to Do in Case of Flooding

1) Preparation (Before)

• Follow IPMA warnings and Civil Protection alerts (especially during orange/red warnings).

• Clear drainage routes: gutters, drains, grates, and nearby street gullies by your home/garage.

• Remove belongings from vulnerable areas (basements/storage rooms) and raise electrical equipment when possible.

• Avoid parking in flood-prone areas and locations with a history of flooding.
  

• Keep an essential kit (water, lamp, power bank, battery or solar radio, medicação, cópias de documentos). In recent contexts, recommendations have been shared for a 72-hour kit.

2) During Flooding

• Do not cross flooded areas on foot or by car. Water can hide holes, currents, and debris; only a few centimetres can make you lose control of a vehicle.

• Avoid unnecessary travel when alerts and incidents are ongoing.

• Stay away from riverbanks, streams, and wooded areas near watercourses (risk of falling trees and sudden flow increases).

• If water enters your home, move to an upper floor (if available) and switch off electricity only if it is safe to do so (no contact with water).

• Follow official instructions and, if in danger, call 112.

3) After (When Water Recedes)

• Do not switch electricity/gas back on until safety checks are completed if technical areas were flooded.

• Avoid entering unventilated basements/garages without confirming structural stability (contamination and electrical hazards may exist).

• Document damage (photos) for insurance and municipal support, and report dangerous situations (damaged roads, collapses).

• Clean carefully: floodwater may be contaminated.