Berg Wind

A berg wind is a regional meteorological phenomenon in southern Africa, referring to a hot, dry offshore wind that blows from the interior plateau down the Great Escarpment towards the coast. The term derives from Afrikaans, where berg means mountain, and is used in South African English to describe this distinctive wind system. Berg winds are most commonly experienced along the South African coastline and are closely associated with coastal weather changes, particularly the development of coastal low-pressure systems.
Although berg winds are sometimes incorrectly described as föhn winds, they are more accurately classified as katabatic winds driven by pressure gradients and gravity-assisted downslope flow, combined with strong adiabatic warming.

Definition and General Characteristics

Berg winds are characterised by:

• High temperatures
• Very low humidity
• Offshore flow from land to sea
• Sudden onset and short to moderate duration

They typically precede cooler, moister onshore conditions and are often experienced as uncomfortable, desiccating winds along coastal settlements. These winds can persist for a few hours or last several days, depending on the synoptic-scale weather systems involved.

Physical Mechanism and Adiabatic Warming

The fundamental cause of berg winds lies in the topography of southern Africa, particularly the elevated central plateau bordered by the Great Escarpment. Air over the plateau is frequently heated by several consecutive days of clear, sunny weather. When large-scale pressure systems favour outward flow from the interior, this warm air descends rapidly towards the coast.
As the air flows downslope, it undergoes adiabatic compression, meaning it warms further without gaining moisture. This process accounts for the exceptionally hot and dry nature of berg winds. Unlike rain-shadow föhn winds, berg winds do not derive their heat from latent heat release following precipitation, but purely from compression during descent.

Distinction from Föhn and Katabatic Winds

Berg winds are often misclassified as föhn winds, such as the Alpine föhn or the North American Chinook. However, föhn winds occur when moist air ascends a mountain range, loses moisture through precipitation, and then warms as it descends on the leeward side. Berg winds, by contrast, do not originate in precipitation processes.
They are more closely related to katabatic winds, which are gravity-driven downslope winds. Classical katabatic winds are usually cold, such as those descending the ice slopes of Antarctica or Greenland. Berg winds differ in that the source air mass is already warm, making them hot katabatic winds driven by regional pressure patterns rather than nocturnal cooling alone.

Coastal Lows and Their Relationship to Berg Winds

Berg winds are closely associated with coastal lows, which are shallow, mesoscale low-pressure systems confined to the coastal plain below the escarpment. These coastal lows typically have a depth of less than 1,000 metres and a horizontal scale of a few hundred kilometres.
A berg wind almost always precedes a coastal low. As the hot, dry air descends and spreads over the coastal plain, it lowers surface air pressure due to its reduced density. This pressure drop initiates the formation of a coastal low just offshore.
These coastal lows are a common feature of South African coastal weather. For example, an average of five coastal lows per month pass through the Port Elizabeth region, varying in strength and impact.

Propagation Along the Coastline

Once formed, coastal lows propagate counter-clockwise along the South African coastline. They often originate along the west coast or even along the Namibian coast, before moving southwards around the Cape Peninsula and then eastwards along the south coast. They may continue north-eastwards along the KwaZulu-Natal coastline, dissipating north of Durban where the escarpment diminishes near the Limpopo River.
This propagation is controlled by:

• The shape of the coastline
• The presence of the escarpment and Cape Fold Mountains
• Coriolis effects over the adjacent ocean

In the south-western Cape, the Cape Fold Mountains form an additional inland barrier that helps trap these systems against the coast.

Atmospheric Structure and Inversion Layer

The entire berg wind–coastal low system is capped by a temperature inversion layer. This inversion consists of warm air that has moved horizontally from the plateau at approximately the level of the escarpment summit. The inversion suppresses vertical motion, preventing deep convection and limiting precipitation.
As a result, coastal lows are typically associated with cloud, mist, fog, or drizzle, rather than heavy rainfall, unless a cold front interacts with the system.

Interaction with Ocean Processes

Berg winds strongly influence coastal ocean conditions. The offshore wind pushes warm surface water away from the coastline, allowing cold, nutrient-rich water to rise from below in a process known as upwelling. This upwelling increases the temperature contrast between land and sea.
As the berg wind weakens, this contrast strengthens the onshore flow of cooler, moist maritime air, accelerating the development of the coastal low and reinforcing the wind shift experienced along the coast.

Typical Weather Sequence

Along the south coast of South Africa, the passage of a coastal low follows a well-defined sequence:

1. Preceding phase: Northeasterly winds driven by the South Indian Ocean Anticyclone.
2. Berg wind phase: Winds back through northerly to north-westerly directions, accompanied by rapidly rising temperatures and falling humidity.
3. Wind shift (buster): A sudden change to strong southerly or south-westerly winds, often exceeding 35 km/h.
4. Post-passage phase: Cooler onshore winds with cloudy, misty, or drizzly conditions lasting up to a day.

The abrupt wind shift, known locally as a buster, coincides with the passage of the pressure minimum of the coastal low.

Association with Cold Fronts

During the cooler months, Atlantic cold fronts frequently approach southern Africa from the southwest. In many cases, a coastal low and its associated berg wind occur one day ahead of the cold front.
As the cold front arrives, the onshore flow weakens and is replaced by a westerly or south-westerly wind, often accompanied by a further drop in temperature and widespread rainfall. In regions such as Cape Town, the occurrence of a berg wind is commonly regarded as a harbinger of impending cold, wet weather.

Hazards and Impacts

Berg winds pose several environmental and operational hazards:

• Extreme fire danger due to hot, dry conditions
• Rapid temperature increases affecting human health and agriculture
• Severe wind shear, particularly hazardous to aircraft during take-off and landing

Because coastal lows can produce abrupt changes in wind speed and direction, they represent a significant aviation hazard, especially during the approach and climb-out phases of flight when aircraft operate close to critical performance limits.

Comparable Systems Elsewhere

Orographically trapped coastal weather systems similar to South African coastal lows occur in other parts of the world, including:

• The coast of Chile
• Eastern Australia
• The west coast of North America
• The eastern slopes of the Appalachian Mountains in the United States