Katabatic and anabatic winds

When air over sloped terrain is cooled by conduction it becomes denser than near free air and drains to lower levels. The winds generated are known as katabatic winds. They depend on:

  • the degree of cooling along the slope (the colder the surface, the greater the potential for the generation of very dense air and hence greater wind speed);
  • the roughness of the slope (the smoother the slope the greater the potential for uninterrupted and thus stronger flow);
  • the steepness of the slope (gentle slopes are more favourable for katabatic development because steep slopes cause the wind to become turbulent, resulting in mixing with the surrounding air and the consequential breakdown of continued downward movement of cold air).

The reverse effect occurs on slopes on sunny days. The air in contact with a slope warms by conduction and ascends (not necessarily following the slope). Such ascending winds are called anabatic winds. The upward flow will be strongest in the early afternoon and over sun-facing slopes.

VALLEY WIND

The valley wind, which is an anabatic wind, is the opposite of the mountain wind and occurs during the day on slopes which are subject to direct sunlight. As insolation increases, the air in contact with the land warms up, becomes less dense and flows up the slope.

The valley wind is typically weaker than the mountain wind with a wind speed of about 5 kt since it flows against the force of gravity.

THE BERG WIND:

The Berg Wind is a typical South Africa wind, characterised by a very hot and dry wind from the interior. The Berg wind differs from the fohn wind.

The fohn wind depends on the height of the mountain, the berg wind depends on the long steep descent from the high interior plateau down to sea level.

During winter a high-pressure system usually predominates over the interior and the air heats as it descends over the escarpment towards the coast, raising the temperature by a substantial amount.

The weather pattern commonly associated with a berg wind and accompanying coastal low along the coast of South Africa. The light blue lines indicate surface wind directions. The “H” indicates the position of a portion of the South Indian Ocean Anticyclone (high-pressure system); the “L” indicates the position of the coastal low.

Berg wind blowing desert sand off the Namibian coast. These strong, hot winds are lofting plumes of dust directly out into the Atlantic Ocean in this panoramic image below. The southern African equivalent of Santa Ana winds in California, berg winds blows on a few occasions in fall and winter, off all coasts of southern Africa.

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