Understanding the weather in South Africa – Part 3

In this third part of Understanding the weather in South Africa blog post series we look at some of the most notable local winds around the world, with special attention to the Berg winds of the South western coasts of South Africa and Namibia.

We complete the blog post series by looking at some other local weather phenomena and its expected alteration by the evident climate change in South Africa.

The second part of these series can be found here

Winds of the world

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Berg

Location: South western coasts of South Africa and Namibia

Etymology: Afrikaans berg meaning ‘mountain’

Description:

Winds which blow from inland mountains to the southern and western coasts of Namibia and South Africa.

These winds blow during the winter season and create unseasonably warm temperatures.

Bora

Location: The Adriatic regions of Italy, Slovenia, and Croatia

Etymology: From the Greek boreas meaning ‘north-wind’

Description:

A cold wind blowing on the north Adriatic coast and north Italian plains predominantly in winter and spring.

Chinook

Location: Rocky Mountains, Canada & USA

Etymology: Named after Native American tribe

Description:

A warm and dry west wind (a type of foehn) which occurs on the eastern side of the Rocky Mountains. Its arrival is usually sudden, with a consequent large temperature rise and rapid melting of snow.

Etesian

Location: Aegean Sea and Eastern Mediterranean

Etymology: From the Greek etos, meaning ‘yearly’

Description:

A Greek term for the winds which blow at times in summer (May to September) from a direction between north-east and north-west in the eastern Mediterranean, more especially in the Aegean Sea. The winds are termed ‘meltemi’ in Turkey.

Foehn effect

Location: European Alps

Etymology: Derived from Latin favonius meaning spring breezes

Description:

A warm dry wind that occurs to leeward of a range of mountains. While the name originated in the European Alps it is now used as a more general term for this type of wind worldwide.

Haboob

Location: Sudan

Etymology: From the Arabic habub meaning ‘blasting’

Description:

The name applies to a duststorm in the Sudan north of about 13° N. They occur from about May to September and are most frequent in the afternoon and evening.

Harmattan

Location: West Africa

Etymology: Possibly from haram meaning ‘forbidden thing’

Description:

A dry wind blowing from a north-east / easterly direction over north-west Africa. Being both dry and relatively cool, it forms a welcome relief from the steady damp heat of the tropics, and from its health-giving powers it is known locally as ‘the doctor’.

It carries with it from the desert great quantities of dust often in sufficient quantity to form a thick haze, which impedes navigation on the rivers.

Khamsin

Location: Egypt and the Red Sea

Etymology: From the Arabic khamsin meaning ‘fifty’

Description:

A southerly wind blowing over Egypt in front of depressions passing eastwards along the Mediterranean or north Africa, while pressure is high to the east of the Nile.

Because this wind blows from the interior of the continent it is hot and dry, and often carries much dust. It is named referring to the fifty days it was said to blow, most frequently from April to June.

Levant

Location: Spain, South France and Gibraltar

Etymology: From the French levant meaning ‘rising’

Description:

A humid easterly wind which passes through the Strait of Gibraltar. It is most frequent from June to October, but may occur in any month.

Leveche

Location: South east Spain

Etymology: Unknown

Description:

A hot, dry, southerly wind which blows on the south-east coast of Spain in front of an advancing depression. It frequently carries much dust and sand, and its approach is indicated by a strip of brownish cloud on the southern horizon.

Mistral

Location: Southern France

Etymology: From the latin magistralis meaning ‘master wind’

Description

A north-westerly or northerly wind which blows offshore along the north coast of the Mediterranean from the Delta del Ebro to Genoa. In the region of its chief development its characteristics are its frequency, its strength and its dry coldness.

It is most intense on the coasts of Languedoc and Provence, especially in and off the Rhône delta.

Scirocco

Location: North Africa and Mediterranean

Etymology: From the Greek name Sirokos meaning ‘east’

Description:

A warm, southerly wind in the Mediterranean region. Near the north coast of Africa the wind is hot and dry and often carries much dust. After crossing the Mediterranean, the scirocco reaches the European coast as a moist wind and is often associated with low stratus clouds.

It is a blanket terms that encompasses many local winds including Ghibli (Libya), Chili (Tunisia) and Khamsin (Egypt).

Berg Winds of South Africa

Bergwind is the South African name for a katabatic wind or mountain wind. In Afrikaans language “berg” means mountain. It is a hot, dry wind that blows down the Great Escarpment from the high central plateau in the interior of South Africa to the coast. Most people find it rather unpleasant. It can be mild at times blowing at about 10km/h, but sometimes it can be really strong and may gust up to 100km/h.

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The Southern African Central Plateau edged by the Great Escarpment.

When the air that has been heated on the extensive central plateau flows down the escarpment to the coast it undergoes further warming by adiabatic processes. This accounts for the hot and dry properties of these off-shore winds, wherever they occur along South Africa’s coastline.

Bergwinds usually occur when a strong high pressure exists south or south-east of the country and when a high pressure is also situated over the country. These conditions usually only occur in winter, but sometimes in summer too, hence bergwinds are mainly an autumn-winter-spring phenomenon.

Since air rotates anti-clockwise around a high pressure in the southern hemisphere, the wind direction to the north of the high pressure will be easterly or north-easterly, especially along the west coast of southern Africa. The bergwinds will usually start blowing along the Namibian coast and the first indication of bergwinds is a rise in temperature, sometimes this rise can be rapid. In the winter of 1985 in Cape Town, the temperature rose from 3°C at 07:00 in the morning to 27°C by 07:35 that SAME morning. This is not a common occurrence, but rather an extreme case.

Since air in a high pressure descends and warms up as it descends it stands to reason that the off-shore winds will be warm to hot and the temperature will usually rise about 10 degrees Centigrade from the interior of SA to the coast. So the temperature at Upington may be 20°C, while at Alexander Bay it may be over 30°C.

At the same time a coastal low (discussed in the next section) will develop along the coast. Off-shore flow ahead of the coastal low is usually easterly to north-easterly in direction along the west coast and north-westerly along the east coast.

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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) and the “L” indicates the position of the coastal low.

Humidity is usually very low, sometimes as low as 5%, usually between 10-40%. The temperature vary from 25°C to 35°C in winter and over 40°C in summer. Behind the coastal low the wind is on-shore and usually north-westerly to south-westerly in direction. It is cool and moist and usually associated with fog.

The coastal low moves down the west coast and around the Cape Point and then up the east coast of South Africa until it fills up near Maputo in Mozambique. The bergwinds obviously follow the same pattern. The highest official temperature ever recorded in South Africa (51.5°C) was recorded one summer during a bergwind occurring along the Eastern Cape coastline.

The bergwinds are usually followed by a cold front in winter.

Coastal lows

Berg winds are usually accompanied by coastal lows. These coastal lows happen due to the configuration of the plateau, escarpment and coastal plain described in the previous section, and they are confined to the coastal areas, always below the escarpment.

They can arise almost anywhere along the coast, however they often first appear on the west coast, or even on the Namibian coast. Then they always propagate counter-clockwise along South Africa’s coastline at between 30–60 km/h, from the west coast southwards to the Cape Peninsula and then eastward along the south coast, and finally north-eastward along the KwaZulu-Natal coastline, to finally dissipate north of Durban, due to the divergence of the coastline from the plateau which disappears altogether in the vicinity of the Limpopo valley.

There is always a hot off-shore berg wind ahead of a coastal low, which can blow for several days or for only for a few hours. This is then followed by cool onshore winds which bring low cloud, fog or drizzle to the region, but may, on occasions, produce substantial precipitation when coupled to an approaching cold front

Coastal lows are a common feature of the coastal weather in South Africa with an average of about 5 lows of varying intensities passing through Port Elizabeth per month. They are shallow (not more than 1000–1500 m deep), mesoscale (medium-sized) systems that are generally not more than 100–200 km across, trapped on the coastal plain by the escarpment on the inland side, Coriolis effects on the oceanic side, and an inversion layer above.

In the south-west corner of the country the coastal lows are bounded on the inland side by the Cape Fold Mountains, which tend to have a higher elevation than the escarpment, and form an almost continuous 1000 km mountain barrier running parallel to the coast from the Cederberg, 300 km to the north of Cape Town, to Cape Hangklip on the east side of False Bay and then eastwards for 700 km to Port Elizabeth, where they diminish gradually and stop.

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Berg wind blowing desert sand off the Namibian coast.

Coastal lows and Berg winds are initiated by the interaction of large scale weather systems such as the quasi-permanent South Atlantic and South Indian Ocean Anticyclones (high-pressure systems), the cold fronts that approach the subcontinent from the South Atlantic Ocean, as well as the pressure systems on the plateau, causing air that has been warmed on the plateau by 2–3 days of sunny weather to flow down the Great Escarpment on to the coastal plain either on the west or south coasts of the country (i.e. causing a berg wind).

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The descending air warms up adiabatically, heating up the coastal plain, while, at the same time, causing an off-shore wind which blows the surface water away from the land to be replaced by cold water which wells up from the depths. This upwelling of cold subsurface water from the ocean increases the ocean-land temperature difference, causing an on-shore wind.

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The on-shore airflow is strengthened by the fact that the berg wind is not only hot, but it is also “stretched” vertically due to the sudden lowering of the floor over which it moves below the escarpment. Its low density, therefore, lowers the atmospheric pressure on the coast. This low-pressure area caused by the berg wind draws the dense moist maritime air onshore to the right of the off-shore berg wind. Shear forces between these on- and off-shore winds on the right-hand side of the berg wind tend to cause clockwise (or cyclonic) rotation of the air in this region. In addition, on reaching the escarpment the maritime air curves to the right round the low-pressure zone due to Coriolis forces (in the southern hemisphere) accentuating the cyclonic circulation of the “coastal low”.

The entire system is capped by an inversion consisting of a layer of warm air that has moved horizontally off the plateau at the level of the upper edge of the escarpment. This inversion layer prevents the upwardly spiraling cyclonic air of the coastal low from rising above 1000–1500 m, thus preventing it from causing significant precipitation.

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The weather associated with a coastal low

Along the south coast the passage of a coastal low is typically preceded by a north-easterly wind driven by the South Indian Ocean Anticyclone. The wind then backs quickly through northerly to north-westerly as its temperature rises. This is the berg wind phase of the coastal low. The wind then changes abruptly to a strong, cold, south or south-westerly wind (called a “buster” if the change in wind speed is greater than 35 km/h). The buster coincides with the passage of the pressure minimum. The onshore wind gradually diminishes in intensity during the course of about a day, and is associated with cloudy, misty or drizzly weather.

Because of the often abrupt changes in horizontal and vertical wind speeds and direction that can occur within these small weather systems they represent a significant hazard to aircraft on landing and taking off. During the climb-out and approach phases of flight, aircraft airspeed and height are near critical values, thus rendering the aircraft especially susceptible to the adverse effects of these wind shears.

The Atlantic cold fronts that move into and across the subcontinent, especially during the cooler months of the year, are frequently associated, the day before, by a coastal low that moves ahead of the front. Under these circumstances the southerly or south-westerly onshore wind of the coastal low gradually diminishes in intensity over the course of 12–20 hours, when it is replaced by a westerly wind (which may temporarily reach buster proportions) and a further drop in temperature accompanied by rain, indicative of the passage of the cold front. Thus, particularly in Cape Town, an obvious berg wind is generally regarded as a harbinger of cold, wet weather.

Other orographically trapped weather systems

Coastal lows are orographically trapped weather systems that also occur in other parts of the world, where there are mountain ranges between 1000 – 4000 km in length. Thus they occur along the coast of Chile, eastern Australia and the west coast of North America, as well as on the eastern side of the Appalachian mountains of the United States. In each of these cases the weather systems are trapped vertically by stable stratifications, and laterally by Coriolis effects against the mountains. However, only the South African and the South American coastal disturbances are “coastal lows”; the remainder are generally produced by coastal ridging

Tornados in South Africa

Extreme weather phenomena are on the increase in South Africa, with fierce storms, tornadoes and heatwaves hitting parts of KwaZulu-Natal, Gauteng and the Eastern Cape recently.

A few weeks after KZN was lashed by a series of tornadoes, the country reeled from a heatwave that recorded 53.2°C in Vioolsdrif in Namaqualand in the North West.

Tornadoes can occur basically anywhere where a thunderstorm is possible. From an analysis of the occurrence of South African tornadoes it became clear that most of them have been observed in Gauteng, the Free State, KwaZulu-Natal (along a line from Pietermaritzburg to Ladysmith) and the northern region of the former Transkei. In figure 1 the eastern part of the country is depicted, showing the more significant events (F2 and F3) from 1905 to 1997.

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Some 65% of the South African tornadoes are classified as F0 or F1 (light damage), while more than 90% are classified as F0, F1 or F2 (considerable damage) or less. The tornado which occurred at Harrismith on 15 November 1998 was classified as F2 and the Mount Ayliff tornado which occurred in the Eastern Cape on 18 January 1999 was classified as F4.

Tornadoes can occur basically anywhere where a thunderstorm is possible. From an analysis of the occurrence of South African tornadoes it became clear that most of them have been observed in Gauteng, the Free State, KwaZulu-Natal (along a line from Pietermaritzburg to Ladysmith) and the northern region of the former Transkei. There seems to be a preference to mountainous areas.

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The effect of tropical cyclones on South Africa

The tropical cyclone season in our part of the world is from November to April, with the peak frequency in January and February. Only tropical cyclones moving into the Mozambique channel influence South Africa’s weather. When this happens, we usually experience dry weather over the interior because of the subsiding air surrounding a tropical cyclone. Only a few move in over or close enough to the land to cause destruction, and then usually north of the 25°S latitude. In such cases, the Northern Province, Mpumalanga and KwaZulu-Natal may experience destructive winds and the risk of flooding. Significant tropical cyclones that had such an effect on South Africa was Domoina which occurred in January 1984, Imboa in February 1984 and more recently Eline in February 2000.

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Cyclone Leon–Eline was the longest-lived Indian Ocean tropical cyclone on record, traveling over 11,000 km during its 29‑day duration

Climate change in South Africa and its consequences on the weather patterns

Inter-annual and decadal climate variability and extreme weather events are natural phenomena. This means that South Africa from time to time experiences years that are unusually wet and cool compared to the long-term average.

At other times the country experiences relatively very dry and warm periods. This type of variability is part of the Earth’s natural climate dynamics and is partially caused by oscillations and complex configurations of global and regional climate systems working in concert to produce our weather.

Climate change is different from inter-annual climate variability. Climate change is a result of global warming which is caused by human activities that have resulted in the emission of various pollutants, principally carbon dioxide. This has altered Earth system dynamics in a way that affects the climate system, ultimately causing trends and changes to climate and weather systems.

Climate models project that, given the current rate of carbon dioxide emissions into the atmosphere, and some unique features of South Africa’s climate system, like our location in the subtropics and the important role that high pressure systems play in controlling the system, temperatures in southern Africa are likely to increase by at least 1.5 times the global average rate of temperature increase.

According to a National Climate Change Adaptation Strategy published by the Department of Environmental Affairs last year, there is evidence that extreme weather events in South Africa are increasing, with heat wave conditions found to be more likely, dry spell durations lengthening slightly, and rainfall intensity increasing.

Climate zones across the country are already shifting, ecosystems and landscapes are being degraded, veld fires are becoming more frequent, and overused natural terrestrial and marine systems are under stress.

As part of its report, the Department of Environmental Affairs provided a summary of projected future changes in temperature and rainfall in South Africa.

Before the end of the current century (to 2099) it expects temperature increases greater than 4°C across South Africa, with increases greater than 6°C possible in the western, central and northern interior.

There is more uncertainty around rainfall projections than in temperature projections. A large number of projections predict generally wetter conditions over the central and eastern interior while other projections predict generally drier conditions.


References

https://visibleearth.nasa.gov/

https://www.mindat.org/climate.php

https://en.climate-data.org/africa/south-africa-61/

http://koeppen-geiger.vu-wien.ac.at/

https://www.researchgate.net/publication/45714537

http://www.wasaproject.info/

http://veaonline.risoe.dk/wasa/

https://backend.orbit.dtu.dk/

http://citeseerx.ist.psu.edu/

https://www.sa-venues.com/no/weather.htm

https://www.windguru.cz/

https://www.metoffice.gov.uk/weather/

http://www.sajs.co.za

https://en.wikipedia.org/wiki/Berg_wind

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