Dust in the Free State (South Africa)
On 19 September 2013 a dust storm was observed on the Dust RGB over central South Africa.
04 October 2023
16 October 2014
By Estelle de Coning, Gregor Feig, Seneca Naidoo, Lee-ann Simpson, Jannie Stander (SAWS)
The Meteosat-10 Dust RGB imagery (Figure 1) shows that on 16/17 October a persistent dust cloud travelled northeastwards for 800km.
Although dust storms are far from being a regular event in South Africa, they do occur on rare occasions over the central interior of the country. The land in central parts of South Africa is largely semi-arid range land and farming areas, where large tracts of land have no or little vegetation during the spring months. Persistent, strong, one directional winds have the ability to lift this dry soil above ground level, suspend it in the lower parts of the atmosphere, and move it away from its point of origin. What is uncommon in South Africa is a dust storm which moves more than 800km across the country, leaving residents running to close windows and doors after they realise that 'that big dust cloud' is actually about to move over them.
On 16 October, a surface cold front was located over the central interior of South Africa (Figure 2) accompanied by a developing upper cut-off low (Figure 3) as forecast by the Unified Model.
Figure 2 shows a strong surface pressure gradient in the sea-level pressure field associated with the cold front.
This strong pressure gradient occurred over the central parts of South Africa where a 1004hPa low was situated. A strong surface high pressure system of 1028hPa was situated south-west of the country.
Figure 3 shows the development of an upper cut-off low pressure system over the central interior of South Africa, as indicated by the 500hPa geopotential height field (gpm) at 12:00 UTC.
Strong surface south-westerly winds of 28–37km/h (15 to 20kts) — due to the strong pressure gradient — can be noted in Figure 4 behind the cold front over the central part of the country at 12:00 UTC, as forecast by the Unified Model.
These synoptic weather conditions resulted in strong surface winds over the central parts of South Africa, which was ideal for causing small dust particles to be suspended in the atmosphere. It is important to note that, climatologically, the central interior of South Africa is characterised by dry winters. It is also the main maize crop farming area of South Africa.
Crop fields are ploughed at the end of winter (Jul/Aug) in anticipation for the spring and summer rains (from September onwards). Due to very little to no rainfall over this area up to this period in the rainy season, and the availability of dust particles, this was the ideal situation for dust storms. Figure 5 further illustrates the dry surface synoptic conditions associated with the strong winds. Dew point temperatures expected by the Unified Model were below zero over the affected areas (dark brown shading).
The Dust RGB uses a combination of infrared channels to distinguish features such as dust in the atmosphere — shown as pink areas. The first appearance of a dust cloud was noticed at 11:00 UTC — over the central part of the country (Figure 6).
As shown in the animation, Meteosat-10 Dust RGB , from 11:00 UTC to 22:00 UTC the dust cloud became increasingly more distinct and moved northeastwards to the northern part of South Africa. Overnight, the dust cloud persisted, as it moved into the northeastern parts of South Africa, until it reached the border with Zimbabwe early on 17 October.
The dust cloud could be expected to have persisted until the wind moderated and was no longer able to suspend the dust particles. On Figure 7, Natural RGB, 17 October 05:00 UTC, the dust cloud can still be clearly seen as a light brown cloud on the border of South Africa and Zimbabwe.
The South African Weather Service operates six ambient air quality monitoring stations in the Vaal Triangle Area, located to the south of Johannesburg (Figure 8).
The instrumentation consists of Thermo Scientific FH62 instruments fitted with either a PM10 or PM2.5 sampling head. The instruments are set to a measurement range of 0-1000µg/m3, except the PM10 instruments at Sharpeville and Sebokeng which are set to a measuring range of 0-1500µg/m3.
The PM10 mass concentrations at the monitoring stations showed a rapid increase starting at 15:49 UTC (17:49 local time) at Zamdela (the southwestern most station) to 16:17 UTC (18:17 (local time) at Kliprivier (the northwestern most station).
A tenfold increase in the PM10 mass concentration was observed within the space of approximately 20–30 minutes, at which point the instrument range settings were exceeded.
The PM10 mass concentrations had returned to background levels by 19:30 UTC that evening (Figure 9). These ground measurements of the sudden change in dust particles in the air corresponds well with the images of the same time on the 24-hour Dust Microphysics images.