Weather Conditions in Southern Africa
Extraordinary soaring flights, such as Klaus Holighaus’ world record over a 1400 km triangular course and Helmuth Fischer’s speed world record over a 1000 km triangular course of 169,7 km/h, have put the spotlight on South Africa as one of the prime “hunting grounds” for world records.
Due to the lack of understanding of Southern Africa’s weather mechanism, the potential of this area has not been fully explored. Record attempts were mainly undertaken during relatively short periods and launch points were not systematically chosen. This is not surprising as to date no in-depth analysis exists of the South African weather conditions in respect of soaring flights.
With this paper the author attempts to shed some light on the subject.
The Weather Pattern of the South African Summer
Most of the interior of South Africa is situated at an elevation of between 3.000 and 5.000 ft MSL. Therefore, only atmospheric conditions above 950 – 850 mb are of interest for the soaring areas under discussion. During the summer months, the interior is screened off from the effect of frontal systems by coastal mountain ranges, in particular to the South of the country.
Two oceans surround the “Southern Cape” of Africa. The Atlantic Ocean to the West and the Indian Ocean to the East and South East. Two currents determine the temperature of the two oceans. Originating from the Antarctic region, the cold Benguela current brushes the Western cost of the country. In contrast a warm current emanating from an area South of Madagascar affects the Indian Ocean to the East and the South East of the country.
The major weather engine for the Southern African region is a strong high pressure system over the Atlantic Ocean, which periodically sweeps around the country to link up with a high pressure system over the Indian Ocean. The outstanding and, for soaring, most important feature in this “concert of air masses” is the formation of a heat low over the interior of Southern Africa during the summer months. Due to its elongated shape this is frequently referred to as the “trough line”.
Figures 1 and Figures 2 show the typical summer weather pattern of Southern Africa at the 850 mb level and at 13.000 ft respectively. The corresponding weather pattern is as follows:
Warm moist air is moving in from the East. Dewpoints in the area around Johannesburg would be typically in the region of 15 – 17 degrees Celsius. The inversion East of the trough line is predominantly weak or non-existent, which frequently leads to the development of thunderstorms. Therefore, the interior of South Africa is also referred to as the summer rainfall region.
Figure 3 shows the lightning density that supports the abovementioned statement. For example, Johannesburg has one of the highest lightning densities in the world.
Having lost most of its moisture, the subsiding air to the West of the trough line, together with a strong high pressure system in the upper air, produces cumulus clouds with the characteristically high cloud base for which South Africa and Namibia are well known in gliding circles.
Figure 4 shows the average rainfall in Southern Africa. It can be seen how the trough line separates arid regions to the West from prosperous agricultural land in the East. Maize and cattle farming characterise the area East of the trough line whereas sheep farming dominates the area West of the trough line.
The Significance of the Trough Line for Soaring.
The influx of cool unstable air, which in central Europe during spring can lead to extraordinary soaring conditions, is unknown in South Africa. Only convective processes as a result of high temperatures play a role. Cool Antarctic air pushed into the interior from the South or South-West by the Atlantic High, is an absolute “killer” for soaring conditions.
As a result of regular thunderstorms the high moisture content of the soil East of the trough line has a similar detrimental effect. This, together with the high dewpoints, leads to a low cloud base (4.000 to 5.000 ft AGL) and weak thermal conditions in the East, e.g. Johannesburg area.
This situation is vastly improved closer to the trough line where due to moisture loss of the air the cloud base is lifting. However dewpoints are still high enough and together with a weak inversion, thunderstorm development prevails. Subsiding air just West of the trough line forms an inversion – so vitally important for successful soaring flights. This subdues the thunderstorm activity and together with the now significantly dryer air (dewpoints near De Aar between 3 and 7 degrees Celsius), leads to the formation of flat cumulus clouds with a cloud base of between 14.000 and 18.000 ft MSL as well as strong thermal activity – a glider pilot’s dream.
Figure 5 and Figure 6 show typical summer tephigrams of the Johannesburg area and just West of the trough line (De Aar) respectively which illustrate the above effects.
Even further West conditions for soaring deteriorate with the Atlantic High now coming into effect with more stable air and a drop in the inversion level.
Therefore, the position of the Atlantic High, but more importantly, the position and the vertical extension of the heat low (trough line) and their movements during the course of the day, become the critical planning parameters not only for the setting of the task, but also for the selection of the launch point.
The …