Weather Conditions Terrain & Task Area
Generally, Gariep and the Karoo has outstanding weather conditions for gliding. Summer temperatures regularly reach 35 deg C and quite often 40 deg C. Cu is typically 1/8 to 4/8. Cloud base is normally from 12000ft to 18000ft amsl.
Click here for today’s gliding weather, click ‘central Interior’ and check the expected thermal strength for any given time. The map shows Gariep.
There are no coastal effects in Gariep’s task area. During December and January the relative humidity is low, but occasionally there may be isolated thunder showers in the afternoon. Mostly showers and storms are localised, and are of no concern to glider pilots on long flights. Gariep and the Karoo is much drier than the northern parts of South Africa, such as Mmabatho in summer, which often suffers from moist air masses originating in Namibia.
Winds are normally from the NW or W and may be up to 15 knots during the middle of the day. With an airfield surrounded by grassland and a nature reserve, plus tarmac runways, airfield dust is almost zero.
Left Photo: DG505 cruising under heavenly skys.
Thermal strengths are often over 6m/s on the high cloud base days. This means very high true airspeeds and record breaking at Gariep is a common occurrence.
On most days soaring starts at 10:30 and lasts until 19:00. 1000km flights have been started as late as 11:30 and some very long flights have been started at 09:30. Its not uncommon to fly two 300km tasks the same day.
You don’t need an open class glider or thousands of hours experience to fly long, fast tasks at Gariep!
Typical Cu at FL150
Final glide at 1930
TERRAIN AND TASK AREA
The airfield is situated in a wide grassy valley next to a few small hills on one side and a man-made lake on the other side. The lake affects gliding operations very little, but is a beautiful landmark.
Tasks are mostly to the west and as one flies further in this direction the grassland become progressively more arid, until after a couple of hundred kilometers it begins to look like a semi-desert.Further west the grass is replaced with scrub and rock.
During any flight, the scenery is constantly changing. You fly over lakes, rivers, rocky mountains, little old pioneering towns, until far in the west one is climbing on Kalahari dust devils to 4000m above the ground.
Outlanding opportunities vary according to the direction and distance from Gariep. In some areas there are suitable fields almost everywhere (usually to the wetter north and east), and sometimes suitable fields are 20 kilometers apart, but this is not a problem when the cloud base is say 15000ft ! Good airfields are usually less than 50km apart and we have marked all these on a map. In the last 15 years we have experienced no serious outlanding damage. Being far away from the big cities, the local country people are very friendly and helpful if you land out. Entertaining a foreign glider pilot could be the year’s highlight for a Karoo farmer’s family and his staff.
Cellular (mobile) telephone coverage is suprisingly good over most of the area. 4×4 Vehicles are not normally required, the road network is good and trailers don’t fall to bits on a retrieve! Unlike some other sites, you don’t need a motorised open class glider to operate safely from Gariep.
These tasks are mostly set south of Bloemfontein and Kimberly, since afternoon thunderstorms are fairly frequent from Bloemfontein northwards, and the airspace becomes complicated. Our tasks therefore stretch as far west as Olifantshoek or Prieska and as far as Victoria West to the south. We have a huge uncontrolled airspace available to us and for those who don’t mind chatting to our friendly ATC, possible task distances exceed the daylight hours available. Transponders are only necessary if you want to be able to divert into or over the busier hubs. (also see The “Magic” of the Trough Line )…
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 …