Introduction | South Africa boasts one of the greatest variety of
climatic types of any country in the world. The tropical conditions over the
eastern coastal regions change rapidly into savannah over the eastern highveld
before changing into semi-arid and eventually arid conditions on the West Coast.
The extreme south-western areas enjoy a Mediterranean climate. Mafikeng lies
in the region between the savannah and semi-arid conditions. |
Weather Patterns Affecting the Region during Mid-summer | By mid-summer the
sub-tropical high pressure belt which dominates the weather during the winter
months has moved somewhat southerly and split over the central interior of the
sub-continent. This results in tropical sourced air to flow southwards from the
equatorial regions over the central and eastern interior. The moisture content
of this air mass can vary from very moist to somewhat dry, depending on its
track from the source region. Dry, somewhat, stable superior air is found to the
west of the moist air and is very often separated by a very narrow dividing
line. The position of the dividing line or surface convergence is the main
factor in the weather conditions affecting the regions as it is in this region
that over-development will occur. The intensity of the over-development is
dependent on properties of the two interacting air masses and the influence of
thermal heating. The position of the convergence line pendulums backwards and
forwards from a fulcrum somewhere over north-west Botswana in sympathy with the
surging of the Atlantic high pressure cell along the south coast of the
sub-continent. On occasions the southerly flow of tropical air mass can be so
intensive that it can result in overcast conditions with general rain. |
Mid-Summer Weather Conditions | Under the Influence of
Tropical Air-mass Under such conditions the sky is generally clear in the morning. Between 10h00 and 11h00 cumulus cloud at a base between 1000m and 1300m will begin developing. By 13h00 the cloud cover can be expected to be between 4 and 5 octas at about 1500m and 1800m. Over-development, if any, will be limited to isolated storms only. On occasions cool, moist air from the north-east can result in overcast conditions with a low cloud base in the morning. On most occasions the cloud will burn off by 09h30 and be followed by normal convective cloud. On such occasions the change of over-development is very small. Flying day 10h00 to 18h00. Under the Influence of the Superior Air-mass. When the superior air-mass is dominating the region, the skies will be generally clear with small amounts of convective cloud developing after 13h00 at cloud bases of 2000m to 3000m. No over-development is expected. Flying day 11h00 to 18h00. In the Vicinity of the Convergence Area. The day starts off much the same as when the tropical weather dominates. However, once the convective activity begins, the clouds over-develop rapidly with thunderstorms often by 13h00. It must however, be remembered that in general the activity is only a few hundred kilometres wide. Even under these conditions a task can be flown, provided it is set correctly. Under a Strong Influence of Deep Tropical Air Overcast, rainy conditions prevail under these conditions, making flying impossible. However, on occasions, depending on the intensity of the air-mass, the cloud can begin dispersing in the late morning as a result of the thermal influences. Convective cloud at a base of 800m to 1200m will then develop. This is not a regular occurrence. |
Weather Data | |
| Meteorological details have been extracted for points in the task area from the national 70 year database for the time period over which we intend to hold the Championships. |
Cloud cover averages between 3/8 and 5/8. Cloud bases can be as high as 19000ft (a.m.s.l.), but average 12000ft. Thunder storm activity can be expected on 38% of the flying period. The majority of these storms would be small with high cloud bases that would not pose a significant problem for task setters or pilots. The likelihood of hail is minimal. The average hours of available flying time per day is 6 and the maximum 10. Rainfall. Out of the contest period 14% will experience rainfall of more than 10mm, 38% will experience between 1mm and 10mm, and 47% of the days will have less than 1mm. Average maximum and minimum air temperatures are 31°C and 16° C respectively for the competition period. Highest monthly average temperatures can be expected to be between 36°C and 20°C with diurnal peaks as high as 40°C. During the competition period the temperature will be above 20°C for all but two of the days and can be expected to exceed 35°C on 3 of the days. Diurnal relative humidity varies between 40 and 60 percent. Diurnal wet bulb temperature spread will vary between 4°0 and 10° over the competition period. |
Copyright © Soaring Society of South Africa 1997. Pages
updated by:
Stephen Thomas