Coen Welsh | Nov 14, 2017 | 0
Weather 12 February 2016
The topography of southern Africa has a marked impact on local weather. The inland plateau is on average some 1400 to 1600 metres above sea level with a prominent escarpment both on the western side and on the eastern side. Along the western coastline, the coastal plain averages about 140 km, in the southern Cape it is very narrow, i.e. less than 50 km gradually widening further to the east until it becomes an extensive lowland in Mozambique, in many places some 400 km wide.
This elevated central plateau reaches into western Zambia and southern Angola before it starts falling away to the central African lowlands. It is a major obstacle that any air travelling from the Indian Ocean, must cross before it can have an effect over the southern African interior. Namibia lies on the western side of this large plateau so whatever weather develops over southern Africa, it reaches us and the northern Cape last. It translates into reduced atmospheric moisture since the bulk has been spent over Mozambique, Zambia and Angola.
Then there is the very dominant South Atlantic high pressure cell which tends to oppose and dispell the movement of air from the Indian Ocean. The effect is most pronounced (and visible) on the surface, but as happened this week, it can also reach into the upper levels between 45,000 and 56,000 feet.
The central plateau is prone to excessive heating during summer. It creates a so-called heat-low during the day but the moment the sun passes noon, the energy subsides and the system very slowly contracts back to the surface. When high pressure control exists above this heat-low, the depth of the atmosphere very very slowly compacts, and it leads to the excessive late afternoon heat Namibians are so familiar with. This occurrence is also detrimental to rain, again witnessed this week.
This week saw a typical hamburger atmosphere. On the surface high pressure conditions reigned, reflected by a cloud base at around 16,000 feet. The airflow was westerly to south-westerly. In the mid-levels up to about 35,000 feet, the airflow was northerly to north-westerly advecting cloud from Angola into Namibia along a so-called convergence line that was not very well defined. The result was isolated showers of reduced intensity. In the upper levels above 45,000 feet, a high pressure system was present causing a strong zonal airflow from west to east.
This activity in the upper levels was greatly enhanced by a strong vortex just east of Madagascar. This low pressure system started as a tropical depression but as the week progressed, it developed into a Level 4 tropical storm. It also migrated towards the south eventually morphing into a string of low pressure cores south of the island. It is the very low pressures inside this vortex that created the huge suction pump literally sucking in all the upper air from west (high pressure) to east (low pressure).
The result is very limited moisture on the surface level, moderate moisture in the mid-levels but almost zero moisture in the upper air. The final result – no rain over Namibia.
The series of tropical storms south of Madagascar fizzles out as the migrating South Atlantic high pressure cell passes the southern Cape. The inland plateau remains under low pressure control due to heat. The core of this system covers Botswana where again it will be blisteringly hot with a similar but slightly reduced effect over Namibia especially along the Botswana border.
Thus over the weekend it remains very hot over the whole country with only very limited penetration of moisture from Angola and Zambia and then only north-east of the imaginary line that runs through Ondangwa, Otjiwarongo and Mata Mata.
It is only by Wednesday next week that the high has shifted far enough to the east to make room for a low pressure intrusion from Angola. Consequently, although isolated showers may occur over the north-eastern quadrant, the general prospects only improve during the latter half of next week.