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SOME COMMON SYNOPTIC WEATHER SYSTEMS AFFECTING SOUTHERN AFRICA
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introduction to weather events that occur in southern Africa, such as Tropical cyclone
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THEME 6: INTRODUCTION TO SOME COMMONSYNOPTIC WEATHER SYSTEMS
AFFECTING SOUTHERN AFRICA
6.1 HIGH-PRESSURE SYSTEMS (ANTICYCLONES)
6.1.1 THE NATURE OF A HIGH-PRESSURE SYSTEM
In nature an anticyclone is an area of high pressure surrounded by lower
pressure. An anticyclone is characterized by subsidence throughout the larger
portion of the troposphere, divergence at the surface and anticlockwise
circulation in the Southern Hemisphere (Figure 6.1). On a chart it may be
presented by a set of closed isobars with the highest pressures in the middle. As
the air in an anticyclone subsides it is warmed adiabatically (compressional
warming) and dries out as a result (difference between temperature and dew-
point temperature increases). The general subsidence of air occurs very slowly in
comparison to the uplift taking place in lows/depressions.
Figure 6.1 Atmospheric circulation associated with an anticyclone.
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,6.1.2 REGION OF OCCURRENCE
Although anticyclones occur randomly in the spaces between low-pressure
systems, they are mostly located in the subtropical belt between 20 and 35
degrees latitude (i.e. between the two planetary belts of low pressure). The
pressure is also fairly high over the Polar regions and one or more closed high-
pressure systems are frequently observed.
6.1.3 WEATHER ASSOCIATED WITH A HIGH-PRESSURE SYSTEM
Anticyclones are generally much larger than low-pressure systems and are
associated with weak winds and uninteresting weather. Subsidence and the
resultant adiabatic heating sometimes leads to the formation of an inversion in
the vertical temperature profile (inversion implies that the temperature increases
with increasing height). The air is mainly stable and dry and as a result little/no
storms or precipitation occurs.
In the case of a well-developed inversion a shallow layer of stratiform cloud may
occur over a widespread area due to turbulent mixing of the air in the lowest
layers, but dry air is found above the temperature inversion. During the winter
continental high-pressure systems may give rise to serious air pollution since
polluted air is trapped beneath the inversion and not transported away due to the
weak winds.
In the summer anticyclones usually bring bright sunny days over landmasses due
to strong surface heating and vertical mixing up to heights of 2 – 3 km under
conditions of low relative humidity.
Since South Africa is located in the subtropical high-pressure belt, anticyclones
play a very important role in our weather. The subtropical high-pressure system
of the southern Atlantic Ocean shifts eastward and extends a ridge to the south
of the country during the summer and mostly over the South Coast in the winter.
When a high-pressure system moves around the country, an onshore flow of
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, moist maritime air occurs to the front (eastern side) of the system. This moist air
is piled up all the way to the escarpment so that a reasonably thick layer of cloud
is formed from which rain or drizzle falls. An important part of the rainfall over the
southern and eastern coastal regions may be attributed to this occurrence. Under
such conditions rainfall occurs during pressure rises in contrast with the rule for
frontal systems in the mid-latitudes where rainfall occurrence is observed with
falling pressures (as trough moves through).
High-pressure systems, moving around the coast in summer, causes warm,
unstable air from the land to flow south and eastwards and to be lifted over the
wedge of cold, maritime air which builds up over the coastal regions. The latter
process is known as undercutting and is often a trigger for the occurrence of
heavy rain and thundershowers from continental air. It is clear, therefore, that the
combination of a cool onshore airflow and South Africa’s exceptional topography
work together in order to produce this precipitation in the front part of an
anticyclone.
6.1.4 TYPES OF HIGH-PRESSURE SYSTEMS
Distinction is made between two types of anticyclones, namely cold and warm
systems.
6.1.4.1 COLD ANTICYCLONES
In a cold anticyclone cold air is found to be present in the bottom layers of the air
column. Since cold air has a higher density, surface pressures can attain high
values. Normally cold anticyclones will be situated polewards of the polar front
(Figure 6.2) and are frequently located over the large continents of the Northern
Hemisphere (e.g. over Siberia and Canada) or between two moving low-pressure
systems in the Southern Hemisphere. Over the continents of the Northern
Hemisphere the sea-level pressure may rise to 1060 or 1070 hPa at times, whilst
it has never been higher than 1050 hPa in the Southern Hemisphere. Cold
anticyclones move fairly fast - at an average speed of 25 knots. Above 2.5 –
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