The pattern of weather
The sun controls the weather due to radiated energy towards earth. Only a fraction of this energy penetrates the atmosphere. Harmful ultra-violet light is absorbed by the ozone layer and other rays are reflected and scattered back into space by clouds, dust particles and water particles in the atmosphere.
The actual amount of heat to reached the earths surface varies according latitude, altitude, season and the time of the day. The earths spherical form and movement also prevent even heat distribution. Because earth spins on its own axis, a period of 24 hours, thus each half of the sphere gets heated for half the period. Earth is tilted on its orbit around the sun, thus the intensity of heat around the globe changes during the year, hence the seasons. The world maintains a balance of incoming and outgoing heat. The heat is redistributed through the air and ocean by convection and via conduction through the ground. Convection heat absorbed by evaporating surface water is taken towards the poles by certain wind systems (see Chart 4: Wind Patterns). Warm air rises towards higher altitudes and cools. When sufficiently cooled the water vapor held in the air condenses to form droplets. Clustering of the droplets takes place, cloud formation results and when enough clustering and colliding takes place, rain falls.
Wind is the result of air flowing from an area of high pressure to an area of low pressure. Air pressure is dependable on the temperature and the density of the air. Hot air rises ire the pressure is low, cold air sinks so pressure is high. Air pressure at the equator is low (hot air). At latitude 30° the upper air is cool and begins to sink earthwards, causing higher surface pressure. At latitude 60° very cold polar air forces the warmer air in front of it upwards and the air pressure is reduced. At the poles, due to the cold dense air, the air pressure is high.
Movement of air from a high-pressure cell towards a low-pressure cell is influenced by the rotation of the earth which will deflects the winds to the left of their direct path in the southern hemisphere and to the right in the northern hemisphere.
In the Southern hemisphere the surface winds are:
- the doldrums – a narrow belt of light, changing winds over the oceans around the equator;
- the trade winds – consistent south-easterlies from 30° towards the doldrums;
- the horse latitudes – a belt of changing winds between the trade winds and the westerlies;
- the westerlies – between 40° and 60° – known also as the roaring forties;
- a belt of wind is sandwiched between the westerlies and the weak polar easterlies, pushed out by the high pressure polar area.
The climate of Southern Africa
The weather varies substantially due to the topographic nature of the country. In general one would say the climate is from warm to hot, lush vegetation to a semi desert, Mediterranean to near zero temperatures, but seldom freezing temperatures. Snowfall do occur in winter months in particular areas.
The northern geographical latitude boundary is 22° South (i.e. within the Tropic of Capricorn), and the southern latitude border is 35° South, placing the land in the sub-tropical climate region. Generally the summer temperature varies from moderate average of 25° southernly to a very hot 35° in the northern parts, and the winter temperatures very seldom will be below freezing points and varies between southernly 15° and northern borders between 2° to 5°. Chart 1 and 2 indicates the average temperature measured in degrees Celsius for the month of January and July respectively.
Other factors involving the climatic temperature is the temperature of the sea currents along the eastern, southern and western coasts. Along the eastern and southern coasts flows the warm Mozambique and Agulhas current which brings warm tropical water from the equator down to the southern tip of the land. Along the west coast the cold Benguela current flows which brings cold water up from the southern ocean to the coast of South Africa. The influence that the water temperatures have on the air temperatures can be observed on the isotherm charts 1 and 2. If temperatures fall below freezing point, a condition called frost forms. These currents influence the annual rain fall, thus the vegetation, the animal life, the agriculture and influence the establishment of towns and industry.
South Africa, the land of sunshine. How true, because the average days of sunshine is 185 days, and an amazing 330 days of sunshine in the western area. “Come ye sun lovers and enjoy this glorious land of ours.”
Glorious sunshine, yes, but that means also low rainfall over a large area of the country as can be seen on the charts. About two thirds of the country receives less than 600mm of rain and mostly during summer. The area on the west and southwest coasts have a rainfall maximum during the winter months and a small inverted-T area (see chart) is lucky enough to receive rains right through the year, with an average between 800mm and more than 1000mm. Summer rainfall areas, eastern and inland areas, experience thunder storms and lightning and sometimes also accompanied hail storms. The average hail storms in this area add up to more or less 8 to 10 per year. In the south west and southern parts the rain is more intense and over a longer period. Snowfall along the higher mountains of the southern and eastern areas and the highveld of the northern areas. Evaporation takes its toll especially in the low rainfall areas such as the northmen cape, characteristic with drought and sparse vegetation.
To understand the pattern of rainfall, one should look at the circulation of air above the continent. Around earth there are two high pressure systems, the one in the vicinity of 30° northern latitude and the other at 30° southern latitude. South Africa is situated within the southern high pressure system. There are three wind circulation systems that will have an effect on the climate of South Africa, namely the Hadley cell, the Ferrel westerlies and the Polar cell. The afore mentioned are movement of air that creates the high pressure and low pressure systems of our climate. Around the equator warm air rises and moves high above the earth and descends at latitude 30° South where it cools and drops, and blows across the ocean in an easterly direction, thus completing the Hadley cell. South of this the Ferrel westerlies, movement of air creating low-pressure systems, allows cold air to move northwards from the Polar cell to bring rain to the southwest Cape. A high pressure means descending dry air and no rain, while a low pressure means ascending air, uptake of moisture, cooling and rain.
The rainfall pattern of Southern Africa can be simplified and divided into three main categories, namely:
- Winter rainfall area;
- Rain all seasons;
- Summer rainfall areas, which can be subdivided into three categories:
- maximum rain in January;
- maximum rain in January to March;
- maximum rain in March.
Climatic regions of South Africa
The southwestern Cape has a Mediterranean climate with hot, dry summers and cool rainy winters. Further east the climate is temperate, with warm summers and cool winters, and rain throughout the year. From the Border in the southeast to northern Natal, the subtropical coast has humid wet summers and warm winters and slightly drier winters.
The west coast from Namaqualand to Angola is hot and dry desert, with little rain in summer and fog along the coast.
Most of the interior is a semi-arid plateau, with cool, dry winters and hot summers with little rain.
East of this area is the eastern plateau with cool wet summers and cold dry winters. The north is hotter and drier.
The slopes of the plateau extend inland, and these areas has warm, wet summers and cool, dry winters. The subtropical Lowveld has hot, wet summers and dry, warm winters.
The subtropical plateau is hot and dry with some summer rainfall, which diminishes towards the west coast. Winters are rainless. More rain falls in the north, where it is also hotter.