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Index to African Weather Prospects
Africa's most significant climatic feature is its variable rainfall, driven by the continent's position between Earth's anticyclonic belts and its location astride the equator. In the southern part of the continent, the rainfall is highly seasonal, with a pronounced dry season in the winter (June) and an equally prominent wet season in the summer. December (start of southern hemisphere summer) is well into the rainy season, though not the wettest month, and eclipse watchers must contend with a very different climate from that which occurred for the 2001 eclipse.
In December ,tropical moisture pushes well southward as high temperatures under an overhead Sun drag Earth's weather equator, the Inter-tropical Convergence Zone (ITCZ), into northern Zambia and central Mozambique ( Figure 19). The Indian Ocean air behind the ITCZ is a generous reservoir of moisture, always ready to push southward when opportunity permits, but normally held at bay by the easterly trade winds. A second source of sub-tropical moisture can be found on the Atlantic side of the continent, where west and northwest winds cover Congo, Angola, and northwest Zambia with a moist airmass in December. This airmass is also opposed by the easterly trade winds; the boundary between the two is known as the Inter-Ocean Convergence Zone (IOCZ).
The anticyclonic (high-pressure) belt can be found, on average, along the 35th parallel, just south of the tip of the continent. The mean pressure pattern for December ( Figure 19) shows anticyclones on both the Atlantic and Indian Ocean sides of the continent with a weak low pressure trough running north-to-south over Angola, Zambia and Botswana (the Botswana low). The images suggests that they are relatively static, but daily weather charts show that they move from west to east in a semi-regular cycle, bringing an ever-changing pattern of cloud and sun. The anticyclones are of critical importance to the eclipse chaser for they are the main controllers of sunny skies, bringing the dry trade winds from the Indian Ocean onto the continent.
Overlying the anticyclones is a zone of westerly winds that carry weather disturbances of their own. These upper level troughs and lows have a great influence on the weather below as they destabilize the air column and allow convective showers and thunderstorms to grow much more readily than in a stable environment.
Moist air from the northwest and dry air from the southeast moderate the changeable weather and cloudiness across southern Africa in December. Under the influence of passing anticyclones, waxing and waning northerly winds, upper level disturbances, occasional frontal systems, and winds deflected by mountainous terrain, a dynamic climatology of cloud and sun competes for the eclipse watcher's attention. But in spite of the variability of the weather systems there is a generous supply of sunny weather in December in southern Africa to attract the eclipse-watcher. The best areas are in northern South Africa and southeast Zimbabwe. Should movement be necessary as eclipse day approaches, the many countries that are spread out along the track will complicate last-minute adjustments in observing sites, but the main controls on the weather should be predictable for several days in advance.
For Angola, this is the second eclipse in as many years, and for a few small coastal communities north of Benguela, a second chance to see totality. The two tracks actually cross a hundred kilometers offshore, but are close enough on the coast that a very narrow strip defined by the south limit of the eclipse of 2001 and the north limit of 2002 will enjoy a rerun of the spectacle. Beyond this interesting coincidence however, Angola has little to offer the eclipse-seeker because of its prolonged civil war. Recent events have seen an effective counter-offensive by government troops, but the area of fighting is traversed by the eclipse track and millions of land mines make travel very hazardous.
Leaving Angola, the Moon's shadow skips along the Zambian border and then crosses a narrow extension of Namibia known as the Caprivi Strip before heading southeastward into Botswana. The Caprivi landscape features an expansive broad-leafed forest that bespeaks of its relatively generous precipitation and several prominent rivers, particularly the Okavango. The eclipse path lies across East Caprivi, about 40 km west of the town of Kongola. The track is easily reached by the Golden Highway between Kongola and Katima Mulilo. The narrowness of the Caprivi Strip and the lack of north-south roads limit the ability to move to clearer skies if the day is cloudy.
In Botswana, the track crosses the marvelous Chobe Game Park but the wet season severely restricts travel within the Park. December is still early in the rainy season and the eclipse may be accessible with four wheel drive vehicles, but it is likely easier (though farther) to travel from Victoria Falls or Livingstone, Zambia, through Zambia to Sesheke and view the eclipse from the Caprivi Strip. Farther south, the track zigzags back and forth between Botswana and Zimbabwe, passing through Hwange National Park, one the finest conservation areas in the world. The center line within the park is relatively remote and areas to the southeast, closer to Bulawayo where the road network is denser, will be more accessible.
In southern Zimbabwe, the shadow path moves into better weather prospects. Between Plumtree and Beitbridge the center line can be followed for nearly 400 km on a network of roads, allowing a generous selection of eclipse sites and easy movement in case of adverse weather. Bulawayo lies only 50 km outside the north limit. From this city, major roads lead southwest to Plumtree and southeast to Beitbridge, both of which lie very nearly on the centerline. Acces ot this area is also possible from South Africa but the deteriorating political situation in Zimbabwe should be carefully reevaluated, especially in the weeks leading up to the eclipse.
Leaving Beitbridge, the eclipse track crosses briefly into South Africa, passing through the Venda Region and the northern part of Kruger National Park. The terrain becomes much rougher than the other parts of the eclipse track, crossing the peaks of the Soutpansberg, a mysterious land of magical lakes, rock engravings, legend and beauty. The Soutpansberg is richly vegetated, in contrast to the low veld over Zimbabwe, but the greenery is also a sign of the frequent mists and rains.
Kruger National Park is likely to be one of the primary destinations for the 2002 eclipse. A jewel of South Africa, the park is regarded as one of the finest wildlife management areas in the world. The center line travels across the drier part of the park, though in December the vegetation is a lush green because of the summer rains. Punda Maria and Shingwedzi are the two major camps within the zone of totality; Shingwedzi is barely north of the centerline. Of all of the National Parks crossed by this eclipse, Kruger is the easiest to reach and provides the greatest range of facilities.
Leaving South Africa, the track descends onto the coastal lowlands of Mozambique. Here, access to the center line can be obtained by following the coastal highway northward from the capital Maputo, crossing the Limpopo River near Xai-Xai (Shy-Shy) and proceeding onward another 40 km. The region has numerous palm-fringed beaches and resorts that should provide convenient bases for eclipse chasers. Floods in 2000 severely damaged roads and bridges in the Xai-Xai area though most of the route from Maputo to the center line was left in relatively good condition. Malaria is a severe problem in southern Africa during the wet season and no part of the eclipse track is immune from the threat. The disease is becoming increasingly resistant to drugs and stringent precautions should be taken by all eclipse observers to minimize the chances of being infected. Light-colored clothing, mosquito repellants, and an appropriate drug regimen will help reduce the risk. Most African hotel beds are provided with mosquito netting for security at night. With good medical advice and sensible precautions, the risk of malaria can be minimized.
The eclipse track is squeezed between the moist air circulations in the north and the drier trade winds in the south. At its early part, in Angola, the track is affected by the Atlantic monsoon winds, but farther southeastward the path moves gradually into the drier trade wind climate as it approaches the Indian Ocean. The presence of the easterly trades is no guarantee of fine weather however, as the controlling anticyclones have their own embedded disturbances that team up with the northern winds to bring the seasonal rains that define December's weather.
Most of the eclipse track is found in the zone of easterly trade winds. These winds are caused by the circulation around the belt of high-pressure anticyclones located south of the continent; the dry air in the center of each anticyclone is a virtual guarantee of sunny skies. Unfortunately, each high is separated from the other by a trough of lower pressure that usually contains the remnants of an ocean cold front and brings cloudy, wet weather if sufficient moisture is present. The eastward trek of the anticyclones and the intervening troughs brings an alternating pattern of rain and sun to south Africa. Secondary highs and high-pressure ridges also complicate the weather pattern, frequently reaching northward to extend the anticyclones' influence to the eclipse track.
The boundaries between Congo, Indian Ocean, and anticyclone airmasses create zones of wind convergence where air is forced upward. Since rising air cools and becomes moist, the boundaries are marked by extensive cloudiness and precipitation, usually in the form of thunderstorms. These convergence zones are not fixed in position, but slosh back and forth under the influence of high and low pressure systems as they pass across the continent. In general northerly flows are moist and southerly are dry.
One of the major controls on the weather across the eclipse track is a persistent heat low that forms over Botswana in the summer months. The circulation around this low brings northerly winds to the mid-continent and has the effect of pushing moist and unstable Angolan and Indian Ocean air southward into Botswana, Mozambique and northern South Africa when it intensifies. The moisture is usually capped by a temperature inversion that suppresses the development of convective clouds, but when upper level troughs or lows are also present, much more instability is created and the airmass erupts in showers and thunderstorms. These bring much-needed rain to the area, but also form extensive cloud shields that can spread across much of the eclipse track.
As in all climatologies, Africa has its share of typical weather patterns. Recognition of these patterns could help in establishing an eclipse site where sunshine is most likely.
One of the more effective rain-producing patterns begins with a high-pressure system located over the eastern eclipse track, usually with sunny skies overhead. On its western side, satellite images often show the formation of a long band of cloud angling from coastal Angola across Namibia and Botswana into central South Africa. This cloudiness is associated with upper level changes in the winds that occur near the Botswana low. The cloud band in the satellite images shows that the cloud is linked to the tropical moisture behind the IOCZ. A generous rainfall is produced if the Botswana low strengthens along this convergence boundary.
Southwest of the continent, some hundreds of kilometers distant, a band of frontal cloudiness marks the approach of an inter-anticyclonic trough. The approach of this trailing system induces the Botswana cloud band to move eastward to replace the sunny skies over South Africa's northern provinces, ending good eclipse weather. Occasionally the frontal cloud band will join with the leading cloud band, but the effects of the union are usually felt over southern regions, well away from the eclipse track.
Another significant pattern develops when an anticyclone invades the eastern parts of South Africa or strengthens in the waters offshore while a similar ridge prevails along the Atlantic coast. Between the two highs, the Botswana heat low deepens and stronger northerly winds develop on its east side. These northerlies carry high humidity tropical air southward into South Africa where mountainous terrain and instability can induce flooding rains. The presence of the high along the Indian Ocean coast often prevents this rainfall from moving into the eastern parts of the eclipse track, at least in the early stages of the event. In many respects this pattern is similar to the first example above, except that the intensity is more pronounced and the area of precipitation is likely narrower, in spite of higher rainfall amounts. A major difference however is that this second pattern tends to linger in place rather than moving eastward.
A third pattern begins when an cold low forms in the upper atmosphere, typically over the west coast of South Africa from where it drifts slowly eastward over a period of several days, isolated from the usual westerly flow aloft. The low induces a wide assortment of unfavorable weather to much of southern Africa. Moist tropical air flows southward across the mid continent, blocked from eastward motion only by the western slopes of the Drakensberg Mountains, bringing widespread rains. Shortly thereafter, and on the opposite side of the mountains, surface winds bring moisture onshore where the rising terrain induces a second extensive rainfall. This pattern usually lingers for a few days before the low weakens and the upper westerlies return.
To an eclipse watcher, the most important feature of December weather systems is the traveling high-pressure systems that move across South Africa. When an anticyclone is centered over Mozambique and eastern South Africa, the Botswana low is weakened and pushed well to the north over Angola. Winds are generally easterly to northeasterly across the eclipse track and fine weather dominates. If the pattern persists for an extended period, it can bring extreme dryness and even drought. Even in the wet patterns described above, dry areas can be found in the midst of the highs that characteristically inhabit the east end of the eclipse track and that is where the best weather prospects are to be found.
Discussion of the weather systems affecting south Africa would not be complete without mention of tropical cyclones. December is within the cyclone season; on average, there are eleven per year in the southwest Indian Ocean. The cyclones tend to form northeast of Madagascar and track west and southward, occasionally entering the Mozambique Channel before dying against the African coast or curving back to the east. This is an infrequent occurrence Ð about once every two yearsÐbut there are years when two cyclones manage to reach into the Channel. They may even be of some benefit to the eclipse seeker, as a sunny anticyclone is usually found over northern South Africa when a cyclone first moves into the Mozambique Channel.
Table 20, a collection of useful climatological statistics, Figure 20, a satellite-based map of mean cloud cover, and Figure 21, a graph of mean cloudiness along the track extracted from Figure 20 together present the cloud cover statistics in considerable detail. Two columns in Table 20 speak best for the assessment of cloud cover. These are the percent of possible sunshine and the sum of clear and scattered cloud. The highest value on the eclipse track for the percent of possible sunshine (the number of sunshine hours divided by the time between sunrise and sunset) lies at Beitbridge (59%), a small community in Zimbabwe on the border with South Africa.
Correspondingly high values for the sum of clear and scattered cloud can be found at Shingwedzi in Kruger National Park. The cloud cover map in Figure 20 and the graph in Figure 21 confirm these results, showing a minimum in mean December cloud cover in the same area. Careful examination of these data shows that the best prospects can be found in an area stretching from Beitbridge in Zimbabwe across Kruger National Park in South Africa to Massingir in Mozambique. Beitbridge is an arid area of Zimbabwe, though prodigious rainfalls have been known to occur when a rare tropical cyclone manages to reach the area.
Most of the cloudiness in the southern African summer is convective. Satellite images show that the cloud is generally organized into large systems, so that one area might be nearly overcast while adjacent parts several hundred kilometers away are completely clear. And though areas of thunderstorms tend to have holes in the cloud cover on a finer scale, these are small, difficult to find, and impossible to predict more than a short time ahead. Convective clouds build through the day as the ground warms, with the result that a cooler morning eclipse is likely to have less cloudiness than an afternoon one. The frequency for clear, scattered, broken and overcast cloud in Table 20 accommodates this diurnal trendÐthe numbers apply to the morningÐbut the percent of possible sunshine is probably slightly pessimistic.
Temperature is intimately related to sunshine, altitude and moisture and the eclipse track shows evidence of all three influences. Low elevation coastal regions in Mozambique have the warmest daily highs, reaching up to the mid thirties Celsius. On the interior plateau average highs range through the upper 20s to the lower 30s, about five degrees cooler than coastal regions. Over coastal Angola, the daily maximum is suppressed because of the presence of cool water offshore. Nighttime temperatures are about 10 degrees below those of the day, a small difference that reflects the relatively short nights, frequent cloudiness and high moisture content of the atmosphere. In the most promising region, from Beitbridge in Zimbabwe to Kruger National Park in South Africa, eclipse observers can expect daily temperatures to range from the lower 20s in the morning to the lower 30s in the afternoon.
The rainfall statistics in Table 20 match the pattern of sunshine, with much lower amounts and frequencies in the sunniest regions. At Beitbridge and Messina, monthly amounts are less than 60 mm. West of Beitbridge the mean monthly precipitation rises abruptly to over 150 mm, and then increases more slowly to over 233 mm at Huambo in Angola. Along the Angolan coast rainfall decreases abruptly (to 61 mm at Lobito) because of the stabilizing influence of the cold Atlantic water. On the opposite side of the continent, east of Messina, the increase in rainfall is more gradual, averaging 88 mm at Shingwedzi in Kruger National Park and just over 100 mm at Maputo and Panda on the Mozambique lowlands. The rainfall minimum is a caused by the frequent anticyclones that inhabit this area and the higher terrain to the southeast that blocks the flow of moisture from the Indian Ocean. Also noteworthy in the statistics in Table 20 is the very low frequency of rainy days at Beitbridge.
Rainfall in southern Africa is highly variable, and the mean statistics in Table 20 greatly smooth the alternating wet and dry years. At Bulawayo the maximum rainfall reported in one set of climatological compilations shows a minimum rainfall of only 3 mm in one December and 273 in another. The range for Messina is between 4 and 141 mm while that for Maputo ranges from 10 to 244. Eclipse chasers could encounter a year with exceptionally dry conditions and very sunny skies or the exact opposite if the weather is wet. Fortunately, and in spite of the range between maximum and minimum, rainfall patterns are actually less variable over northern South Africa than in most other parts of the country and approximately normal conditions can be expected along the eclipse track in about 80% of Decembers.
Winds are generally light along the eclipse track, save for occasional gusts from thunderstorms and the rare presence of a weakening cyclone along the coast. Mean wind speeds range from 7 to 15 km/h along the track, with lighter values inland away from the coast. The prevailing direction is easterly over Mozambique and much of northern South Africa and then turns to northeasterly and northerly over Zimbabwe, Botswana and Angola. This northerly change is a result of the mean position of the Botswana low. Gale force winds (greater than 36 km/h) are a coastal phenomenon and occur only once or twice in the average December.
The multiple signals in the climatological record and the satellite imagery point to sites between Beitbridge in Zimbabwe and Massingir in Mozambique as having the best viewing prospects. The single best location, based on the available data, seems to lie at Beitbridge, a community of 6000 on the north bank of the Limpopo River. Because Beitbridge marks the main border crossing between Zimbabwe and South Africa and is very close to where the center line crosses, viewing sites can be selected in either country. The relatively high sun angle and convective nature of the cloudiness, which will be at a minimum for a morning eclipse, suggest a probability of success around 60 percent.
Adapted from Adapted from NASA TP 2001-209990 "Total Solar Eclipse of 2002 December 04."
Permission is freely granted to reproduce this information and data when accompanied by an acknowledgment of the source:
"From Total Solar Eclipse of 2002 December 04 by Fred Espenak and Jay Anderson, NASA"
WebMaster: Fred Espenak Planetary Systems Branch - Code 693 e-mail: espenak@gsfc.nasa.gov NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771 USA
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