The impacts of climate change on human and natural systems have become a cause for concern at the global, regional as well as national level. The effects of climate change on food, water resources, human settlements and health has initiated a lot of studies in assessing the extend of the impacts and looking for options on mitigating or how communities can adapt to these effects of climate change.
These assessments are only possible when information is available and the Department of Meteorological Services is a one stop shop in the provision of all the information required for operational efficiency and decision making. Changes in temperature, the performance of seasons, the frequency of drought are all a cause for concern and the Department is in a position to provide this information. The Department is willing to work with various organizations in coming up with the tailor-made products that can be incorporated into current and future plans.
The Climate of Zimbabwe
1.a What is climate?
Climate is the average state of the atmosphere over a particular place or region of the earth's surface, related to a particular period of time and taking into consideration the average and extreme variations to which the atmospheric state is subject. In other words, both average and extreme values and frequencies of weather elements constitute climate. Climatic data are usually expressed in terms of an individual calendar month and/or season and are determined over a period of 30 years or more.
The main climatic elements are: precipitation, temperature, humidity, sunshine, wind velocity, fog, thunder, hail and soil temperature.
The climate of a place is mainly determined by its latitude, position relative to continents and oceans, position relative to large scale atmospheric circulation patterns, altitude and orography.
The broad classifications of climate derived from some of the above factors are: tropical continental, tropical maritime, polar continental and polar maritime.
Season refers to any of distinct periods into which the year may be divided, particularly in terms of climatic conditions, as a result of changes in duration and intensity of solar radiation. the beginning and end of a season usually coincide with or are accompanied by biological and phenomenological changes which are caused by changes in climatic conditions.
In temperate regions the year is divided into four seasons, essentially reflecting the life cycle of cultivated plants: winter (dormancy), spring (sowing), summer (growth) and autumn (harvest). In the tropics the division of the year into seasons is usually made in terms of rainfall and temperature.
1.c Factors governing the climate of Zimbabwe
Zimbabwe lies wholly in the tropics - stretching from 15.5 to 22.5 degrees south latitude. The sun is overhead twice a year. The least angle of elevation of the sun is 45 deg. over the south most parts of the country.
b) Position relative to the oceans and lands
Zimbabwe is completely surrounded by land. At its closest to the nearest ocean (the Indian ocean), Zimbabwe is at least 200 km away. The country does not experience direct effect of the ocean.
An important geographic landmark to the climate of Zimbabwe is the presence and position of Madagascar. The SE trades originating from the subtropical anticyclone are not afforded a thorough flow to mainland Africa. They deviate northward over Madagascar but curve southward over the Mozambique Channel, then turn northward over mainland Africa. The island also acts as a deviator of a large number of tropical cyclones which might directly change the climate of Zimbabwe an indeed, that of the whole subcontinent.
c) Position relative to the general circulation features
The southern hemisphere subtropical anticyclone and the ITCZ are the main features which influence the surface circulation and hence the climate of Zimbabwe. These two features normally oscillate at zero phase and in sympathy with the movement of the sun on the earth's surface.
The subtropical anticyclone gradually shifts northward over South Africa from April through to June and is usually north most in July. The axis of this zonal high pressure belt lies along the 27-deg south latitude. During the same period the ITCZ will be also moving northward. Although it does not move en masse as a belt, the ITCZ's winter position, latitudinally opposite Zimbabwe, is over southern Sudan in July and/or August. Zimbabwe's position relative to the subtropical anticyclone ensures that the surface flow has an easterly component throughout the year. In winter (mid-May to mid-September) the SE trade winds from the subtropical anticyclone predominate over the country. However the Eastern Highlands act as a strong mechanical deviator of the trades as they track northward and westward. On reaching the Eastern Highlands, the trades are deflected northward along the slopes and curve westward after the northern edge of the Highlands. Thus the trades reach Harare and surrounding areas as an easterly flow.
The subtropical anticyclone recedes south-eastward and south-westward from Southern Africa in August through to October and is replaced by a trough. The trough normally stretches from Zaire through Angola, Namibia and western Botswana and has its southern limit over the Orange Free State. It is locally referred to as the leader of the ITCZ over Southern Africa. It has no remarkable weather associated with it. As the anticyclone recedes and the equatorial trough replaces it the airflow across Zimbabwe gradually backs and becomes NE'ly over the northern areas and easterly in the south. The mean position of the ITCZ proper at this time is along the 5-deg south latitude. Between November and December, a series of cold fronts pass over South Africa stretching from the Southern Sea to as far north as the 30-deg south latitude.These fronts usually link with the equatorial trough to form what is locally referred to as westerly waves. Behind the southern limit of the waves there is always a high pressure system. As this combination continues moving eastward the wind flow over Zimbabwe first becomes wholly NE'ly when the part of the wave nearest to the country is over east Botswana. The flow changes to SE'ly in the southern districts as the southend of the wave passes South Africa and the high pressure system covers the country (i.e. South Africa). The SE'ly flow of course starts from eastern South Africa and works its way to southern and then northern Zimbabwe and sometimes as far as Zambia. The westerly waves have a period of 5 to 7 days.
In January and February, the wind flow is predominantly NE'ly over the whole of Zimbabwe as the equatorial trough dilates over central Southern Africa, to cover Zimbabwe, Botswana and Transvaal as well. Then it is the ITCZ. As the ITCZ tracks northward beginning from mid-February, the wind flow over Zimbabwe veers to E'ly until mid-May and the cycle starts again.
Occasionally, however, the equatorial trough and the ITCZ move further south than the above described position and the subtropical anticyclone recedes further poleward than is normal. The wind flow over Zimbabwe becomes more northerly or even NW'ly which is very moist. On the other hand, sometimes the subtropical anticyclone remains anchored over Southern Africa thereby blocking the southward shift of the equatorial trough. In this case the airflow across Zimbabwe remains predominantly E'ly which is relatively dry.
The altitude of Zimbabwe ranges from 500m in the major river valleys to 2600m over the highest peaks of the Eastern Highlands. The main watershed bisects the country in the SW-NE direction from Plumtree to Bindura. Another plateau connects the watershed to the Eastern Highlands from Harare through Marondera and Rusape to Nyanga. The watershed lies between 1200 and 1500 m. The Eastern Highlands are the highest land in the country with the highest peaks extending to 2600m above mean sea level.
The climate of Zimbabwe derives its characteristics from the geography of Southern Africa as a whole (and indeed that of the whole world) and not just the country's alone. It is therefore presumptuous to stop at the geography of Zimbabwe alone as the only prerequisite to the knowledge of its climate. The geography of Southern Africa is briefly discussed here to complete the synoptic picture of the factors which influence the climate of the country. Southern Africa is defined here as that part of the African continent south of 10 deg south.
The topography of Southern Africa may be divided into 3 groups: the lowveld, middleveld and the highveld. The lowveld lies between 0 and 1000m above mean sea level (amsl). This comprises of the coastal plains and the main river valleys of the Zambezi, Cunene, Limpopo and the Orange rivers. The western coastal plain is roughly 75 km wide and extends inland through the Cunene and the Orange river valleys. The eastern coastal plain is 120 km wide in the south and broadens to about 250 km in the north.Its extensions inland are the Limpopo-Save and the Zambezi rivers.
The highveld is generally land at 1200m or more amsl. It has 3 cells: the Namibian, Zimbabwean and South African highlands. The Namibian escapement runs in the NW-SE direction parallel to the west coast. It is represented by a chain of broken hills in Angola. The South African highveld has its axis parallel to the east coast. It has the highest peaks in the subcontinent. These go as high as 3600m amsl. The highveld of Zimbabwe is a continuation of the south African plateau - only separated from the later by the Limpopo river valley.
The rest of the subcontinent is the middleveld and is found in the interior separated from the coastland by the highveld.
The river valleys serve as routes of air of maritime origin into the interior. On the other hand, the highlands, running parallel to the coast, block the interior of the subcontinent from direct large scale sea influence. This tends to localize and differentiates the climate conditions of the two sides of the plateau.
e) General Circulation
An air mass is a large body of air which has more or less uniform horizontal characteristics of temperature and humidity. An air mass acquires its distinctive characteristics in a source region where there is a large and fairly uniform surface, either water or land, over which air remains fairly stagnant for a period of at least a few days. Source regions are most commonly areas over which air is subsiding. In moving away from their source regions, air masses will be modified by the surface over which they pass and thus their later properties will depend not only on their source region but also on the nature of the surface they pass over and their age since being formed.
Air masses are classified primarily according to their source region, which is considered to come within one of two latitude belts: Antarctic or polar and tropical, and either of maritime or continental characteristics. (This differs slightly from the northern hemisphere classification of polar, Arctic and tropical maritime or continental air masses. The southern pole is completely surrounded by the Antarctic and therefore the two (pole and Antarctic) are treated as one origin.
There are two major 'foreign' air masses which reach Zimbabwe. These are polar maritime and tropical maritime. The air mass which is otherwise resident over the country is tropical continental. The SE trades originating from the southern hemisphere subtropical anticyclonic cells and whose fetch begins from Antarctic, constitute the polar maritime air mass. The SE trades are a winter feature normally. They are cooler and moister than the resident tropical continental air during winter. However, should they prevail over the country during the summer season, they still remain cooler but are drier than the local air.
The tropical maritime air mass that reaches Zimbabwe is the NE trades originating from the northern hemisphere subtropical anticyclone usually centered over central Indian ocean (over the equator). The NE trades blow over the country during the southern hemisphere summer from the Indian ocean into the equatorial trough usually situated to the west of Zimbabwe but occasionally to the south of or over the country. In some cases, however, the NE'ly wind is nothing but a E'ly wind originating from the same latitude but only being deflected by orographic features to assume the north-easterly direction on reaching Zimbabwe. Winter NE trades signify the absence of the southern hemisphere subtropical anticyclone over Southern Africa. This is a breakdown of the monsoon over the subcontinent and usually means that SE trades will flow over Zimbabwe from time to time during the following summer. In such cases a drought or at least below normal rainfall is almost inevitable during the summer.
f) The El Nino
Literally the El Nino is a warm current of equatorial water that usually appears around Christmas time off the coast of western South America. Closely associated with the El Nino is the Southern Oscillation (SO). The SO is described as an intermittent 2-4 year quasi-periodicity observed in sea level pressure, surface wind and sea surface temperature over a number of places on the globe. In meteorology these two phenomena have linked and their union is popularly referred to as ENSO. ENSO is a whole set of complex ocean-atmosphere interactions associated with a remarkably coherent pattern of ocean and atmospheric anomalies. ENSO is simulcast with torrential rains over places like California, Peru, etc, and severe droughts over Southern Africa, Australia, India, etc. In fact it has been observed that droughts appear in most monsoon areas - of which Zimbabwe is one - during an ENSO episode.
For Southern Africa the pressure see-saw is between Indonesia and Southern Africa itself. It has been observed that low pressure over Indonesia appear concurrently with high pressure over Southern Africa and vice versa.
2 The climate parameters
2.a Surface temperature
The temperature range of a locality is primarily determined by the latitude, altitude and position relative to the nearest water mass. Other derivative factors also contributing to the temperature range of a place. These include cloudiness and precipitation and nature of the surface.
Surface temperature undergoes an annual sinusoidal cycle which closely responds to the obliquity of the sun's rays on the earth's surface and the length of the day (sunshine time) during any time of the year. As the sun is overhead at the tropic of Capricorn on December 21, the temperature should be at its maximum in the southern hemisphere and minimum on June 21 when the sun is overhead at the tropic of Cancer. March 21 and September 23 are the intermediate dates when the temperature should be halfway between the June minimum and December maximum.
The sun is overhead in Zimbabwe twice a year because of its latitude. As mentioned in section ...., the least angle of elevation of the sun is 45 deg over the extreme south in June. The temperature should be least in the south and gradually increasing northward, were this the only factor at play. In reality, however, the hottest month for most of Zimbabwe is October and the lowest temperatures are often recorded within the fortnight after the 21st of June. This modification of the temperature-solar obliquity relationship is caused by variations in humidity (general circulation), altitude, etc.
Cloudiness and precipitation
As mentioned above, the highest temperature is not experienced when the angle of elevation of the sun is maximum, i.e. in December and or January. Cloud cover and precipitation over Zimbabwe are maximum in December, January and February. Low and medium clouds block the sun's rays from reaching the earth's surface through reflection and absorption. Precipitation cools the air it passes through as it falls. This further decreases air temperature. Minimum cloud cover and precipitation are experienced in Zimbabwe during September and beginning of October, which is why, in part, the maximum temperatures are recorded then.
Temperature decreases with increasing height according to the first law of thermodynamics. The mean October temperatures for Chiredzi (420m amsl) and Nyanga (1870m amsl) are 32.5 and 21.5 degrees Celsius, respectively. Theoretically in a dry atmosphere, temperature falls by about a degree Celsius for every 100m rise in altitude. In practice, however, the atmosphere is never completely dry, hence the calculated lapse rate is always less than the theoretical figure (see comparison given above).
Nature of surface
The solar radiation, which reaches the earth's surface may be absorbed there, be transmitted downward if it encounters a material which is transparent to it or be reflected. The albedo or reflectivity of the surface depends on its substance and texture. For example, of the solar radiation incident on the earth's surface, desert sands reflect 35%, green grass or forest 10-25%, dry ploughed field 12-20% and moist soil about 10%. In part, the southwest of Zimbabwe is one of the hottest regions for this reason.
Daily temperature variations
The day-to-day temperature variations are very small, of the order of 0.5 of a degree Celsius or less. Occasionally, however, temperatures on consecutive days may differ by as much as 15 deg Celsius due to incursion of polar maritime air in winter.
Maximum temperatures occur just after mid-day and minimum temperatures just before sunrise. Diurnal temperature changes are of the order of 12 deg in summer and 15 deg or more in winter on a clear day. On the other hand, the changes may be as low as -2 deg in cloudy and wet conditions. The Eastern Highlands often experiences such small diurnal temperature changes.
Radiation is a direct function of temperature. It traces a similar annual cycle to that of temperature. The highest and lowest radiation values correspond with the highest and lowest temperatures, respectively. Maximum radiation in the country is received over the low lying areas in the main river valleys and the Kalahari. It is maximum in October for most parts of Zimbabwe. The south and southwest have their maximum radiation in January. As for temperature, radiation should be maximum during the December - January period. The onset of the rain season with its cloudiness and precipitation results in a marked drop in the amount of radiation received.
In meteorological practice sunshine is measured in number of hours of bright sunshine. It is determined by the sun's obliquity and cloudiness. It therefore undergoes seasonal variations inversely related to those of cloud cover. Zimbabwe has a maximum average of 9.5 hours of sunshine per day from May to October and a minimum of 5.5 hours in December when cloud cover is maximum.
In the country's cities with a number of manufacturing industries, industrial smoke and smog tend to reduce the number of sunshine hours especially around sunrise and sunset.
The common forms of precipitation in Zimbabwe are drizzle, rain, thunderstorm and hail.
Drizzle is light precipitation of drop diameter between 0.2 and 0.5 mm. It falls from stratus and stratocumulus clouds of thickness rarely exceeding 500m. Patchy drizzle occurs anywhere in the country after persistent rains-showers or thunderstorms but moderate to heavy drizzle occurs over south of the main watershed and the Eastern Highlands.
Rain is precipitation with drop diameter greater than 0.5mm and falls from extensive layer clouds, e.g. altostratus and nimbostratus, which form from precipitating convective clouds.
A thunderstorm is one or more sudden electrical discharges manifested by a flash of light and a sharp or rumbling sound and is associated with convective clouds (cumulonimbus) and is, more often, accompanied by precipitation in the form of rain-showers or hail. Thunderstorms account for a large proportion of the rainfall in Zimbabwe.
Hail is solid precipitation in the form hard pellets of ice of variable sizes, which falls from cumulonimbus cloud.
Types of clouds and rainfall
Clouds are the result of condensation in the free atmosphere. Condensation results from air being cooled adiabatically to its dew-point by ascent. There are three principal causes of air rising (and cloud formation) in the atmosphere.
When air which is moving horizontally encounters rising terrain - a hill or mountain for example, it must pass over or around the barrier if the air is not to pile up behind it. The air will initially slow down and begin to pile up behind the barrier, but the following air will then have to rise higher in order to pass this. As this process continues, the rising air cools and at some level will reach its dew-point and cloud will form. Further rising and condensation of the air may result in precipitation. The cloud and precipitation so formed are called orographic.
The form of precipitation resulting from the above process is usually drizzle in Zimbabwe. Drizzle is a constituent of guti. Guti results from high pressure systems moving past the east coast of South Africa, while pressure over Zimbabwe, Mozambique and neighbouring countries in the north is relatively low. The air flowing around the anticyclone has the Southern Sea as its source. The air is thus moist and cool. As the air rises from the sea overland it cools further resulting in cloud formation and sometimes with drizzle falling. The air after precipitating becomes dry as it moves further north and west after the main watershed. This weather may persist for as long as the polar air prevails over the area. Usually the drizzle falls only in the mornings as high afternoon temperatures tend to slacken the pressure between Zimbabwe and surrounding areas and the east coast of South Africa and the drizzle that may be falling is evaporated before reaching the ground.
The second principal cause is the horizontal convergence of air. It involves the meeting of two different air masses in which case the one which is less dense, is forced to rise over the other. The two air masses involved in the case of Zimbabwe are the NE and SE trades. The NE trades are warmer, moister and less dense than the SE trades which originate from the mid-latitudes ( south of 35 deg south). The meeting area which is a relatively narrow zone which circles the globe is in fact the Inter-tropical Convergence Zone. The ITCZ is associated with cloudy and showery weather. It is subject to day-to-day oscillations of position and variations of activity.
The third cause is the warming of air near the ground making it less dense than air just above it so that it rises. This is the process of convection.
Most of the rainfall of Zimbabwe comes from processes two and three above. The resultant clouds from these processes are usually of cumuliform. or heap clouds whose vertical extent is usually greater than their horizontal spread. (There is one exception here of cumulus humilis or cumulus of fair weather which has ceased to grow and has developed vertically through a shallow layer only.)
A rain day is that day when the total amount of rainfall received measures at least 0.3mm. Different countries have (slightly) different threshold points in the definition.
The Eastern Highlands have the highest number of rain days per year - an average of 100 days. The Gwanda area has the least rainy days at an average of 35 per year. By the rule of the thumb, the number of rain days bears a proportionality relationship with rainfall amount.
Most of the rainy days for Zimbabwe are for the months of October to April. The period from May to September have an average of less than 5 days in the areas north and west of the main watershed, 30 in the Eastern Highlands and about 15 rain days in the Bikita-Ndanga area. Most of these falls are due to guti.
As mentioned in section before, the bulk of Zimbabwe's rainfall is in the form of thunderstorms which are mostly of high intensity and short duration. Low-intensity drizzle and rain occur mostly in the area lying south and east of the main watershed.