The Albertine graben has a sharp variation in rainfall amounts, mainly due to variations in the landscape. The landscape ranges from the low lying Rift Valley floor to the rift escarpment, and the raised mountain ranges. The highest landscape is the mountain ranges of Rwenzori, the Rwenzori mountains towering at over 5000m above mean sea level (a.m.s.l).
The Rift Valley floor lies in a rain shadow of both the escarpment and mountains, and has the least amount of rainfall average of less than 875mm per annum much lower than that of the highland area. Rainfall records by Directorate of Water Resources Management indicate that Moyo in the extreme north-east received an annual rainfall mean of 1174.8mm over a seven year period between 2003 and 2009. Over the period the highest annual mean rainfall was recorded in 2006 (1593.9mm) while the lowest was recorded in 2004 (623.6mm) indicating a high variation range in the mean annual rainfall received. Butiaba around Lake Albert in the center north-east receives 750mm, while Kasese in the central part of the graben receives a slightly higher mean rainfall of 970mm. No records are available for areas in the extreme south-western parts of the graben in Rukungiri and Kanungu. However, the area similarly receives rainfall amounts lower than that in the highland area flanking the Rift Valley. On the highland areas of the rift escarpment, rainfall averages increase largely due to orographic influence. For example, Masindi receives an annual average rainfall of 1,359mm, while Hoima receives 1435mm.
Rainfall amounts are even higher on the slopes of the Rwenzoris, in most cases increasing to over 1500mm. There is however, a serious lack of coverage of climatic measuring instrumentation, which is a common problem in mountainous regions worldwide. As a result of this, information on the spatial distribution of rainfall in the Rwenzori mountains remains scanty.
There is also scanty rainfall information in the graben but a high variation in the rainfall received both along and across the Rift Valley. Mean Rainfall amounts in the Murchison Falls Conservation Area for instance vary from 1,500mm per year at Chobe in the east to about 1,100mm at Paraa on the western part of the Rift Valley. Likewise, the mean annual rainfall recorded at Pamoti (Moyo District Farm Institute) in Moyo from 2003 to 2009 was 1174.8mm. The long-term mean rainfall amount recorded at Wadelai in Nebbi is 1,029mm, 750mm at Butiaba and 970mm at Kasese; giving a mean range of 425mm between the most northerly and southerly points where rainfall has been measured in the rift system. There is also significant seasonal variation in the rainfall pattern, mainly as a result of variation in factors influencing rainfall and especially the periodic shifting of the Inter-Tropical Convergence Zone (ITCZ) and the wind blows from the Atlantic Ocean through the Congo basin in central Africa. In the northern part of the region, there are two seasons of high rainfall, associated with the passing of the ITCZ over the region. Generally, rain occurs in all months, but with two peaks occurring between April and May, and August through to October, with two relatively drier spells around January and June.
Temperature and humidity
The Albertine graben region lies astride the equator. The region experiences small annual variation in air temperatures; and the climate may be described as generally hot and humid, with average monthly temperatures varying between 27°C and 31°C. The temperature maximas are consistently above 30°C and sometimes reach 38°C. Average minimum temperatures are relatively consistent and vary between 16°C and 18°C. The recorded lowest and highest monthly mean temperatures in the year vary along the Rift Valley: In Pamoti in Moyo, the lowest mean temperatures are recorded in August (22.6ºC) while the highest are recorded in February (27.1ºC). Southwards at Wadelai, the lowest mean temperatures are recorded in January (8.7ºC) and the highest in February (39.0ºC) indicating an extreme change in temperatures within a period of one month. At Butiaba, the lowest mean temperatures were recorded in September (18.0ºC) and the highest in February (35.6ºC). Further south at Kasese, the lowest mean temperatures were recorded in July (10.5ºC) and the highest in February (36.0º). The high air temperatures result in high evaporation rates causing some parts to have a negative hydrological balance.
The relative humidity in the Albertine graben is higher during rain seasons with maximum levels prevalent in May. The lowest humidity levels occur in dry seasons with minimum levels occurring in December and January. The average monthly humidity is between 60% and 80%. The relative humidity recorded at Wadelai at 0600 hrs ranges between 70% in February and 88% in August while the record at 1200 hrs ranged between 35% from January to February and 55% from August to September. The average humidity recorded at 0600 hrs for Butiaba ranged between 67% in January and 80% in August while at 1200 hrs, the humidity records ranged between 66% in January and 71% in October. At Kasese, the average humidity recorded at 0600 hrs ranged between 79% in January and February and 85% from April to July, while records at 1200 hrs ranged between 49% in July and 61% in November. Relative humidity records for Moyo and the areas in the extreme south-west of the graben in Rukungiri and Kanungu are not available. It can be concluded therefore , that variation in relative humidity is generally moderate, except for Wadelai where both low and high relative humidity figures have been recorded (35% and 88% respectively).
Wind speed and direction records indicate a high incidence of strong winds especially in the Rift Valley. The prevailing winds commonly blow along the valley floor in a north-east to south-west direction or vice versa. Winds also blow across the Rift Valley in an east to west direction. On the escarpment and mountain slopes, prevailing wind-blow is largely multi-directional. The long-term wind speed records from the East African Meteorological Department (1975) indicate average annual wind speeds of 4 knots and 6 knots at 0600 hours and 1200 hours, respectively, for Butiaba; 3 knots and 5 knots, respectively, for Hoima and; 2 knots and 6 knots, respectively, for Kasese. The wind speed values indicated, therefore, represent conditions of moderate to strong or turbulent conditions. The average number of calms experienced in the area, are indicated to be experienced for 41days at 0600 hours, and 14 days at 1200 hours, respectively, at Butiaba; 99 days and 27 days, respectively, for Hoima; 181 days and 44 days, respectively, for Kasese; and 99 days and 27 days, respectively, for Masindi. The general conclusion from these climatic figures is that for most of the year, the area experiences moderate to strong and gusty winds, increasing in the afternoon. Both wind speed and direction have important implications on oil exploration and production activities particularly the dispersion potential for oil pollutants.