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Climate of Lesotho

Climate of Lesotho

Lesotho is land locked and surrounded only by the Republic of South Africa. It is situated approximately between 28° S and 31° S latitudes and longitude 27° E and 30° E. The land area is about 30 000 km². The arable land area is about 12% of the total area. However, the arable land is vulnerable due to severe soil erosion. It is estimated that 40 million tons of soil per year is lost through erosion (6th development plan).

The climate of Lesotho is primarily influenced by the country’s location in the Karoo Basin, spanning altitudes ranging from about 1,400m to above 3,480m above sea level. The oceanic influences of both Atlantic Ocean and Indian Ocean have insignificant contribution to the climate of Lesotho, but the altitudinal height and latitudinal position have significant contribution. Since the latter is in the influence of the sub-tropical high-pressure zone, the basic air mass circulation is anti-cyclonic, with a westerly air current superimposed at heights of 3,000m above sea level.

Winters are generally dry and cold, and are characterized by the domination of high-pressure systems with the resulting clear skies, dry air and warm to moderate temperatures during the day, with a sudden cold temperature just after sunset. During this winter period precipitation is mainly in the form of snow and is very low if any. It snows annually over the highlands, and sometimes over the lowlands. This is as a result of the southerly polar cyclones, which frequently result in frontal systems, which sometimes traverse the entire country. Frontal systems often result in low temperatures, and cold rainfall in the lowlands and heavy snowfalls in the highlands. Heavy snowfalls often occur either at the beginning or the end of the winter season.

Summers are hot and humid, due to the proximity of the Inter-Tropical Convergence Zone (ITCZ) and the Kalahari low-pressure area which draw in land, the moist tropical air masses from the Congo Basin. The combination of the landscape topology and the convention currents uplift these moist air masses producing showers which constitute 85% of the country’s total annual precipitation. During summer months, cloud cover for most of the days, ranges from partly cloudy conditions to overcast conditions, normally with the widespread showers. Short and intermittent thundershowers and hailstorms with strong winds are also common during the summer season.

Precipitation is highly variable both temporally and spatially. Annual precipitation ranges from as low as 500mm in the Senqu River Valley area to as high as 1,200mm in a few localities in the northern and eastern escarpment, which form the border with the Republic of South Africa. Precipitation totals show considerable variation from year to year, and comparison of annual distribution shows a little similarity between any two years. Most of the precipitation comes in the seven-month wet summer season from October to April. The peak rainfall period is from December to February when most parts of the country record over 100mm per month and an average of 6 days with a precipitation of at least 5.0mm. During the rainfall peak period, 70mm of precipitation can be received with a return period of 2 years in the northern lowlands and 150mm of precipitation can be received in 15 minutes with a return period of 2 years in the northern highlands.

The lowest rainfall occurs in June when the monthly totals of less than 15mm are recorded at most stations.

Monthly mean totals of evaporation range from 60mm to 70 mm during June – July period, to between 175mm and 225mm in during December – January period. The annual mean for the whole country ranges from between 1,400mm in the highlands to 1,600mm in the lowlands. In general, evaporation is greater than rainfall over most of the year, with the deficit at its greatest in summer.

In general Lesotho enjoys relatively low humidity, and very clean air. Humidity increases in summer when there is an influx of moist air from the Congo Basin.

Due to the increased use of biomass fuel and fossil fuels in winter, the atmospheric transparency is reduced by smog especially in the lowlands where most of the population resides. The situation is encouraged by low-level inversion a process of warming with height, which is favoured by the high-pressure dominance during winter.

Sunshine records indicate that the country receives between 60% and 80% of the maximum possible sunshine throughout the year. The lowlands enjoy annual average sunshine hours of around 3,211. The annual total solar radiation over the country is estimated to be between 5,700MJ/m² and 7,700MJ/m². It is clear therefore that total radiation, particularly in the lowlands, is no constraint to plant growth, and solar energy can be utilised with greater success for most of the daily human activities.

Temperatures are highly variable, on diurnal, monthly and annual time scales, and are generally lower than those of other inland regions of similar latitude in larger landmasses of both north and southern hemispheres. This is partially due to the tapering of the African sub-continent and overall altitude of the country. Normal monthly winter minimum temperatures range from –6.3°C in the lowlands to 5.1°C in the highlands. However extremes of monthly mean winter minimum temperatures of –10.7°C can be reached, and daily winter minimum temperatures can drop as low as -21°C. Sub-zero daily minimum temperatures can be reached even in summer both in the lowlands and in the highlands.

Mean annual temperature range from 15.2°C in the lowlands to 7°C in the highlands. January records the highest mean maximum temperatures throughout the country, ranging from 20°C in high altitudes to 32°C in the lowlands. On the other hand, mean minimum temperatures of around 0°C are common in June, the coldest month, with the lowlands recording the monthly mean temperatures ranging from -3°C to -1°C in the lowlands and ranging from -8.5°C to –6°C in the highlands.

On average, the first and last days of frost occurrence in the lowlands are respectively 18th May and 6th September, while those for the mountains are 16th February and 19th November. These respectively give a frost risk of 111 days for the lowlands and 276 days for the mountains. Under extreme conditions, however, the first and last days of frost occurrence are respectively 2nd April and 4th October for the lowlands, and 1st January and 31st December for the mountains, implying a frost risk of 276 days for the former ecological region and 365 for the latter.

Monthly mean wind speed range from 1.4m/s in October to 8m/s in August and are generally westerly varying between 200° and 300°. High winds of up to 20m/s can sometimes be reached during summer thunderstorms.

Ecological Regions of Lesotho

The variations in geomorphology and topography including the micro-climatological influences have significant impact on the ecology of a region. These factors characterize the formation of distinct ecological zones in Lesotho, and these are the lowlands, the foothills, the highlands and the Senqu River Valley.

The topology of the country consist of a high altitude plateau, which intrudes from the western parts of the country at an altitude of roughly 1,500m, forming a narrow strip known as the lowlands. The altitude then increases through the foothills [5] to an elevation of 2,000-2,300m, and then finally rises to the highlands. The highlands extend to the eastern escarpment where substantial areas of the country exceed altitudes of 3,000m.

The Lowlands

The lowlands cover the western part of the country and occupy about 5,200 km² which is 17% of the total surface area. This region is a narrow strip of land extending at some places just 10km from the border to 60km at some places and it which lies between 1400m and 1,800m. The northern and central lowlands are characterized by large deposits of rich volcanic soils, while the southern or border’ lowlands are characterized by poor soils and low rainfall. The foothills, on the other hand, consist of very fertile land that is associated with high agricultural productivity.

The Foothills

The foothills are defined as the area between the lowlands and the highlands and occupy an estimated area of about 4, 600 km² which lies between 1,800 and 2,000m above sea level and forms 15% of the total land area.

The Senqu River Valley

The Senqu River valley forms a narrow strip of land that flanks the banks of the Senqu River and penetrates deep into the highlands, reaching lower parts of the main tributaries of this river. This region covers 9% of the total surface area. The soils of the Senqu River valley vary from rich to very poor, making this the most unproductive region in the country

The Highlands

This is the largest ecological area and is known as the Maluti Mountains, is the highlands which cover an area of 18,047 km² (about 60% of the total land area) of the Drakensberg range. This region has been extensively dissected by the headwaters of the Senqu River and its tributaries which drain in a north-south direction, and, together with an extensive network of mountain wetlands, which today forms an important segment of the Southern African region’s water resources. The drainage pattern of the highlands region has produced deep river valleys, gorges, and gullies that, in general, make human very life difficult. The highlands region forms the main livestock grazing area in the country.