Sierra Leone - Climate
Sierra Leone has been ranked as the third most vulnerable nation after Bangladesh and Guinea Bissau to adverse effects of climate change. The vulnerable population has low capacity to adapt to climate change and the rural populations will be the most affected because of their high dependence on rain-fed agriculture and natural resource-based livelihoods. According to the science of climate change, these impacts are likely to continue to affect Sierra Leone in the future, despite the country being least responsible for the problem since Sierra Leone’s contribution to global emissions of greenhouse gases is negligible.
The climate of Sierra Leone is tropical, with constantly high temperatures and, throughout most of the year, high humidity. Two seasons determine the agricultural cycle. These seasons are the Rainy season that lasts from May to November and the Dry season from December to April. During the dry season there is a short spell of cool dry winds that blow in off the Sahara Desert. This period is refered to as harmattan period. At times during the night, temperatures can go as low as 16 °C (60.8 °F).
Rainfall is adequate every-where but received during a single wet season that alternates with a dry period of somewhat shorter duration. The amount of precipitation and growing conditions permitted development of a natural vegetation cover of rain forest over most of the country and moist savanna woodland in the northeast. This vegetation has been extensively altered. however, by human activities that have reduced all but a few forest reserves to bush and poor secondary forest and much of the woodland to lightly treed savanna and savanna grassland.
The annual wet season former occurs generally from May to November and is related to the flow from southwest to northeast of the tropical maritime monsoon, a mass of moisture-laden air that originates over the South Atlantic Ocean. The dry season is caused by the hot, dry, and dusty air of the harmattan, a trade wind that develops over the Sahara region and blows from the northeast generally southwest ward. The movement of these two air masses over Sierra Leone is part of a broader air movement over West Africa associated with the seasonal variation in the heating of the continental and ocean surfaces.
As the front between the two air masses retreats and advances across the country, it is regularly accompanied by strong convectional disturbances in the form of squalls and thunderstorms of varying intensity. These storms produce some precipitation, but most of the rainfall is deposited during a period of steady monsoonal rains usually beginning sometime in June and lasting until about the end of September.
Rain in relatively appreciable quantity begins in eastern Sierra Leone in April, and by May the entire country has entered the wet season. From this time until the end of November more than 85 percent of the total rainfall is received everywhere except for a very small area in the east-central part. The heaviest rainfall is along the coast and in the interior lowland plains; twenty inches or more of rain fall in each month during a three- to four-month period. In the higher area rising to the plateaus in eastern and northeastern Sierra Leone only during one to two months is there rainfall above twenty inches; the plateaus themselves do not ordinarily experience rainfall over twenty inches in any month.
The mean annual rainfall for roughly four-fifths of the country is 100 inches or over. Only in the northern areas does it drop below that amount, and only a small zone along the Guinea border receives less than eighty inches. Total precipitation increases from the interior to the coast, where a large zone receives from 120 to 200 inches; the Sierra Leone Peninsula’s upper elevations have recorded close to 218 inches, and the average annual rainfall at Freetown is about 150 inches.
The December-April dry season is in sharp contrast to the wet period. During at least four months of this time the coastal and interior lowland zones have under 2.5 inches of rain, and for two to three of these months the average rainfall is under one inch. The eastern plateau area has a somewhat shorter extremely dry period, in part because of the greater amount of rainfall produced by convectional disturbances in this more elevated region.
The mean amount of dry-season rainfall for most of the country actually does not exceed ten inches, and a considerable area receives five inches or less; reliable rainfall ranges from about five inches to less than one inch during this period. Although the growth of vegetation slows at this time, the rainfall received annually is adequate under normal conditions to support a profuse cover of moist vegetation types throughout the country.
Temperatures are consistently high throughout the country, ranging roughly between daytime readings of up to the middle 90°sF and readings in the middle 60°sF at night, depending on season and locality. Absolute minimums in the low 50°sF occur at times in the savanna area of the eastern plateaus, and lower readings are presumably reached at upper elevations in the Loma and Tingi mountains. The coastal and interior lowland plains areas have mean annual temperatures of 79°F or 80°F and the eastern plateau region a mean of close to 77.5°F.
Diurnal variations are most marked during the dry season, when there may be a difference of up to 30°F in the interior between monthly mean maximum and minimum temperatures. Absolute maximums may be above 95°F and minimums between about 50°F and the lower 60°sF at this time. The coolest temperatures usually prevail during the wet season in August except in the eastern plateaus, where they may occur at the onset of the dry period. In Freetown daytime temperatures during the rainy season are reported to run about 80°F and those at nighttime about 76°F. In the dry months corresponding temperatures average about 92°F and 74°F respectively.
The humidity, like the temperature, is usually high, although some amelioration is experienced during the harmattan period. In early morning during the rainy season the humidity is close to or above 90 percent throughout the country. By midafternoon it has decreased in the western half to 80 percent or slightly less and in the far northeast savanna to somewhat below 75 percent. In the dry period early morning humidity usually varies between 80 percent and almost 90 percent. A marked drop occurs as the day progresses, however, and at varying times during this season different localities reach lows of around 50 percent or under by midafternoon (in the area around Kabala in the northeast savanna country the humidity drops below 30 percent).
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