🇸🇱Sierra Leone

Sierra Leone (about 72,000–73,000 km²) occupies West Africa's Atlantic coast between 7°–10°N, bordered by Guinea to the north and northeast and Liberia to the southeast, encompassing one of the world's wettest tropical monsoon climates with 402 km of coastline.

The country spans diverse topography: coastal Guinean mangroves forming critical protective belts now degraded by deforestation and development; wooded hill country extending inland; an upland plateau region; and the eastern mountains culminating at Mount Bintumani (1,948 m), the country's highest peak.

The climate is Köppen Am (tropical monsoon) with two distinct seasons: a severe rainy season (roughly May–November) delivering 3,000 to 5,000 mm annually along the coast (among Earth's highest rainfall totals) and decreasing to under 3,000 mm inland, followed by a dry season marked by the Harmattan: dry, sand-laden winds blowing from the Sahara (December–February) that reduce visibility and blanket the landscape in dust.

Mean annual temperatures range from about 26°C inland to 27°C along the humid coast. December and January bring the coolest nights inland, though days remain hot, while July and August, at the height of the rains, have the lowest daytime highs; March and April are the hottest, with highs of about 31°C on the coast and 32 to 34°C inland, and humidity that pushes the heat index above the actual temperature.

Freetown (coastal Western Area) has highs of about 30°C in January and 29°C in July, with nights of 23–24°C, and well over 3,500 mm annual rainfall, experiencing oppressive year-round humidity and extreme wet-season downpours. The severe August 14, 2017 Freetown mudslides killed 1,141 people and left over 3,000 homeless when Sugar Loaf mountain partially collapsed following three consecutive days of torrential rain (August 11–14) that delivered 1,000 mm from July 1 through the disaster, nearly tripling the seasonal average; the Regent district was buried under saturated debris flows carrying mud, boulders, and tree trunks, exacerbated by a decade of reckless deforestation (800,000 hectares cleared), unregulated hillside construction, and inadequate drainage infrastructure.

Bo (southern interior) records highs of 32°C and lows of 19°C in January and highs of 29°C and lows of 22°C in July with 2,600 mm precipitation. Kenema (eastern plateau) has a daily mean of about 26°C with limited seasonal variation and 2,700 mm. Makeni lies in the northern interior. The 2015 floods killed 10 people and displaced thousands, presaging the 2017 catastrophe.

The severe April 2024 West African heatwave pushed temperatures across the Sahel to record-breaking extremes (48.5°C at Kayes, Mali). Accelerating coastal erosion threatens Freetown's Aberdeen and Lumley beach communities as rising sea levels and intensifying storm surges combine with mangrove loss (critical protective ecosystems destroyed for fuelwood and rice cultivation) to destabilize shorelines.

Climate projections indicate further intensification of monsoon rainfall extremes, more frequent severe flooding and mudslides on deforested slopes, extended dry seasons with harsher Harmattan conditions, and continued coastal retreat threatening infrastructure and communities.

Our archive covers 4 Sierra Leonean cities with daily ERA5 reanalysis data going back to 1940. The hottest July afternoons are in Bo, with average highs around 27.7°C, while Koidu has the coldest January nights, with average lows near 20°C. Comparing the last decade against the 1940–1970 baseline, mean temperatures across these cities have risen by about 1.3°C.