🇸🇱Sierra Leone

Sierra Leone (73,253 km²) occupies West Africa's Atlantic coast between 7°–10°N, bordered by Guinea to the north and northeast and Liberia to the south and southeast, encompassing one of the world's wettest tropical monsoon climates with 402 km of coastline rapidly succumbing to erosion and mangrove loss.

The country spans diverse topography: coastal Guinean mangroves forming critical protective belts now degraded by deforestation and development; wooded hill country extending inland with elevations to 400 m around the capital Freetown; an upland plateau region; and the eastern mountains culminating at Mount Bintumani (1,948 m), West Africa's highest peak outside the Cameroon Highlands.

The climate is Köppen Am (tropical monsoon) with two distinct seasons: a severe rainy season (June–October) delivering 3,000 to 5,000 mm annually along the coast (among Earth's highest rainfall totals) and decreasing to 2,000–2,500 mm inland, followed by a dry season (November–May) 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 25°C in highland areas to 27°C along the humid coast, where the average sea temperature reaches 30°C. December to January are the coolest months, though temperatures still exceed 40°C; March and April bring severe heat and humidity (33–36°C with 50% humidity), creating dangerous heat index conditions.

Freetown (coastal Western Area, sea level to 400 m elevation) averages 26°C in January and 27°C in July with 3,650 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, Goderich, and Tacugama districts were 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, 150 m elevation) records 24°C in January and 25°C in July with 2,400 mm precipitation. Kenema (eastern plateau, 150 m) averages 23°C in January and 24°C in July with 2,700 mm. Makeni (northern interior, 130 m) sees 25°C in January and 26°C in July with 2,200 mm. The 2015 floods killed 10 people and displaced thousands, presaging the 2017 catastrophe.

The severe April 2024 West African heatwave pushed temperatures across the region to record-breaking extremes, straining power grids and water supplies. 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 warmest July averages occur in Bo, around 27.7°C, while Koidu records the coldest January nights near 20°C. Comparing the last decade against the 1940–1970 baseline, mean temperatures across these cities have risen by about 1.3°C.