Map from Google - Sierra_Leone

In Sierra Leone the climate is tropical, hot all year round, with a dry season in winter and a rainy season, due to the African monsoon, which runs from May to November in the north, and from April to November in the south. The annual quantity of rainfall is considerable, from 2,000 to 3,000 millimetres (80 to 120 inches), with a maximum in the coastal area.
The western part of the country is flat, while the eastern part is covered by hills and mountains; the highest peak is Mount Bintumani, 1,945 metres (6,381 feet) high. The altitude tempers the heat, especially above a thousand metres (3,300 feet).
Winter in Sierra Leone is hot and sunny: the night-time temperatures are around 22/24 °C (72/75 °F) along the coast, and 18/20 °C (64/68 °F) in the interior, while the daytime temperatures are around 30 °C (86 °F) on the coast and 32/34 °C (90/93 °F) in the interior. Sometimes it can get uncomfortably hot: the temperature can touch 38/40 °C (100/104 °F) even in winter, but luckily the air is quite dry, because the prevailing wind, the Harmattan, blows from the Sahara and lowers the humidity, especially in inland areas, and less frequently along the coast. On the contrary, in the night it can get a little cool, especially in inland areas, where the temperature can drop to around 10 °C (50 °F), while in the mountains it can even get cold.
Here are the average temperatures of Freetown.
Average temperatures - Freetown
FreetownJanFebMarAprMayJunJulAugSepOctNovDec
FreetownJFMAMJJASOND
Min (°C)242424252424232323232424
Max (°C)303031313130292829303030
Min (°F)757575777575737373737575
Max (°F)868688888886848284868686

Starting from February, the temperature rises, under the influence of the tropical sun, but then the rains come quite soon: the first showers occur in March in the south, when already 90 mm (3.5 in) of rain fall in Daru, and only 15 mm (0.6 in) in Freetown, in the centre. Rainfall increases gradually, reaching a peak from June to September, when it exceeds 250 mm (10 in) per month even in inland areas, while on the coast the rains are more violent, and become torrential, especially in July and August: in the capital Freetown, even 800 millimetres (31.5 in) of rain fall in August, as happens in an average rainy city in an entire year. The coast south of Freetown, flat and covered with mangroves, in this period is often flooded.
Here is the average precipitation in Freetown.
Average precipitation - Freetown
FreetownJanFebMarAprMayJunJulAugSepOctNovDecYear
FreetownJFMAMJJASONDY
Prec. (mm)34154517532573579048526590152945
Prec.(in)0.10.20.61.86.912.828.931.119.110.43.50.6115.9
Days001415222727242192152

Even the amount of sunshine in the summer months becomes very poor, when the rains occur almost daily. The temperature during the rainy months often remains below 30 °C (86 °F) during the day, while at night it remains high, and the humidity makes the air quite sultry, even though along the coast the heat is tempered by the breeze.
During the month of November, the monsoon withdraws, the rains cease and the dry winds from the north-east start to blow again.

Freetown

For swimming, the sea is warm all year round, as we can see in the following table.
Sea temperature - Freetown
FreetownJanFebMarAprMayJunJulAugSepOctNovDec
FreetownJFMAMJJASOND
Sea (°C)272726272828282727282928
Sea (°F)818179818282828181828482

When to go

The best time to visit Sierra Leone is winter, only December and January in inland areas (because in February it starts to get hot), and from December to March along the coast, where the temperature is more stable. The worst months are those of summer, from July to September, when the rains, often torrential, create extensive flooding.

What to pack

In winter: in Freetown and the coast, light clothes, a light sweatshirt for the evening, scarf for the wind-borne sand and dust. In inland areas, it may be useful to bring a sweatshirt and a jacket for the evening.
In summer: tropics-friendly, loose fitting clothing, a light sweatshirt and a light raincoat for the long and heavy downpours, possibly an umbrella (not really effective during the most intense rains). In inland areas and mountain ranges, you can add a heavier sweatshirt for the wet and rainy days.