The climate in Benin, a small African country overlooking the Gulf of Guinea, is tropical, with a dry season in winter and a rainy season due to the African monsoon, which runs from May to September in the extreme north, and from mid-March to October in the south; along the coast, there's a pause in the rainy season in August.
Precipitation is below 1,000 millimeters (40 inches) per annum in the extreme north, where the dry season lasts nearly seven months, while in the rest of the country it ranges between 1,000 and 1,300 mm (40 and 51 in).
Here is the average precipitation of Cotonou, the largest city, located on the coast.
Cotonou - Average precipitation

And here are the average temperatures. As we can see, on the coast it is hot all year round, and the coolest months are actually those of summer, when the prevailing currents come from the sea.
Cotonou - Average temperatures
Min (°C)252627262524242424252525
Max (°C)323232323230292829303232
Min (°F)777981797775757575777777
Max (°F)909090909086848284869090

Along the coast the sun shines quite often in the dry season, however, a bit of cloudiness can form even in this season. In the rainy season, the sun shines quite rarely, even during the August break.
Cotonou - Sunshine

Winter, from November to January, is hot: in the north it's often scorching, and highs can exceed 35 °C (95 °F), while along the coast it's less hot, with highs around 31/32 °C (88/90 °F), but more humid, and with a bit of cloudiness in the morning.
In the center and north, a wind from the desert blows in winter, called Harmattan, which can fill the sky with dust and make it white, creating a peculiar mist in the early hours of the day. Moreover, nights are cool in the north, and sometimes even cold, so much so that the lowest records are about 5 °C (41 °F).
Here are the average temperatures of Kandi, located in the north.
Kandi - Average temperatures
Min (°C)182226272624232323232017
Max (°C)343840393734323031343635
Min (°F)647279817975737373736863
Max (°F)931001041029993908688939795

From February to May, the temperature increases further in the north, often reaching 40 °C (104 °F): in the extreme north, the W National Park is particularly hot, where the temperature can reach 45 °C (113 °F); the Pendjari National Park is very hot as well, however, here the rains arrive a few weeks earlier, bringing a bit of relief.

Pendjari National Park

The first showers begin in March in the far south, and gradually move from the coast to the north. In the capital, Porto-Novo, near the eastern part of the coast, the wettest month is by far June, with more than 300 mm (12 in) of rain, then from mid-July to late August the rains decrease (while cloudiness remains widespread), while they rise again in September and October, but without excesses, around 140/150 mm (5.5/6 in) per month. On the contrary, in inland areas the wettest period is from July to September; in these months, the daytime temperature remains around 28/29 °C (82/84 °F) along the coast and around 30/32 °C (86/90 °F) in the north, but relative humidity is high and cloud cover is frequent.
Here is the average precipitation in Kandi.
Kandi - Average precipitation

In the north, the sun shines very often in the long dry period, from October to April, but even in the rainy season there are several sunny periods. However, the least sunny month is the wettest, namely August.
Kandi - Sunshine

The rains cease in early October in the extreme north, and gradually recede towards the south; on the coast they cease about a month later. With the return of the warm northeast winds, the temperature rises again, while the air returns to be dry in inland areas.

The temperature of the sea in Benin is high throughout the year, although it is a bit warmer in the winter months, as can be seen from the following table.
Cotonou - Sea temperature
Temp (°C)282829292928262526272928
Temp (°F)828284848482797779818482

Best Time

The best time to visit Benin is from November to January: the air is dry and the weather is usually good. It is true that it's hot during the day, especially in the north, but less than in the following period, from February to May. The rainiest and sultriest months are May and June on the coast, while they are July, August and September in the center and the north.

What to pack

In winter: in Cotonou and the coast, bring light clothes for the day, a light sweater for the evening, a light scarf for the wind-borne sand and dust. In the north, light clothing, a sweatshirt and a light jacket for the evening, a scarf for the desert wind.
In summer: bring light clothing of natural fibers, a light sweater and a light raincoat for the rain showers; for women it's better to avoid shorts and miniskirts.