Map from Google - French_Guiana

In French Guiana, a country located just north of the Equator, the climate is tropical, hot and humid throughout the year, with a relatively dry and slightly warmer season from July to November, and a rainy season from December to June.
The temperatures are high all year round: lows are always above 20 °C (68 °F), while highs in Cayenne range from 29 °C (84 °F) from January to May, to 32 °C (90 °F) in September and October, which are the hottest months, because they are the driest and sunniest. However, at this time the humidity is a bit less high, around 65% during the warmest hours, while in the rest of the year it exceeds 70%. In the hottest days, however, the temperature can reach 37/40 °C (99/104 °F), from May to November.
Here are the average temperatures of Cayenne, the capital, located in the central part of the coast.
Average temperatures - Cayenne
CayenneJanFebMarAprMayJunJulAugSepOctNovDec
CayenneJFMAMJJASOND
Min (°C)232323232322222221212222
Max (°C)292929292930303132323130
Min (°F)737373737372727270707272
Max (°F)848484848486868890908886

Precipitation in French Guiana is definitely abundant, given that it exceeds 2,500 millimetres (100 inches) year, and in many areas even 3,000 mm (120 in). Within the rainy season, we can distinguish between three periods:
- a period of the short rains, in December and January, due to the fact that the intertropical convergence zone (ITCZ) passes over the country from north to south; the rains, sometimes intense but generally short-lived, on the coast occur mainly at night and in the morning; the northeast trade winds blow with moderate intensity on the coast, and the sea can be rough;
- a short dry season, called the short summer of March, which may also occur in February, due to the fact that the ITCZ is by now south of the country, so the weather tends to be sunny, but for its brevity (one or two weeks) it's not reliable, however, it's more evident in the northwest (see St. Laurent du Maroni) than in the south; in this period, the wind can even bring dust from the Sahara desert;
- a period of heavy rains from April to June, with the peak in May, when the ITCZ passes again, this time from south to north, bringing all the heat and humidity of the Amazon rainforest. The wind is weaker and the humidity makes the heat stifling. On the coast, the rains occur mainly at night and in the morning. The rains are more abundant, and often torrential, along the central and eastern part of the coast (see Cayenne, Saint-Georges-de-l'Oyapock), where they can easily create flooding. In May and June, more than 300 mm (12 in) per month fall in St. Laurent du Maroni, and even more than 500 mm (20 in) in a single month (May) in Cayenne and Saint-Georges-de-l'Oyapock, in the east. In this period, it rains almost every day.
During the dry season from July to November, the ITCZ is located to the north, but not by much, and in any case thunderstorms can form in inland areas, which can then head towards the coast. In September and October, the least rainy months, rainfall amounts to around 40/50 millimetres (1.6/2 inches) per month in Cayenne, a little more in Maripasoula, 70/80 mm (2.7/3.1 in) per month, in the west, and even more in St. Laurent du Maroni, in the far north-west, with about 100 mm (4 in) per month, but still less than in the rest of the year.
In Cayenne the total annual rainfall is even 3,200 mm (124 in). Here is the average precipitation.
Average precipitation - Cayenne
CayenneJanFebMarAprMayJunJulAugSepOctNovDecYear
CayenneJFMAMJJASONDY
Prec. (mm)38032038038051039020010040501202903160
Prec.(in)1512.6151520.115.47.93.91.624.711.4124.4
Days2117222227241910551118201

Moisture is high throughout the year, making the heat sweltering, especially in the interior, while at least on the coast the breeze blows. The most humid period is that of the heavy rains, from April to June, while the relatively drier period is the least rainy, but also the warmest, from August to October.
The sunshine amount is not very good in the wet season, from December to June, when there are about 5/6 hours of sunshine per day, while it becomes good in July, August and November, when it rises to 7/8 hours per day, and it reaches 9 hours on average in September and October.
Off the coast, there are some small islands, such as the famous Devil's Island, with a climate similar to that of the coast.
French Guyana is outside of the path of hurricanes, which pass more to the north, over the Caribbean Sea.

Devil's Island

The sea is warm enough for swimming all year round, since the water temperature fluctuates around 27/28 °C (81/82 °F). The sea water is not particularly clear and has often a brown colour (except in the islets that are located offshore), because of the large quantity of water coming from rivers, although it is generally clean (except in the vicinity of the cities). It's better to swim at high tide, because during low tide the part of the beach that remains uncovered can be muddy.
From December to April, sometimes the coast can be reached by abnormal waves, generated by Atlantic depressions passing far to the north.
Sea temperature - Cayenne
CayenneJanFebMarAprMayJunJulAugSepOctNovDec
CayenneJFMAMJJASOND
Sea (°C)272727282828282828282827
Sea (°F)818181828282828282828281

When to go

The best time to visit French Guyana is from mid-August to mid-November, and in particular the months of September and October, which are the least rainy and the sunniest, especially on the eastern part of the country. As mentioned, this is also the hottest period, but it is less humid than the rest of the year, and in any case what matters most is to avoid the heavy rains.

What to pack

Throughout the year: lightweight clothing of natural fibres, possibly with long sleeves for mosquitoes; a light sweatshirt for rain showers or sea breezes; sun hat; umbrella (though useless for the stronger showers) or lightweight and breathable raincoat.
From December to March, a sweatshirt for the evening.