The climate in Haiti is tropical, hot throughout the year, with a less hot period from November to March, when the trade winds from the northeast prevail, and a muggy period from May to October, when the heat becomes unpleasant, though along the coasts it is tempered by the breeze. Temperatures are a bit higher in inland plains and south-facing coasts, and slightly lower along the north-facing coasts.
From December to March, sometimes the north-west cool wind, coming from the United States, can lower the temperature by a few degrees, bringing the night temperatures a few degrees below 20 °C (68 °F).
Here are the average temperatures of the capital, Port-Au-Prince, located in a particularly sheltered and warm area.
Port-au-Prince - Average temperatures
Min (°C)202021222223232323222221
Max (°C)313132323233343433323131
Min (°F)686870727273737373727270
Max (°F)888890909091939391908888

As regards to the rains, there is usually a dry season from December to February, and a rainy season from April to October, with with two peaks at the beginning and the end of the period, and a relative pause in July. This is the trend that is observed in the Port-au-Prince, where a total of 1,250 millimeters (49 inches) of rain fall per year.
Here is the average precipitation in Port-au-Prince.
Port-au-Prince - Average precipitation

In other areas, there are microclimates that have some variations from this pattern. The coasts exposed to the northeast trade winds are rainy even in winter, especially in the first part; actually, the rainiest period is from November to January. This happens on the northern coast (see Cap-Haïtien), and to a lesser extent on the north-facing coast of the Department of Grand Anse (see Jérémie).
Here is the average rainfall in Cap-Haïtien.
Cap Haitien - Average precipitation

Moving inland, we find some hills, and in the south also some mountains, among which the highest is Pic la Selle, 2,680 meters (8,793 feet) high; in this area, there are also sheltered valleys and plains, which are almost arid, where precipitation drops below 1,000 mm (40 inches) per year.

Citadelle Laferrière

As generally happens in tropical climates, the rains in Haiti occur mostly in the form of short but intense showers or thunderstorms, especially in the evening, which do not affect much the amount of sunshine, except perhaps in September and October, which are typically the rainiest months, and when thunderstorms are more frequent and may last longer.
In fact, the amount of sunshine is good all year round, at least along the coasts, as can be seen in the following table.
Port-au-Prince - Sunshine

The sea in Haiti is warm all year round: on the northern coast, the sea surface temperature ranges from 26 °C (79 °F) between February and April to 29 °C (84 °F) between July and October, while at Port-au-Prince it's just slightly warmer, as we can see from the following table.
Port-Au-Prince - Sea temperature
Temp (°C)272727272828292929292928
Temp (°F)818181818282848484848482


From June to November, in Haiti, as commonly happens in the Caribbean, it's the period of hurricanes, which, though rarely directly hit a particular location, are dangerous and may sometimes be destructive. However, hurricanes are most likely from August to October.
The country was hit hard by the "Dominican Republic hurricane" in September 1930, by the "Jérémie hurricane" in October 1935, by Hurricane Flora in early October 1963, by hurricane Georges in September 1998, and by hurricane Matthew in October 2016.
However, not very intense tropical storms can produce heavy rains in inland mountainous areas as well, and given the fragility of the soil that may cause landslides and floods.

Best Time

The best time to visit the capital and the south-central area of Haiti is from December to March. In summer, the sun frequently shines, but the heat is sweltering, in addition, there is some risk of hurricanes; in this period, the best month (or the least bad) is July because there's usually a certain decrease in rainfall, furthermore, hurricanes are less likely than in August and September.
On the northern coast of Haiti, given the frequent rains lingering until December, you can choose January, February and March. In July, the northern coasts experience a decrease in rainfall as well.
Winter is warm enough for a beach holiday, even though Haiti is not a peaceful country in socio-economic terms, and it's not equipped for this type of activity.

What to pack

From November to April: bring light clothes, a sweatshirt for the evening; for the mountains, a jacket and hiking boots.
From May to October, bring lightweight clothing, and possibly a light sweatshirt and a light raincoat for thunderstorms; a sweatshirt or sweater for the mountains.