Map from Google - Belize

In Belize, a small country in Central America which occupies the south-east of the Yucatan Peninsula, the climate is tropical, hot all year round, with a rainy and muggy season from June to October, and a less hot season from November to February, when the northeast trade winds prevail, but they still unload rainfall, which gradually decreases, so that from February to April it's the driest period. From March to May, before the rainy season, the heat gradually increases, so that in April and May the hottest days of the year occur, and there may be torrid days, especially in inland areas; in May, even thunderstorms become a bit more frequent.
The rains fall mainly in the form of downpour or thunderstorm in late afternoon, at least in the hottest period, while in December and January they are less abundant but more prolonged. Due to rainfall and humidity, most of Belize is covered by a tropical forest.
During the coolest months (December, January and February), the average temperature is around 23 degrees Celsius (73 °F) in the north, 24 °C (75 °F) in the south and 25 °C (77 °F) in the island of Ambergris Caye, with highs around 26/27 °C (79/81 °F). From December to March, the country is sometimes reached by the Norte, a wind coming from the United States, which can bring a bit of cloudiness and rainfall, and also a bit of cool in the evening and at night, so that lows can drop to around 15 °C (59 °F). In the rainy and sultry season, from June to October, the average temperatures are around 27/28 °C (81/82 °F), with highs around 30/32 °C (86/90 °F).
Here are the average temperatures of Belmopan, the capital city, which is located in the plains, 50 km (30 mi) away from the coast.
Average temperatures - Belmopan
BelmopanJanFebMarAprMayJunJulAugSepOctNovDec
BelmopanJFMAMJJASOND
Min (°C)181819202223222222212019
Max (°C)282931333432323232312928
Min (°F)646466687273727272706866
Max (°F)828488919390909090888482

The rains are more abundant in the south, where they exceed 3,000 millimetres (118 inches) per annum, but in the far south, on the border with Guatemala, in the area of Punta Gorda they even exceed 4,000 mm (157 in); in the north-east they hover around 1,800 mm (70 in), while in the north-west they drop to around 1,300 mm (51 in). In the south, from June to October more than 300 mm (12 in) of rain fall per month, while in the north and west there'a relative decrease in rainfall in August.
Here is the average precipitation in Belize City, located in the north-est, on the coast.
Average precipitation - Belize City
Belize CityJanFebMarAprMayJunJulAugSepOctNovDecYear
Belize CityJFMAMJJASONDY
Prec. (mm)1356040551102001651702453052251851895
Prec.(in)5.32.41.62.24.37.96.56.79.6128.97.374.6
Days12645713151415161214133

In the south, in the area of the Maya Mountains, the sky is often covered with clouds. Here the dry season is less pronounced, because the northeast trade winds are forced to rise over the mountains, and form clouds and rains. The weather becomes a bit cooler towards the two peaks of Belize: Doyle's Delight and Victoria Peak, both around 1,100 metres (3,600 feet) high.
In the period from June to November, Belize can be affected by tropical storms and hurricanes, especially between August and October.
The sunshine amount in Belize is never excellent, however, it's just acceptable in winter (about 6 hours of sunshine per day), while it becomes good in the dry season (about 7/8 hours a day), except in the aforementioned Maya Mountains, which are often cloudy.

Belize

The sea is warm all year round, ranging from 27 °C (81 °F) from January to March, to 29 °C (84 °F) in the summer months.
Sea temperature - Belize City
Belize CityJanFebMarAprMayJunJulAugSepOctNovDec
Belize CityJFMAMJJASOND
Sea (°C)272727282929292929292928
Sea (°F)818181828484848484848482

When to go

The best time to visit Belize, for beach life, and for exploring the atolls and the coral reefs, goes from February to April, being the period in which the rains are more scarce, and the heat is not as muggy as in the following months. January falls in the cool period, although it is a bit more rainy, but overall you can also choose this month.
To go in search of the Mayan ruins, in March and especially in April, the heat can be intense in the interior, though, on the positive side, we are in the middle of the dry season.
From May to October, in addition to the rainy season, you must take into account the heat, which is sultry and unpleasant. From June to November, as mentioned, there's some risk of hurricanes, which, however, are most likely between August and October.

What to pack

From December to March: light clothes, but also a sweatshirt or sweater for the evening and for windy days.
From April to November: tropics-friendly, loose fitting clothing, light shirts and pants, comfortable and breathable shoes, sun hat, sunglasses, a light sweatshirt and a light raincoat for thunderstorms.

For the reef, equipment for snorkeling, water shoes or rubber soled shoes.