In Sumbawa, an Indonesian island belonging to the group of the Western Lesser Sunda Islands (Nusa Tenggara), and on the nearby small islands (Moyo, Sangeang, Medang etc.), the climate is tropical, hot all year round, with a muggy and rainy season from December to March, and a dry season from June to September. In the rainy season, the sun shines for a few hours a day, in fact it usually rains a couple of hours in the morning and a couple of hours in the afternoon.
Temperatures are high and quite uniform throughout the year. July is the least hot month. From September to November, and sometimes from mid-August, the hottest period of the year occurs, with temperatures reaching as high as 35/37 °C (95/99 °F), especially on the north side. In the rainy season, on the other hand, high humidity makes the heat muggy.
Here are the average temperatures of the largest city, Sumbawa Besar, located on the northern coast.
S.Besar - Average temperatures
Min (°C)232423232322212122232424
Max (°C)303131323232323334353432
Min (°F)737573737372707072737575
Max (°F)868888909090909193959390

Throughout the year, 1,400 millimeters (54 inches) of rain fall, with a maximum from December to March and a minimum from June to September. Here is the average precipitation in Sumbawa Besar.
S.Besar - Average precipitation

The wettest area of the island is the interior, occupied by hills and mountains, whose highest peak is Tambora volcano, 2,850 meters (9,350 feet) high, famous for the violent explosion of 1815. The eruption produced a caldera, 6 km (3.7 miles) in diameter and 1,100 (3,600 feet) meters deep.
If you are planning some hiking at high altitudes, keep in mind that above 1,000 meters (3,300 feet) it's cool in the evening and at night, especially in the dry season, while above 2,000 meters (6,500 feet) it can get quite cold; in addition, at noon and in the afternoon the chance of thunderstorms in the highlands is higher than on the coasts.
In Sumbawa, there's plenty of sunshine in the dry season, while in the rainy season the sun shines for a few hours a day, between one rain shower and another. In the interior of the island, a blanket of clouds often lies over the mountain peaks.
S.Besar - Sunshine

From November to mid-May, but more often from January to mid-April, Sumbawa can be affected by tropical cyclones, which are formed on the sea of Timor, and then move south-westard, between the Sunda Islands and Australia.

Caldera of Tambora volcano

The sea in Sumbawa is warm enough to swim in throughout the year: the water temperature ranges from 27 °C (81 °F) between July and September, to 29 °C (84 °F) between November and May.
S.Besar - Sea temperature
Temp (°C)292929292928272727282930
Temp (°F)848484848482818181828486

Best Time

The best time to travel to Sumbawa and enjoy its beaches goes from June to August: the sun shines, the heat is not too intense, and in any case, it's tempered by the breeze; there are just some short downpours or thunderstorms every now and then. In September, the rains have not started yet, but it starts to get very hot at low altitude. In late September or in October, the rains often increase in inland and mountainous areas.

What to pack

All year round, bring light clothes of natural fibers, a sun hat, a scarf for the breeze, a light sweatshirt for the evening. From December to March, a light raincoat or umbrella.
When going mountain climbing, bring hiking shoes, a sweatshirt or sweater, a jacket, and a raincoat.