In the western part of the island of Timor, which belongs to Indonesia, the climate is tropical, hot all year round, with a 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.
The eastern part of the island is occupied by the small country of East Timor.
Temperatures are high and uniform throughout the year. They are slightly higher from October to May, and a bit lower from June to August. From September to November, and sometimes from mid-August, the hottest period of the year occurs, with temperatures reaching 35/37 °C (95/99 °F). In the rainy season, high relative humidity makes the heat muggy.
Here are the average temperatures of the main town, Kupang.
Kupang - Average temperatures
Min (°C)242423232322212121232424
Max (°C)303131323231313233343432
Min (°F)757573737372707070737575
Max (°F)868888909088889091939390

The rains are not very abundant, and generally amount to 1,200/1,500 millimeters (47/60 inches) per year, but there are also some areas where rainfall drops below 1,000 mm (40 in). Given that the rains are abundant only from December to March, and that the dry season is quite long, the landscape is not particularly verdant, in fact in some areas it is almost arid. The small neighboring islands, such as Semau, Rote and Savu, have also a fairly arid landscape. Here is the average rainfall in Kupang.
Kupang - Average precipitation

In Timor there's plenty of sunshine in the dry season, while in the rainy season the sun shines for a few hours a day, between a rainshower and the other. In the interior of the island, the sky is more easily cloudy, especially on the mountain peaks.
Kupang - Sunshine

From November to mid-May, but more often from January to mid-April, Timor can be affected by tropical cyclones, which usually hit it in the early stages of their development, and therefore at a moderate intensity; however, they can bring an increase in wind and rainfall.
In the interior of Timor there are hills and mountains. The highest peak is Mount Mutis (or Nuaf Nefomasi), 2,351 metres (7,713 feet) high. If you are planning some hiking at high altitudes, keep in mind that above 1,000 metres (3,300 feet) it's cool in the evening and at night, especially in the dry season, while above 2,000 metres (6,500 feet) it can get quite cold; besides, at noon and in the afternoon the chance of thunderstorms in the highlands is higher than along the coasts.

Lasiana beach

The sea in Timor is warm enough to swim in throughout the year: the water temperature goes from 29/30 °C (84/86 °F) from October to May, to 27 °C (81 °F) in July and August.
Kupang - Sea temperature
Temp (°C)302930302928272728293030
Temp (°F)868486868482818182848686

When to go

The best time to travel to Timor 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 it is often very hot. At the end of September or in October the rains begin to increase in inland and mountainous areas.

What to pack

All year round, bring light clothes of natural fibres, a sun hat, a scarf for the breeze, a light sweatshirt for the evening. From November to March, a light raincoat or umbrella.
When going to the reef, you can bring snorkeling equipment, including water shoes or rubber-soled shoes.
When going mountain climbing, bring hiking shoes, a sweatshirt or sweater, a jacket, a raincoat.