In Lombok, an Indonesian island belonging to the group of the Lesser Sunda Islands, and on the neighboring Gili Islands, the climate is tropical, hot all year round, with a rainy season from November 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 uniform throughout the year, and are slightly higher from October to March, and a bit lower from June to September. In the rainy season, high relative humidity makes the heat muggy.
Here are the average temperatures of the main city, Mataram, located on the west coast.
Mataram - Average temperatures
Min (°C)242424242322212122242424
Max (°C)313132323231303031323231
Min (°F)757575757372707072757575
Max (°F)888890909088868688909088

Throughout the year, 1,500 millimetres (60 inches) of rain fall, with a maximum from December to February, and a minimum from June to September. Here is the average precipitation in Mataram.
Mataram - Average precipitation

The wettest area of the island is the interior, occupied by hills and mountains, whose highest peak is the volcano Rinjani, 3,726 metres (12,224 feet) high (it is the second highest volcano in Indonesia); being closer to the inland elevations, the northern coast is rainier than the southern one. The south-eastern coast is almost arid.
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 on the coasts.
In Lombok 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, a blanket of clouds often lies over the mountain peaks.
Mataram - Sunshine

From November to mid-May, but more often from January to mid-April, Lombok can be brushed by tropical cyclones, which usually do not hit it directly, but pass not far to the south, bringing an increase in wind and rainfall.

Volcano Rinjani and Lake Segara Anak

The sea in Lombok is warm enough to swim in throughout the year: the water temperature goes from 29 °C (84 °F) from November to May, to 27 °C (81 °F) in July, August and September.
Lombok - Sea temperature
Temp (°C)292929292928272727282929
Temp (°F)848484848482818181828484

When to go

The best time to travel to Lombok and enjoy its beaches, goes from June to September: 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. May and October are months you may want to consider as well, in fact they are the best ones among those that do not fall in the high season: they are a bit more muggy and rainy, but they are still out of the rainy season proper. However, in inland and mountainous areas thunderstorms are frequent.

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, you can add 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.