Weather in Indonesia can be split into two seasons - wet and dry - with warm tropical temperatures averaging 28°C during the day, throughout the year. In most regions, the dry season spans from May to September, with the rains falling between October and April.
There are some regional exceptions. In Sumatra it rains from October to January in the north, and November to February in the south. Sulawesi, with its unusual shape, also experiences slightly different weather patterns and rainfall can be expected in the northern regions of the island during June and July.
Generally speaking, the best time of year to visit Indonesia is between May and September when the days are dry and sunny. However, during wet season temperatures remain high and rainfall comes in the form of intense tropical downpours that tend to last for a couple of hours and needn’t spoil your trip.
In some regions the difference between the two seasons is not extreme: Bali and Kalimantan, for example, are relatively unaffected by the season change, but the contrast typically grows as you travel further east into the Nusa Tenggara region (to the islands of Lombok, Flores and beyond) where there is a greater chance of flooding during the wet season and droughts in the dry.
It really depends on your travel plans: muddy back roads can be a problem during wet season if you intend to explore more off the beaten track locations and rough seas can render some islands inaccessible to smaller boats. The best months for diving off the coast of Bali, Lombok or the Komodo National Park are April to September, with rough seas particularly in January & February. If you plan to climb one of Indonesia's many volcanoes, such as Mt Bromo or Mt Rinjani, then dry season is recommended. Expect the temperature to cool as you climb, and pack some warmer layers.