Indonesia has a tropical climate, and divided into wet and dry season. The temperature varies little throughout the year. The average temperature in Jakarta is 26 °C-30 °C with some days getting a bit warmer but temperatures rarely drop below 22 °C. There are however differences in Indonesia between islands and even parts of islands and as Indonesia is pretty mountainous, it can get much cooler once you are ascending. 

 

Generally, the western monsoon brings rains from December to March and the drier eastern monsoon brings relatively dry weather from June to September. Still, heavy rainshowers can occur on every day, but usually don't last longer than an hour in the late afternoon. Some places on Sumatra have extremly wet weather from October to December with 500 mm of rain on average during these months and become drier from January onwards. But during January it can get extremely wet on other islands more east, with Sumbawa hitting a massive 900 mm in this mont! Kalimantan has high rainfall during most months and doesn't have a drier season.