Indonesia has a tropical climate. However, the heat is somewhat reduced by the sea winds. The average annual temperature is about 27°C. Depending on the region, however, the average temperatures show slight differences. Thus they lie in the coastal areas with approximately 24-35°C, in the inland with 20-30°C, in the mountainous country with 16-26°C. The air humidity is generally high. In the mountains, it often cools down sensitively at night. The rain and dry seasons are determined by the east and west monsoon. In Jakarta, the climate is hot almost all year round and the humidity is high.
For the main islands the following information applies:
JAVA/MADURA: On the islands there are very different rain conditions. From June to October (in the west of Java mostly July to September) the dry season with little rain becomes a real dry season, which lasts one to three months on the north side of Java, six months southeast of Surabaya and four to five months on Madura. SUMATRA: On the west coast and in the mountainous hinterland, the main rainy season extends from October to April, partly until June or August (approx. 12-21 rainy days per month). On Sumatra there is no pronounced dry season. In the northwest and inner part of the northern half, however, there is a kind of dry season with low rainfall, which usually lasts from June to August. The main rainy season (about 17 rainy days per month) in Sumatra is from October to January.
LITTLE SUNDA ISLANDS (WITH BALI): In the rainy season from December to March it rains 11-18 days per month (in the mountain areas up to 24 days). KALIMANTAN: Most rain falls from November to May, the least from July to September. IRIAN-JAYA: Rainfall is evenly distributed throughout the year with few exceptions. On average there are 8-20 rainy days per month. The least rain falls from June to October, in the extreme northwest from November to February.
The best travel time is the dry season from May to October.