Upon arrival and disembarkation of the plane, you will immediately notice the sudden rise of hot and humid air. Indonesia is a warm place. In Indonesia there is no spring, summer, autumn or winter, only two seasons: rainy and dry, both of them relative (it still rains during the dry season, it just rains less). Although there are significant regional variations, in most countries (including Java and Bali) the dry seasons is from April to October, while the rainy season goes from November to March. In many regions, rain falls like clockwork, but in recent years, global warming has made the seasons less predictable. One of the advantages of the rainy season is that regular rain washes clean most of the mosquito habitats, especially at the foot of the hills. While local torrential rains are common, the country rarely suffers from typhoons.
Droughts are a major problem in parts of Java and other islands during the dry season, and water becomes a serious problem, but bottled drinking water is still available even in rural areas. Smog from bush or forest fires frequently covers many areas of Sumatra and Kalimantan in the middle of the dry period, usually in June, July and August, and sometimes airports are closed for a day or two. Also, when it is dry in one area, it can still be wet in another.
Temperatures in most places are between 26 and 32 degrees Celsius during the day with little fluctuation from day to day, although nights can be a few degrees cooler. The dry season south of the equator is cool due to the cold southern hemisphere, although the difference may be less noticeable. It is also advisable to bring a jacket when visiting the highlands, as the temperatures will naturally be cooler, and there are even some snowy peaks above 5000m in Papua. You may be amused to see people putting on hats, gloves, jackets or even winter coats when the temperature drops a little, and people usually wear them on their motorcycles, although more often to prevent their skin from darkening.