Do you need to know, when is the best time to go to Indonesia? Then you should take a look here, what weather you can expect there during the year.
The climate of Indonesia is tropical. You can expect hot and humid weather, but more moderate weather in highlands. Temperatures remain fairly constant throughout the year, they are higher at the coast and a little bit lower in the inland.
As Indonesia has a really huge area, the information below relate mostly only to the area around the capital of this country. To see information about other places, please, choose one of these areas: Bali, Java, Kalimantan, Maluku, Nusa Tenggara, Papua, Sulawesi, Sumatra.
Let's look now what weather you can expect in the area around the capital and in many other cities. The warmest month here is May, the average maximum temperature is 32.9 ℃ (91.22 ℉). The average minimum temperature in this month is 25.2 ℃ (77.36 ℉). The coldest month here is January, the average maximum temperature is 30.3 ℃ (86.54 ℉). The average minimum temperature in this month is 24.3 ℃ (75.74 ℉). The most rainy month here is January, the average number of rainy days is 26.1. The least rainy month here is July, the average number of rainy days is 5.1. 
When travelling to some places in Indonesia, always consider the altitude of the the place as well. The highest point here is Puncak Jaya (4884 m) and the lowest point is Indian Ocean (0 m). So there are huge differences in altitudes in this country and the weather is very different in them therefore. Keep in mind, that the temperature usually decreases by 1.2 - 1.9 ℃ (2.2 - 3.5 ℉) per 200 m (656 ft), depending on the other conditions. (And we should add, that in the very special condition named thermal inversion even the temperature may be higher in higher altitudes.)
Typical (usual) weather in Indonesia in January is this: The average highest temperature in Indonesia in January is 30.3 ℃ (86.54 ℉). The average lowest temperature is 24.3 ℃ (75.74 ℉). At the beginning of January you can expect higher temperatures, the average highest temperature is around 31.4 ℃ (88.52 ℉). At the end of January you can expect higher temperatures, the average highest temperature is around 30.65 ℃ (87.17 ℉). The average number of rainy days in January is 26.1. The average rainfall is 384.3 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:28 (hours and minutes), in the middle of the month 12:26 and at the end of the month 12:22.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in February is this: The average highest temperature in Indonesia in February is 31 ℃ (87.8 ℉). The average lowest temperature is 24.2 ℃ (75.56 ℉). At the beginning of February you can expect lower temperatures, the average highest temperature is around 30.65 ℃ (87.17 ℉). At the end of February you can expect lower temperatures, the average highest temperature is around 31 ℃ (87.8 ℉). The average number of rainy days in February is 20.4. The average rainfall is 303 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:22 (hours and minutes), in the middle of the month 12:18 and at the end of the month 12:13.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in March is this: The average highest temperature in Indonesia in March is 31 ℃ (87.8 ℉). The average lowest temperature is 24.9 ℃ (76.82 ℉). At the beginning of March you can expect lower temperatures, the average highest temperature is around 31 ℃ (87.8 ℉). At the end of March you can expect higher temperatures, the average highest temperature is around 31.55 ℃ (88.79 ℉). The average number of rainy days in March is 14.6. The average rainfall is 100.2 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:13 (hours and minutes), in the middle of the month 12:08 and at the end of the month 12:03.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in April is this: The average highest temperature in Indonesia in April is 32.1 ℃ (89.78 ℉). The average lowest temperature is 25.9 ℃ (78.62 ℉). At the beginning of April you can expect lower temperatures, the average highest temperature is around 31.55 ℃ (88.79 ℉). At the end of April you can expect higher temperatures, the average highest temperature is around 32.5 ℃ (90.5 ℉). The average number of rainy days in April is 18.2. The average rainfall is 261.5 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:03 (hours and minutes), in the middle of the month 11:58 and at the end of the month 11:53.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in May is this: The average highest temperature in Indonesia in May is 32.9 ℃ (91.22 ℉). The average lowest temperature is 25.2 ℃ (77.36 ℉). At the beginning of May you can expect lower temperatures, the average highest temperature is around 32.5 ℃ (90.5 ℉). At the end of May you can expect lower temperatures, the average highest temperature is around 32.1 ℃ (89.78 ℉). The average number of rainy days in May is 12.7. The average rainfall is 131.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:53 (hours and minutes), in the middle of the month 11:50 and at the end of the month 11:47.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in June is this: The average highest temperature in Indonesia in June is 31.3 ℃ (88.34 ℉). The average lowest temperature is 25.5 ℃ (77.9 ℉). At the beginning of June you can expect higher temperatures, the average highest temperature is around 32.1 ℃ (89.78 ℉). At the end of June you can expect higher temperatures, the average highest temperature is around 31.65 ℃ (88.97 ℉). The average number of rainy days in June is 16.9. The average rainfall is 82.9 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:47 (hours and minutes), in the middle of the month 11:45 and at the end of the month 11:46.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in July is this: The average highest temperature in Indonesia in July is 32 ℃ (89.6 ℉). The average lowest temperature is 25 ℃ (77 ℉). At the beginning of July you can expect lower temperatures, the average highest temperature is around 31.65 ℃ (88.97 ℉). At the end of July you can expect higher temperatures, the average highest temperature is around 32.15 ℃ (89.87 ℉). The average number of rainy days in July is 5.1. The average rainfall is 30.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:46 (hours and minutes), in the middle of the month 11:47 and at the end of the month 11:50.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in August is this: The average highest temperature in Indonesia in August is 32.3 ℃ (90.14 ℉). The average lowest temperature is 25.2 ℃ (77.36 ℉). At the beginning of August you can expect lower temperatures, the average highest temperature is around 32.15 ℃ (89.87 ℉). At the end of August you can expect higher temperatures, the average highest temperature is around 32.45 ℃ (90.41 ℉). The average number of rainy days in August is 24.4. The average rainfall is 33.8 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:50 (hours and minutes), in the middle of the month 11:53 and at the end of the month 11:58.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in September is this: The average highest temperature in Indonesia in September is 32.6 ℃ (90.68 ℉). The average lowest temperature is 25.9 ℃ (78.62 ℉). At the beginning of September you can expect lower temperatures, the average highest temperature is around 32.45 ℃ (90.41 ℉). At the end of September you can expect lower temperatures, the average highest temperature is around 32.45 ℃ (90.41 ℉). The average number of rainy days in September is 5.9. The average rainfall is 29 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:58 (hours and minutes), in the middle of the month 12:03 and at the end of the month 12:08.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in October is this: The average highest temperature in Indonesia in October is 32.3 ℃ (90.14 ℉). The average lowest temperature is 25.1 ℃ (77.18 ℉). At the beginning of October you can expect higher temperatures, the average highest temperature is around 32.45 ℃ (90.41 ℉). At the end of October you can expect lower temperatures, the average highest temperature is around 31.95 ℃ (89.51 ℉). The average number of rainy days in October is 9.1. The average rainfall is 33.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:08 (hours and minutes), in the middle of the month 12:13 and at the end of the month 12:18.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in November is this: The average highest temperature in Indonesia in November is 31.6 ℃ (88.88 ℉). The average lowest temperature is 24.6 ℃ (76.28 ℉). At the beginning of November you can expect higher temperatures, the average highest temperature is around 31.95 ℃ (89.51 ℉). At the end of November you can expect higher temperatures, the average highest temperature is around 32.05 ℃ (89.69 ℉). The average number of rainy days in November is 22. The average rainfall is 173.6 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:18 (hours and minutes), in the middle of the month 12:22 and at the end of the month 12:26.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Indonesia in December is this: The average highest temperature in Indonesia in December is 32.5 ℃ (90.5 ℉). The average lowest temperature is 24.6 ℃ (76.28 ℉). At the beginning of December you can expect lower temperatures, the average highest temperature is around 32.05 ℃ (89.69 ℉). At the end of December you can expect lower temperatures, the average highest temperature is around 31.4 ℃ (88.52 ℉). The average number of rainy days in December is 12.1. The average rainfall is 85.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:26 (hours and minutes), in the middle of the month 12:28 and at the end of the month 12:28.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.