Do you need to know, when is the best time to go to Sumatra? Then you should take a look here, what weather you can expect there during the year.
Sumatra is part of Indonesia. 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.
When speaking about the usual weather in this area, the information is relevant for example for these popular places: Aceh, Anambas, Banda Aceh, Bandar Lampung, Bangka Belitung, Banyak Islands, Barelang Islands, Batam, Batam Centre, Batusangkar, Belitung, Bengkulu, Berastagi, Bintan, Bintan Resorts, Bukit Barisan Selatan National Park, Bukittinggi, Bungus Bay, Dumai, Gunung Leuser National Park, Harau Valley, Jambi, Karimun, Kerinci Seblat National Park, Ketambe, Kijang, Koto Gadang, Krakatoa, Krui, Kundur, Kutacane, Lake Maninjau, Lake Toba, Lampung, Lhoknga, Lhokseumawe, Lingga Islands, Medan, Mentawai Islands, Meulaboh, Nagoya, Natuna Islands, Nias, North Sumatra, Padang, Padang Panjang, Padang Sidempuan, Palembang, Pangkal Pinang, Parapat, Pekanbaru, Pematangsiantar, Riau, Riau Islands, Sabang, Sekupang, Selatpanjang, Sibolga, Singkil, Solok, South Sumatra, Sumpat, Sungai Pinang, Takengon, Tanjung Batu, Tanjung Pinang, Tanjung Uban, Tapak Tuan, Telaga Punggur, Tesso Nilo National Park, Trikora, Waterfront City, West Sumatra.
Let's look now what weather you can expect in the area around Medan and in many other cities. The warmest month here is May, the average maximum temperature is 34.4 ℃ (93.92 ℉). The average minimum temperature in this month is 23.3 ℃ (73.94 ℉). The coldest month here is November, the average maximum temperature is 30.9 ℃ (87.62 ℉). The average minimum temperature in this month is 23.3 ℃ (73.94 ℉). The most rainy month here is September, the average number of rainy days is 23.5. The least rainy month here is July, the average number of rainy days is 13.1. 
When travelling around Sumatra, always consider the altitude of the the place as well. We should remind you, that the highest point in Indonesia 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 Sumatra in January is this: The average highest temperature in Sumatra in January is 32.4 ℃ (90.32 ℉). The average lowest temperature is 22.1 ℃ (71.78 ℉). At the beginning of January you can expect lower temperatures, the average highest temperature is around 32 ℃ (89.6 ℉). At the end of January you can expect lower temperatures, the average highest temperature is around 32.1 ℃ (89.78 ℉). The average number of rainy days in January is 14. The average rainfall is 90.4 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 11:55 (hours and minutes), in the middle of the month 11:56 and at the end of the month 11:57.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 Sumatra in February is this: The average highest temperature in Sumatra in February is 31.8 ℃ (89.24 ℉). The average lowest temperature is 22.5 ℃ (72.5 ℉). At the beginning of February you can expect higher temperatures, the average highest temperature is around 32.1 ℃ (89.78 ℉). At the end of February you can expect higher temperatures, the average highest temperature is around 32.1 ℃ (89.78 ℉). The average number of rainy days in February is 19.3. The average rainfall is 113.2 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 11:57 (hours and minutes), in the middle of the month 12:00 and at the end of the month 12:02.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 Sumatra in March is this: The average highest temperature in Sumatra in March is 32.4 ℃ (90.32 ℉). The average lowest temperature is 23.4 ℃ (74.12 ℉). At the beginning of March you can expect lower temperatures, the average highest temperature is around 32.1 ℃ (89.78 ℉). At the end of March you can expect higher temperatures, the average highest temperature is around 32.6 ℃ (90.68 ℉). The average number of rainy days in March is 13.2. The average rainfall is 97.9 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 12:02 (hours and minutes), in the middle of the month 12:05 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 Sumatra in April is this: The average highest temperature in Sumatra in April is 32.8 ℃ (91.04 ℉). The average lowest temperature is 23.2 ℃ (73.76 ℉). At the beginning of April you can expect lower temperatures, the average highest temperature is around 32.6 ℃ (90.68 ℉). At the end of April you can expect higher temperatures, the average highest temperature is around 33.6 ℃ (92.48 ℉). The average number of rainy days in April is 18. The average rainfall is 154.6 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 12:08 (hours and minutes), in the middle of the month 12:11 and at the end of the month 12:14.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 Sumatra in May is this: The average highest temperature in Sumatra in May is 34.4 ℃ (93.92 ℉). The average lowest temperature is 23.3 ℃ (73.94 ℉). At the beginning of May you can expect lower temperatures, the average highest temperature is around 33.6 ℃ (92.48 ℉). At the end of May you can expect lower temperatures, the average highest temperature is around 34.2 ℃ (93.56 ℉). The average number of rainy days in May is 22.1. The average rainfall is 179 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 12:14 (hours and minutes), in the middle of the month 12:16 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 Sumatra in June is this: The average highest temperature in Sumatra in June is 34 ℃ (93.2 ℉). The average lowest temperature is 23.7 ℃ (74.66 ℉). At the beginning of June you can expect higher temperatures, the average highest temperature is around 34.2 ℃ (93.56 ℉). At the end of June you can expect lower temperatures, the average highest temperature is around 33.85 ℃ (92.93 ℉). The average number of rainy days in June is 14.8. The average rainfall is 144.2 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 12:18 (hours and minutes), in the middle of the month 12:19 and at the end of the month 12:19.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 Sumatra in July is this: The average highest temperature in Sumatra in July is 33.7 ℃ (92.66 ℉). The average lowest temperature is 24.2 ℃ (75.56 ℉). At the beginning of July you can expect higher temperatures, the average highest temperature is around 33.85 ℃ (92.93 ℉). At the end of July you can expect lower temperatures, the average highest temperature is around 33.25 ℃ (91.85 ℉). The average number of rainy days in July is 13.1. The average rainfall is 164.8 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 12:19 (hours and minutes), in the middle of the month 12:18 and at the end of the month 12:16.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 Sumatra in August is this: The average highest temperature in Sumatra in August is 32.8 ℃ (91.04 ℉). The average lowest temperature is 23.6 ℃ (74.48 ℉). At the beginning of August you can expect higher temperatures, the average highest temperature is around 33.25 ℃ (91.85 ℉). At the end of August you can expect lower temperatures, the average highest temperature is around 32.65 ℃ (90.77 ℉). The average number of rainy days in August is 17.3. The average rainfall is 188.1 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 12:16 (hours and minutes), in the middle of the month 12:14 and at the end of the month 12:11.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 Sumatra in September is this: The average highest temperature in Sumatra in September is 32.5 ℃ (90.5 ℉). The average lowest temperature is 22.1 ℃ (71.78 ℉). At the beginning of September you can expect higher temperatures, the average highest temperature is around 32.65 ℃ (90.77 ℉). At the end of September you can expect lower temperatures, the average highest temperature is around 32.2 ℃ (89.96 ℉). The average number of rainy days in September is 23.5. The average rainfall is 264.8 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 12:11 (hours and minutes), in the middle of the month 12:08 and at the end of the month 12:05.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 Sumatra in October is this: The average highest temperature in Sumatra in October is 31.9 ℃ (89.42 ℉). The average lowest temperature is 23.2 ℃ (73.76 ℉). At the beginning of October you can expect higher temperatures, the average highest temperature is around 32.2 ℃ (89.96 ℉). At the end of October you can expect lower temperatures, the average highest temperature is around 31.4 ℃ (88.52 ℉). The average number of rainy days in October is 21.9. The average rainfall is 387.4 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 12:05 (hours and minutes), in the middle of the month 12:02 and at the end of the month 12:00.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 Sumatra in November is this: The average highest temperature in Sumatra in November is 30.9 ℃ (87.62 ℉). The average lowest temperature is 23.3 ℃ (73.94 ℉). At the beginning of November you can expect higher temperatures, the average highest temperature is around 31.4 ℃ (88.52 ℉). At the end of November you can expect higher temperatures, the average highest temperature is around 31.25 ℃ (88.25 ℉). The average number of rainy days in November is 20.2. The average rainfall is 250.5 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 12:00 (hours and minutes), in the middle of the month 11:57 and at the end of the month 11:56.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 Sumatra in December is this: The average highest temperature in Sumatra in December is 31.6 ℃ (88.88 ℉). The average lowest temperature is 22.2 ℃ (71.96 ℉). At the beginning of December you can expect lower temperatures, the average highest temperature is around 31.25 ℃ (88.25 ℉). At the end of December you can expect higher temperatures, the average highest temperature is around 32 ℃ (89.6 ℉). The average number of rainy days in December is 18.7. The average rainfall is 231.2 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 11:56 (hours and minutes), in the middle of the month 11:55 and at the end of the month 11:54.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.