Weather in Sumatra: Climate, Seasons, and Average Monthly Temperature

Good weather at Lake Toba in Sumatra

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Sumatra feels hot and sticky throughout most of the year. Even the summer dry season is punctuated with heavy downpours, but they don’t last long. You’re pretty well guaranteed to get soaked more than once while searching for orangutans and enjoying the many adventurous things to do in Sumatra. But with the equator nearby and humidity soaring, who cares! Temperatures stay between 80 and 90 degrees F (27 and 32 degrees C) throughout the year and days are consistently around 12 hours long.

If like many travelers you’re bound for Lake Toba and adventures in the north, June is arguably the best month for visiting Sumatra. Temperatures are slightly milder around the big volcanic lake—mornings and evenings can even feel chilly. Generally speaking, Sumatra’s summer months are the driest; however, air quality declines in late summer as annual fires burn unchecked.

Fast Climate Facts

  • Hottest Month: June (83 F / 28 C)
  • Coolest Month: December (81 F / 27 C)
  • Wettest Months: October in North Sumatra (6.5 inches of rainfall); April in South Sumatra (8 inches of rainfall)
  • Busiest Months: May to June (the Lunar New Year in January/February is also extra busy at Lake Toba)

Monsoon Season in Sumatra

Monsoon season in Sumatra begins between September and November. The monsoon can arrive early or be delayed in any given year. Regardless, be ready for heavier rain than usual between September and November. Because of Sumatra’s geography, monsoon season tends to begin about a month earlier in North Sumatra (September) than in South Sumatra (October).

Traveling during monsoon season in Sumatra is still possible, but your outdoor adventures will be probably be affected. Flooding and mudslides can make visiting national parks more challenging, and you’ll deal with more mosquitoes and leeches. The rivers often used to access national parks may be too dangerous to navigate. Regional flights sometimes get canceled or delayed due to weather. The air is cleaner and hotels are cheaper during the monsoon “low” season.

Burning Season in Sumatra

Sadly, Sumatra suffers from an annual “burning season” (typically June to October) when haze from illegal agricultural fires reduces air quality to unhealthy levels. Despite efforts from world leaders, the massive event causes a haze to spread across Southeast Asia, choking Kuala Lumpur and Singapore to the east. Schools close and flights become canceled as a result. Local residents are advised to stay indoors or wear masks.

Haze is often at its worst between July and September until rainy season begins to clear it up. Consider checking the air quality before traveling to Sumatra between June and October, especially if you are prone to respiratory issues. Fortunately, the smoke isn’t as bad around Lake Toba as it is farther south.

Harau Valley in West Sumatra at sunset

Kanoke_46 / Getty Images

Spring in Sumatra

Temperatures in Sumatra tend to stay consistent; highs are around 90 degrees F (32 degrees C) during the spring months while humidity hovers around 79 percent. May is an ideal month for visiting West and South Sumatra for better air quality before the haze begins to build later in summer.

Spring is rainy in most of Sumatra, but showers become less frequent until dry season begins in summer. North Sumatra averages only 3.8 inches of rainfall in April, but precipitation nearly doubles in May before dropping back to 3.4 inches in June.

South Sumatra averages 7.6 inches of rainfall in March and April before becoming a little drier in May with 4.5 inches of rain.

What to Pack: Breathable clothing and rain gear are essential for traveling Sumatra in spring—or any season, for that matter. 

Summer in Sumatra

Heat and humidity continue to build into summer, but that doesn’t stop the bulk of Sumatra’s tourists and surfers from turning up. The trails are in peak form in summer, and summer is the best time to embark on a motorbike adventure to enjoy lakes, villages, and spread-out sights. Summer is also burning season, so expect haze and lower air quality until rain intensifies—particularly around September. Palembang, the capital of South Sumatra, is often plagued by poor air quality in summer.

Although average rainfall is between 3 to 5 inches, summer is a relatively dry time in Sumatra as downpours are shorter and less frequent. South Sumatra is the driest part of the island in summer. Humidity can drop as low as 75 percent.

What to Pack: Take extra precautions for sun protection; the equator cuts through West Sumatra, and the UV index is high. If traveling in late summer, carry a serious mask for protection against airborne particulate matter caused by fires.

Fall in Sumatra

Fall is typically the start of monsoon season in Sumatra as downpours intensify in frequency and duration. The average rainfall for North Sumatra in fall is 6 inches a month; October is the wettest month. Humidity hovers above 80 percent in fall.

The rain arrives a little later for South Sumatra. September is dry and enjoyable, but heavy rain begins in October and peaks in November with more than 7 inches of rainfall on average.

What to Pack: You’ll need some heavy rain gear for traveling during the peak of monsoon season. Have a reliable way to waterproof your luggage, passport, and electronics. An umbrella alone won’t do the trick.

Winter in Sumatra

Unless summiting a volcano, you won’t have to worry about cold temperatures during winter in Sumatra. That said, the heavy rains can push lows into the 70s F at night. Samosir Island at Lake Toba can feel especially cool in early mornings.

January and February are often dry months for North Sumatra (around two inches of rain per month on average) while other parts of the island are inundated. Rain increases through the spring until peaking in May then dropping off again for dry season in summer. The rest of the island is very wet in winter. With an average of 9.6 inches of rain, November is the wettest month for Padang, the capital of West Sumatra.

What to Pack: No need for a coat, but you’ll want an uninsulated rain jacket—something better than the throwaway ponchos that can be purchased locally. You’ll also need waterproof boots for trekking. Bring tall socks to thwart the leeches that thrive on the damp forest floor.

Average Monthly Temperature, Rainfall, and Daylight Hours

  Avg. Temp. Avg. Rainfall Daylight Hours
  January 81 F / 27 C 2.9 inches 12 hours
  February 82 F / 28 C 1.5 inches 12 hours
  March  81 F / 27 C 3.2 inches  12 hours
  April  82 F / 28 C 3.8 inches 13 hours 
  May  83 F / 28 C 6.1 inches 13 hours 
  June  83 F / 28 C 3.4 inches 13 hours 
  July  82 F / 28 C 5 inches 13 hours 
  August  82 F / 28 C 4.7 inches 13 hours 
  September  82 F / 28 C 6.2 inches 13 hours 
  October  81 F / 27 C 6.5 inches 12 hours 
  November  81 F / 27 C 5.4 inches  12 hours 
  December  81 F / 27 C 4.1 inches  12 hours 
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