Since Sumatra is right next to the equator, Sumatra’s climate is very tropical. This means one minute the weather is hot and humid, the next it is raining in sheets. However, in general, throughout the year, the temperature during the day remains in between 70 and 86 degrees. As with all tropical climates, Sumatra has a rainy season and a dry season. The dry season lasts from May to September. The wet season however is harder to define and forecast.
In the North part of the island, the rain begins in October. It is wettest there during December and January. However, in the South, the rain begins in November and reaches its height around January and February. Thus, travelers should be cognizant of where they plan to travel and when, and pack appropriately (to be safe, one should pack a rainjacket regardless).
The most time to travel is within the dry season, between May and September. Since the wet season can be impede some travel options (i.e. depending on if one is in Sumatra for adventure hiking or kayaking, etc.), it is safest to come when the weather is reasonably mild.