Climate
During the wet season, October to March, expect rain at least daily, towards the late afternoon and early evening. Intensity varies, but invariably the monsoonal rains always arrive. Climate is always very humid, so pack a lot of drinking water if you are trekking.
Flora and fauna
Seeing the orangutans is a magical experience. In Bukit Lawang, you can usually see them near the rehabilitation centre and at the feeding platform during the morning and afternoon feeding sessions.
The best experience is an encounter in the jungle where there are many semi-wild and wild animals. The wild orangutan can be difficult to spot unless you go deep into the jungle. There are also white and black gibbons that make an amazing noise calling out to each other, and Thomas Leaf monkeys. If you are lucky, you may be able to see hornbills, toucans, moon snakes, monitor lizards.
Since there are very few still alive, it is very improbable that either the Sumatran Tiger or the Sumatran Rhinoceros will be encountered, although footprints and droppings have been reported. For elephants, go to Tangkahan or near Ketambe you can trek further into the jungle to try and spot wild elephants.