Balikpapan is equatorial and humid year-round, with no true dry season. The drier months (June–October) see less rainfall and are generally more comfortable for outdoor activities and forest walks. The wet season (November–April) can bring heavy daily downpours that make forest trails muddy and river levels high — which can actually aid boat travel upriver to the Mahakam.
Avoid the worst of the haze season (August–September in bad years) when forest fires across Kalimantan can reduce air quality severely, though Balikpapan itself is better positioned than many inland cities.