Bali has a warm, tropical climate with temperatures averaging between 20 to 33 degrees Celsius year-round.
The wet season is typically December to March with heavy rainfall (usually at night and for short periods) and high humidity. During the wet season the days are generally sunny, so you can sit by the pool or on the beach and catch some rays.
The dry season is between June and September with low humidity and hardly any rainfall. The evenings can be cool and there is usually a refreshing breeze during the day. The central mountains, Ubud and higher regions are typically cooler than the lower coaster areas.