You can expect the temperatures to vary between 24°C at night during winter time (June to September) and 34°C during the day. In summer, temperatures at night may reach 30°C, while during the day they can be close to 38°C.
In summer it rains more than in winter. However do not expect rain to be consistently present every day. You can expect a tropical, warm and humid climate all year round with two main distinctive seasons: Dry season and Rainy Season.
Rain is usually stormy, and lasts for a few hours only, getting quickly replaced by the sun that dries the water away in a few minutes. It’s always pretty warm, with cooler evenings in July and August. Also in April and May you can find nice weather. The humidity can start around the end of November – beginning of December and visitors during the rainy season might usefully re-consider their activity schedule since it's very hot & humid. A whole day with no sun is very rare, as is it is not rare to have many consecutive dry dies even during the rainy season, which goes from November to March.
Come to Ubud at any time of the year, as you are guaranteed some nice weather even during the worst season.