Hot and humid would describe the lowlands' climate. The wet season country-wide is from late November to late March or early April, depending on where you are geographically. The quantity of rainfall varies from region to region, but the tropics get most of the rain by far. It can rain any day of the year in the Yungas and parts of the tropics as well. The highlands get very little rain in the winter except when it snows or hails, which are more frequent in the summer - wet season.
Winter has the most beautiful climate and temperatures in the valley regions.
There are primarily two seasons in Bolivia - the dry and the wet. The dry season is from May to October, the winter time months. The wet season is from November to April, the summer time months. It is coldest during the months of June to September and wettest from December to March. The dry season is best for travel due to the better road conditions and generally sunny skies and warm temperatures during the day.