The only thing predictable about Bolivia's weather is its unpredictability. For most people the climate is generally cooler than they have expect. Even in the humid forests of the North frost is not unheard of during a surazo, a cold wind blowing from Patagonia and the Argentine Pampa. Generally speaking, Bolivia has two seasons, wet and dry, but in a country as geographically diverse as Bolivia, local weather patterns vary greatly.
In the highlands, the dry season is between May and October, but even the mountains' wet season isn't always really wet and the dry season sees its share of downpours. In La Paz and Sucre, temperatures during the day in the dry season are perfect for a light fleece and some travelers even try to sneak in a day with only shorts. Uyuni and Potosi, however, can be chilly in the middle of the day, and temperatures can drop below freezing at night - bring lots of layers!
The Amazon experiences...you guessed it, rain and heat and more rain and heat! The average daytime temperature is 30-32° C (85-90° F), while nights can cool down to 20° C (68° F). They do have a wet and wetter season with the wet (dry) season corresponding with the dry season in the highlands.