Summer is the rainy season; it can be miserable in the lowlands.
Overland transportation becomes difficult.
Climbing is dangerous, and trekking and biking tedious.
February to April have plenty of religious festivals.
A great time to visit the Salar de Uyuni.
Good for budget hunters.
Mostly sunny days throughout Bolivia but cooler in the altiplano.
Reliable weather means easier transit, and better climbing, trekking and mountain biking.
Be prepared for festival crowds in August.