Generally speaking, climate varies much more as a result of altitude and topography than it does between different seasons. Nevertheless, there are clear-cut seasonal differences. Winter (invierno) runs between May and October: this is the dry season, and in many ways the best time to visit, though it’s also the high season for tourism, so some prices will be higher and attractions busier. In the highlands it’s noticeably colder at night, particularly in June and July. The days are slightly shorter, but usually sunny, and the skies crystal clear, making this the best time of year for trekking and climbing. Winter is also the best time for visiting the hot and humid lowlands, when temperatures are generally slightly (but pleasantly) lower, although the dry season is less pronounced and rain remains a possibility all year round. A few times a year, usually between July and August, the country is swept by cold fronts coming up from Patagonia, known as surazos, which can send temperatures plunging even in the Amazon. Towards the end of the dry season in late August and September, farmers set fire to cleared forest areas across much of Bolivia, which can obscure views and cause respiratory problems.
Summer (verano) is the rainy season, which runs roughly from November to March and is much more pronounced in the lowlands; in the Amazon, road transport becomes pretty much impossible, as huge areas are flooded and everything turns to mud – though, conversely, river transport becomes more frequent. Heat, humidity and mosquitoes are also much worse. In the highlands, particularly the Altiplano, it rains much less and travel is not as restricted, though delays and road closures still occur, while trekking trails get muddier and clouds often obscure views, particularly in the high mountains, where route-finding can become impossible. Despite this, the rainy season is also a very beautiful time in the Andes, as the parched Altiplano and mountainsides are briefly transformed into lush grassland and wild flowers proliferate.