Map from Google - Bolivia

In Bolivia there are different types of climate depending on area and altitude. In plains and lowlands, the climate is equatorial in the far north and tropical in the south-central, while in the plateau and Andean mountain peaks, it's colder and generally quite dry.
Bolivia lies in the Southern Hemisphere, albeit at tropical latitudes, so its seasons are reversed compared with the Northern Hemisphere.

Areas with an equatorial climate
The northernmost area of Bolivia (Pando Department, northern parts of the departments of Beni and La Paz) is covered by rainforest, and has a nearly equatorial climate: hot and humid all year round, with little variation in temperature, just a few of degrees, from 74 °F in the coldest month to 80.5 °F in the warmest months, heavy rainfall, more than 60 inches per year, and a short dry season from June to August, when less than 2 inches of rain per month fall. The rains are most abundant from Novembre to March.
Here is the average precipitation in Cobija, in the far north, near the border with Brazil.
Average precipitation - Cobija
Prec. (in)

The best time to visit this area runs from June to August, which is not only the driest, but also the least hot: by day it's as hot as in the rest of the year, with maximum temperatures around 88/90 °F, but at night the temperature usually goes, albeit slightly, below 68 °F, besides moisture is a bit lower, so that the heat is more bearable. From mid-August to mid-October, however, before the rainy season, the temperature increases and there is the hottest time of the year.
Here are the average temperatures in Cobija.
Average temperatures - Cobija
Min (°F)727070686463616366707070
Max (°F)888888888486889193908888

Climate of the savanna - Chaco
In the rest of the plains and lowlands, we find the Gran Chaco, a region which includes the south-west of Mato Grosso in Brazil, western Paraguay and northern Argentina, and which is covered by a savannah-type vegetation. Here the climate is tropical, with a dry season in winter, and a wet season in summer. During winter, the temperature drops by a few degrees, especially in the center and south: the average temperature in June and July is 73.5 °F in Trinidad, 68 °F in Puerto Suarez, and drops to 61/62.5 °F in the extreme south, also due to the altitude that in this area is higher, on average around 2,000/2,300 feet. As typically happens in the tropics, this area is hottest in spring, before the rain season, so that heat spikes, from 100 to 105 °F are recorded from September to November. Then the summer, albeit less hot, is sultry and therefore in this season the heat is still annoying.
The rains are more abundant in the western part, near the eastern Andean Cordillera: along the slopes at the foot of the Andes facing north-east, there is no real dry season, because even in winter some showers are possible (see San Buena Ventura).
Here is the average precipitation in Santa Cruz de la Sierra, located in the central part, at 1,300 feet above sea level.
Average precipitation - Santa Cruz
Santa CruzJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (in)

In the southern part, south of Santa Cruz de la Sierra, the climate tends to be drier: the annual rainfall is less than 40 inches per year, and in addition the dry season becomes longer, since it goes from May to September instead of June to August, as happens in the north-central. In this southern area, being further away from the equator, even thermal variations increase: from September to November, the temperature can reach 105 °F, while between May and August at night it can drop even to the freezing point (32 °F).
In fact, the central and southern part of the region, during the winter from May to August, are affected by sudden drops in temperature, due to cold winds blowing from the Argentinian pampas, called Surazos. In Santa Cruz de la Sierra, the night temperature may fall between 40 and 50 °F, but sometimes even below 40 °F, especially in June and July. In the south, lows can drop at times even a few degrees below freezing. Cold winter outbreaks are short, nevertheless they are significant for the latitude (we are still north of the Tropic of Capricorn).
Average temperatures - Santa Cruz
Santa CruzJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)727272686461616364707072
Max (°F)848684817773757981868686

Climate of the Andes in Bolivia
The Andean area is delimited by two cordilleras, between which there is a plateau where you will find the capital and other major cities of Bolivia. The western cordillera is much less rainy than the eastern one. The plateau is fairly barren, and in the south-west, near the border with Chile, it becomes even desert. In the Andean region there are high mountains, like the volcano Sajama, 21,463 feet high, Nevado Illampu, 21,276 ft, and Illimani, 21,003 feet high.
Winter on high mountains is dry, however, brief snowfalls are possible above 13,000 feet in winter and early spring. The snowline is very high, above 16,400 feet, because of the poor snowfall in the coldest season.
In general, in the plateau the diurnal temperature range is high, and after the sunset the air cools quickly. The cold is dry and therefore more bearable, except when the wind blows.
The capital of Bolivia, La Paz, is located at an average altitude of 11,800 feet, so it is the highest capital in the world, but the different districts are located at an altitude between 10,500 and 13,500 feet, so there are different temperatures from area to area. The airport is located in the highest area, at 13,300 feet above sea level: in this part of the city, the climate is cold all year round: the average daily temperature ranges from 41 °F in June and July to 48 °F in November (which is, albeit slightly, the warmest month, because then in summer, rains and clouds lower a little the temperature). Night temperatures easily descend below freezing at the airport in the colder months, from May to September, when they can plunge to 14 °F, while during the day the temperature remains fairly mild, 54/59 °F. In the summer months, the temperature does not rise much, just by a few degrees.
Average temperatures - La Paz
La PazJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)373737363228283032363737
Max (°F)545555555554545555575957

The total annual rainfall is around 22 inches, but since there is no warm month in which the trees could grow, here tundra dominates, like in sub-polar areas. Rain falls mostly from September to April, but from December to March it's more frequent (up to 20 days per month), and can sometimes be abundant, causing floods and landslides. Here is the average precipitation.
Average precipitation - La Paz
La PazJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (in)

The sunshine amount is good from May to August, while the sky is often cloudy from January to March.
At lower elevations, in the central area of the city, which is located at 12,000 feet, the temperature is higher by a few degrees: so much the better, since during the day it reaches 57 °F in winter and 59/63 °F in November and in the summer months; in the lower neighborhoods, at just above 9,800 feet above sea level, the temperature rises to 63/66 °F, and winter frosts, when they occur, are light.
Here are the average temperatures of the lower part of the city, at 10,600 feet of altitude.
Average temperatures - Lower La Paz
Lower La PazJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)434343393734343637394343
Max (°F)636364646463636364666664

The best time to visit La Paz goes from May to August, although it's the coldest, because it's also the driest and sunniest.

West of La Paz, at no great distance to the capital, we find Lake Titicaca, which is the highest navigable lake in the world, being located at 12,500 feet above sea level, while to the south we find the archeological site of Tiwanaku, located at the same altitude. This area has a climate similar to that of the higher part of La Paz.

Lake Titicaca

The other capital, Sucre, is located further south, at an altitude of 9,200 feet. Compared with La Paz, here the temperatures are higher, that is, spring-like all year round, but at night it is often cold in winter, since they hover at night between 40/43 °F in the June-August period, and 51/52 °F from November to March, while the daytime temperatures hover around 71/76 °F throughout the year. Here are the average temperatures.
Average temperatures - Sucre
Min (°F)525250484541414346505252
Max (°F)727272727272727375757573

In an average year, 26.7 inches of rain fall. Even here, summer the rainy season, and the best time is therefore winter, from May to August.
Here is the average precipitation.
Average precipitation - Sucre
Prec. (in)

In Cochabamba, located at 8,200 feet above sea level, the temperatures are slightly higher. Here, daytime temperatures fluctuate throughout the year between 73 and 79 °F, but the diurnal temperature range is high, so nights are cold from May to August.
Average temperatures - Cochabamba
Min (°F)555454504337374348525455
Max (°F)737373777573737577797977

In one year, just 21.5 in of rain fall, and the dry season is very long; the rains are substantial only from December to March. Here is the average rainfall.
Average precipitation - Cochabamba
Prec. (in)

In the southwest, near the border with Chile, the plateau becomes definitely desert. Here we find Salar de Uyuni, a huge expanse of salt, located at 12,000 feet of altitude, where the tropical mountain sun, already strong, increases its effects due to reflection on the white surface: better to bring sunglasses and sunscreen. Since it is not rainy, this area doesn't have particular weather problems, and can be visited throughout the year, even though, given the altitude it's preferable in summer, when the temperature is higher.

When to go

The best time to visit Bolivia in its entirety runs from June to August: even though at high altitudes it's the coldest period, it's also the least hot at low altitudes, and above all it's the driest and sunniest throughout the country.

What to pack

In winter (June to August): in the forest, lightweight clothing of natural fibers, maybe with long sleeves for mosquitoes, a sweatshirt for the evening. In the savannah, light clothing, a sweatshirt or sweater and a jacket for the evening, especially in the south. In the Andes (La Paz, Lake Titicaca), clothes for spring and autumn for the day, and warm for the evening: warm jacket, warm windproof hat; sunglasses, sunscreen. In the high mountains, warm clothes, down jacket, scarf, gloves, hiking boots.

In summer (December to February): in the forest and the savannah, lightweight clothing, possibly a light sweatshirt and a light raincoat for thunderstorms, sun hat. In the Andes (La Paz, Lake Titicaca), clothes for spring and autumn, raincoat or umbrella, jacket, sweater, sunglasses, sunscreen. In high mountains, warm clothes, down jacket, scarf, gloves, hiking boots.