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 23.5 °C in the coldest month to 27 °C in the warmest months, heavy rainfall, more than 1,500 millimetres per year, and a short dry season from June to August, when less than 50 mm 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. (mm)265250235185802525451001652201551850

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 31/32 °C, but at night the temperature usually goes, albeit slightly, below 20 °C, 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 (°C)222121201817161719212121
Max (°C)313131312930313334323131

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 centre and south: the average temperature in June and July is 23 °C in Trinidad, 20 °C in Puerto Suarez, and drops to 16/17 °C in the extreme south, also due to the altitude that in this area is higher, on average around 600/700 metres. As typically happens in the tropics, this area is hottest in spring, before the rain season, so that heat spikes, from 37 to 40 °C 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 400 metres above sea level.
Average precipitation - Santa Cruz
Santa CruzJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)20013012011090706040701001201701280

In the southern part, south of Santa Cruz de la Sierra, the climate tends to be drier: the annual rainfall is less than 1,000 mm 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 40 °C, while between May and August at night it can drop even to the freezing point (0 °C).
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 5 and 10 °C, but sometimes even below 5 °C, 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 (°C)222222201816161718212122
Max (°C)293029272523242627303030

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, 6,542 metres high, Nevado Illampu, 6,485 metres, and Illimani, 6,402 metres high.
Winter on high mountains is dry, however, brief snowfalls are possible above 4,000 metres in winter and early spring. The snowline is very high, above 5,000 metres, 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 3,600 metres, so it is the highest capital in the world, but the different districts are located at an altitude between 3,200 and 4,100 metres, so there are different temperatures from area to area. The airport is located in the highest area, at 4,050 metres above sea level: in this part of the city, the climate is cold all year round: the average daily temperature ranges from 5 °C in June and July to 9 °C 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 -10 °C, while during the day the temperature remains fairly mild, 12 to 15 °C. In the summer months, the temperature does not rise much, just by a few degrees.
Average temperatures - La Paz
La PazJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)33320-2-2-10233
Max (°C)121313131312121313141514

The total annual rainfall is around 560 mm, 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. (mm)120100703010551030405090560

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 3,650 metres, the temperature is higher by a few degrees: so much the better, since during the day it reaches 14 °C in winter and 15/17 °C in November and in the summer months; in the lower neighbourhoods, at just above 3,000 metres above sea level, the temperature rises to 17/19 °C, and winter frosts, when they occur, are light.
Here are the average temperatures of the lower part of the city, at 3,250 metres of altitude.
Average temperatures - Lower La Paz
Lower La PazJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)666431123466
Max (°C)171718181817171718191918

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 3,800 metres 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 2,800 metres. 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 5/6 °C in the June-August period, and 11 °C from November to March, while the daytime temperatures hover around 22/24 °C throughout the year. Here are the average temperatures.
Average temperatures - Sucre
Min (°C)111110975568101111
Max (°C)222222222222222324242423

In an average year, 680 mm 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. (mm)1551151053062212255065120680

In Cochabamba, located at 2,500 metres above sea level, the temperatures are slightly higher. Here, daytime temperatures fluctuate throughout the year between 23 and 26 °C, but the diurnal temperature range is high, so nights are cold from May to August.
Average temperatures - Cochabamba
Min (°C)1312121063369111213
Max (°C)232323252423232425262625

In one year, just 550 mm 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. (mm)1351108025624813254595547

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 3,650 metres 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 fibres, 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.