I climbed in Bolivia in 2009, from late August until the end of September. That's the tail of the dry season, which starts in May or so. Dry really means dry there. I saw virtually no rain or snowfall, neither in La Paz nor in the mountains.
In the Cordillera Real, the weather was so stable, we succeeded on every climb we started! Usually, we had excellent views as well, only once did we find ourselves in clouds on a summit. In the last week, we went to the volcanos of Sajama NP. First, we hiked up on Parinacota (I wouldn't call it climbing - being late in the dry season, there wasn't all that much snow anymore, and we didn't need crampons, ice axe or rope). It was very cold in the early hours. Finally, on Sajama, our last climb, the weather caught up with us. Visibility was excellent, but the winds were coming on strong and we didn't want to be blown off the mountain. Literally. And as if that wouldn't have been reason enough, the windchill made it so cold, I was afraid that we would have risked frostbite on our hands and feet.
The busiest mountain of all is Chacaltaya (5395m). Every day, one or more buses drive tourists from La Paz to a very basic ski lodge less than half an hour from the summit, which offers great views over the Cordillera Real and down to La Paz. Great for acclimatization, but not exactly an alpine adventure.
The busiest big
one by far is Huayna Potosi. In the climbing season there will be guided parties almost daily, many with tourists that never climbed a mountain before. After that, the Condoriri group and Illimani in the Cordillera Real, and Sajama and Parinacota in the Cordillera Occidental see a fair bit of climbers, but you may still find yourself alone on the mountain.Sajama
Ancohuma and Illampu usually don't get much traffic. As expected, we were the only team on Ancohuma, but when we got to Illampu base camp, there just happened to be a course for aspiring mountain guides going on, so there were actually lots of tents! There was a nice atmosphere in that camp actually, and our two porters were happy to spend some time with friends while we climbed.
You can read more about it in my trip report
Saludos, RobHigh tea at Ancohuma high camp