Atacama Pathfinder EXperiment APEX
Annual variation of PWV at Chajnantor

Annual variation of the Precipitable Water Vapour (PWV) content at Chajnantor, based on 10 years of site testing. As can be seen, conditions are worse during the “Bolivian Winter” from the end of December to early April. This time is generally reserved for technical work and commissioning of new instruments. The expected median PWV for the rest of the year is around 1 mm, while conditions of PWV<0.5mm can be expected up to 25% of the time.