The ambient cloud in situ measurement campaign is a cross-comparison of cloud in situ measurement instruments with respect to the variables liquid water content and droplet effective diameter. The campaigns focus on the measurement instruments recommended in ACTRIS, such as the PVM. The campaign is organized every two years, with a measurement phase of 3-5 days at Mt. Hohen Sonnblick. During this time the participants are trained in ACTRIS routines and the standards are further developed. A joint report finally summarizes the results. The participants receive a certificate.