Abstract
A superpressure balloon flying at a constant density level fortuitously encountered (on two consecutive days) an artificial cloud generated by a French nuclear test over the South Pacific. The measurements of temperature and radiation made aboard the balloon show that the two observed encounters had short durations, and that there were rapid divergences between the trajectories of the cloud and the constant density level trajectory of the balloon. The analysis of the meteorological data and the observations made by the balloon allow us to demonstrate the quasi-isentropic nature of the radioactive cloud's trajectory.
