AbstractThis paper reports observations made over the North Atlantic of a system of convective cloud bands found within the cold air behind an occluded front. The observations were made from an instrumented aircraft which additionally released a pattern of dropsondes, measuring temperature, relative humidity and wind, as functions of pressure. These data are analysed to establish some of the mesoscale, cloud‐scale and microphysical properties of the system.Here, in part I, the mesoscale structure is described and compared with the predictions of various theoretical models. The convective cloud structure emerges as a consequence of the mesoscale organization but appears to play only a minor role in its creation and maintenance. One of the theoretical models considered, conditional symmetric instability, provides a piausible explanation of the mesoscale observation