The focus of this paper is the occurrence of dry intrusions in the tropical atmosphere and their origin. Dry layers are observed to have a wide range of thicknesses, so as guidance on the scales of interest the radiative impact of dry intrusions is investigated. Typically such intrusions would contribute an increase in OLR of 3 Wm-2 per 100 hPa thickness in a clear-sky scenario and less if clouds are present. The velocity fields from ERA-40 are used to calculate the back trajectories of air masses from radiosonde profiles. The humidity observed by the sondes is compared with humidity profiles reconstructed by taking ERA-40 specific humidity from the origin of the trajectories, reducing the value only if saturation occurs along the trajectory. The agreement found provides a validation of the trajectory technique used, which is then used to infer the origin of dry air masses during November and December 1992 at Kapingamarangi, in the tropical West Pacific. A variety of behaviors is found, with many dry events clearly associated with air masses descending from the extratropics. Four particular dry events are discussed.