[en] Monthly records of ¹³⁷Cs and ¹³⁴Cs concentrations in air and fallout at Dalat for the period 1986-91 are presented and discussed. The concentration variations exhibit distinct maxima during December-January, when dry fallout dominated. These peaks are explained by the intrusion of more radioactive cold air masses from temperature northern latitudes during the development of large-scale anticyclones frequently observed in the most active winter monsoon period. High dry fallout velocities (about 10 cm/s) determined from these data clearly demonstrate one of the most relevant characteristics of cold air masses: behind the cold front, vertical air motion is descending. (Author)