[en] Meteorological factors which influence the external radiation were analysed. Attention was paid to the relationship between the concentration of the radioactive aerosol and the outdoor radiation level. Twelve kinds of data were obtained at Kumatori Site of Kyoto University. The mean diurnal variation and annual variation of each factors were obtained from the data of 25 months periods from August 1974 to August 1976. Outstanding was the result that the outdoor gamma radiation varied just slightly in spite of the large variation of the airborne activities and others. It implies that the individual effect due to the factor would compensate and cancel each other. Otherwise the major contribution on the variation might distribute at random with respect to time. Contrary to the apparent independence of the outdoor gamma radiation level of the airborne activity, however, the partial correlation coefficient between them was not negligibly small. Analysis of each time series of individual data has revealed that the rainfall has the largest influence on the variation of the outdoor gamma radiation level particularly in its initial period. Airborne activity was also revealed to have a strong connection with the atmospheric stability and the wind velocity though it depends on the rainfall just a little. The atmospheric stability could be expressed by the vertical gradient of the atmospheric temperature which is strongest at the ''inversion''. However problem remains as to the nature of the wind. Which of the atmospheric turbulence and the prevailing wind corresponds to the wind here will have to be analysed referring to the large scale weather charts. Regression analysis showed that the atmospheric temperature was the essential cause of the variation of the radiation level. (author)