Although the rate of stratospheric fall-out probably has some average mean value, the fall-out process may be rather sporadic. Frequent measurements of air activity at ground stations could be helpful in studying the fall-out process in detail. Probable causes of variations in air activity are summarized, and automatic equipment for collecting samples of airborne dust is described. Results are given from measurements made at Kjeller during March and April 1959. The diurnal variation of air activity may indicate the rate of dry deposition of radioactive materials. (auth)