The results of the environmental radiation (ER) monitoring obtained during the Tunguska expedition of July 1999 with a NaI(Tl) scintillation detector are reported. Some interesting meteorological effects related to the airborne radioactive components (radon and thoron decay products) have been observed. The main effects observed are (1) the doubling of the ER during two quite different rain-out episodes and (2) a fairly regular diurnal wave in the intensity of the total radiation. The rain-out phenomenon allows us to approximate the timescale for the removal by precipitation of radioactivity from the air, while the wave phenomenon gives us the opportunity to develop a simplified model for estimating the time-averaged, dry weather surface radon concentration.