Indoor radon concentrations show a strong dependence on weather. Winter tends to be associated with higher than average indoor radon, and summer with lower than average. However, in northern Virginia, the summer of 1988 was wetter than the summer of 1987. Consequently, the regional indoor radon during the summer of 1988 was about 30% higher than during the summer of 1987, and indoor radon during the summer of 1988 actually exceeded the indoor radon level of the 1987-1988 winter.