Concentrations of radon-222 in soil gas measured over about 1 yr at a monitoring site in Denver, Colorado, vary by as much as an order of magnitude seasonally and as much as severalfold in response to changes in weather. The primary weather factors that influence soil-gas radon concentrations are precipitation and barometric pressure. Soil characteristics are important in determining the magnitude and extent of the soil’s response to weather changes. The soil at the study site is clay rich and develops desiccation cracks upon drying that increase the soil’s permeability and enhance gas transport and removal of radon from the soil. A capping effect caused by frozen or unfrozen soil moisture is a primary mechanism for preventing radon loss to the atmosphere.