Several sources and processes contribute to the natural radiation background level, causing significant fluctuations in time. Quantified knowledge on the probability of these variations is desirable for many reasons, e.g. to discriminate between natural and human-induced factors or to support the management of nuclear emergency networks. Frequency distributions of ambient dose rate, as observed by the Dutch National Radioactivity Monitoring Network over the period 1990-1994, have been explained through the joint contribution of five time-varying sources, including counting statistics. Normalised probability density functions (with a normal shape for noise, terrestrial and cosmogenic radiation, and an exponential shape for airborne and deposited radioactivity from radon progeny) add up to one joint probability density function, which agrees with long-term data distributions over four orders of magnitude. This comparison yields parameter values describing the probable impact of rainout and washout of radon progeny and the typical fluctuation band of terrestrial radiation as observed in the Netherlands.