abstract: Statistical distribution of atmospheric precipitation sums may be approximated with gamma function. An easy way of determining the empirical parameters is the method of moments, where they are calculated on the basis of mean value and variance. When precipitation sums are aggregated in longer periods, the parameters change and the distribution function approaches a normal distribution. The changes of variance may be determined after the formula: ln betas = a + b ln (n- 0.2), where betas - standard deviation of aggregated sums, n - length of period (days) a and b - empirical parameters. Knowing two values of variance (e.g. daily and monthly), the variance and both gamma parameters may be evaluated in any chosen period. Values of probability in the limits 1-99% determined this way may be assessed as sufficiently precise for practical purposes.