The aim of this paper is to find the functional dependence between the occurrence of forest fires and the factors inherent to solar activity. It has been shown that the amplitude of number of forest fires in the USA for warm period 2004-2007 is not time dependent. The method of seasonality indices for seasonal components filter was used for the decomposition of time series. In order to test this hypothesis the correlation analysis was held between the factors Hi and the number of fires taking into account time delay (lag) between the onset of fires and solar activity. The results of this analysis show that any correlation coefficient is not higher than 0.2. For determination of the degree of randomness for time series of input and output parameters, the R/S analysis was conducted. The Hurst index was used for determining the depth of their memory. Based on the proximity of the Hurst index for the 10.7 cm solar flux categories and small forest fires, a reasonable assumption can be made that the dynamics of these time series is heavily dependent on the same factors. [Projekat Ministarstva nauke Republike Srbije, br. III47007]