Summary:

The aim of the undertaken research was an evaluation of the frequency of the dry and the wet months in the Opole Voivodeship in the months of May to September in1981-2010 years, in the view of the global warming. The dry and the wet months were characterized on the basis of hydrothermal index K, thermo-precipitation index A, standardized precipitation index SPI and the amount of the precipitation deficiencies and excesses of the late potato. The attempt of answering the question which of these indexes describe the moisture conditions of the vegetative season the best were undertaken.
It was proved that the average frequency of months, in which the different level of draught's intensity was observed, was equal to 25% on the basis of the thermo-precipitation index A, 35% standardized precipitation index SPI, 36% hydrothermal index K, and 34% on the basis of the deficiencies precipitation amount. The average frequency of the wet months were counted to all four indexes and the values were: 23% on the basis of the thermo-precipitation index A, 23% standardized precipitation index SPI, 16% hydrothermal index K, and 29% on the basis of the deficiencies precipitation amount.
The dependence between the four indexes and the yield of the potato were observed. The correlation between the potato yield and the thermo-precipitation index A turned out to be the most statistically significant. It seems reasonable, that in the term of the constant temperature growth and the strong disperse of the precipitation, typical for the climate of Poland, the indexes, connected with the air temperature and precipitation, are the best indicators to classify the wet and the dry months.