The mismanagement of the rainy season observed these last years due to the global climate change has created a lot of disorder in programming of the agricultural activities in many countries, particularly the developing ones, putting them in front of permanent hunger constraints. To contribute in solving this problem the authors of this paper have elaborated a new method of forecasting the beginning and end of rainy season based on the deficit of the virtual temperature of the air using only two parameters, the atmospheric pressure and the temperature of the air registered during the period from 1986 to 2000 in the localities of Ndjamena; Moundou and Sarh in Chad. The acceptable results obtained indicate that the method can be operational. The obtained probabilities of realization of 66.7% and 83% in Sarh and Moundou give our method some strength and fairness. Probably that the method has to be improved using long and more accurate chronological data from other localities.
