Rainfall is one of the most important variables of the hydrological cycle,making its constraints, as the Number of Days with Rain (NDR), of greatimportance in climatological studies. In order to analyze the monthlyand annual trend of Number of Days with Rain in the state of MatoGrosso, we performed a study of its extreme variability and its possiblecorrelation with the El Nino Southern Oscillation (ENSO) phenomenaof moderate or heavy intensity. We used monthly and annual averagesof the NDR of the historical series of rainfall data obtained from theNational Hydrometeorological Network, the National Water Agency andthe Geological Survey of Brazil. It was used the historical series of 30years (1977-2006). For the statistic treatment we calculated the temporalaverages of the number of days with rainfall of 120 rainfall stationsdistributed in the state of Mato Grosso. It was observed that only themonth of March tended to increase the NDR for the historical series studied, leaving the other months and the annual general average withdecreasing trend on NDR in the state of Mato Grosso. Results are presentedin graphical trends of NDR for each month and year, correlatedwith the intensity of ENSO, and also in the monthly histogram with theaverage of 30 years and for each one of the three decades.
