Hydric precipitation as joint statistical process: Dry or wet day and amount of water

Summary The long time series of daily hydric precipitations in Modena, Italy, allows homogeneity tests which do not involve the assumption of particular models. The tests show that the number of dry days which have just preceded any wet day and the amount of precipitated water are independent. This result, which is likely to be valid for the Po Valley, implies that the hydric precipitation considered as a joint process can be factorized into two independent processes. The first concerns the probability that the particular day considered is dry or wet; the second concerns the probability of a certain amount of precipitated water in that wet day. The probabilities of each process have been often studied in the literature, whereas studies on the joint process are absent. Paper presented at the 1° Congresso del Gruppo Nazionale di Fisica dell' Atmosfera e dell'Oceano, June 19–22, 1984, Rome.