The availability of chilling hours below 7°C (HF<7) and 13°C (HF<13) accumulated every 15 days up to certain date for different levels of probability (1, 10, 20, 50, 80, 90 and 99%) in 9 different locations in the central region of Rio Grande do Sul, was determined. Daily chilling hours were measured with thermographes or estimated by minimum and maximum daily temperatures and temperature at 9 pm adapting an analytical method or by multiple regression analysis. A combination of two distributions was used in order to determine the probability of occurring chilling hours. The first was a discrete distribution employed to verify whether there were chilling hours within the interval of 15 days and the other was a continuous distribution to verify the number of chilling hours when occurring. The availability of accumulated chilling hours expected for a given probability was calculated based on the density of gamma distribution. Normal distribution was used only in few cases in which the "g" parameter of the gamma function was greater than 27. It was observed that at least in one of every two years there are above 75 accumulated HF<7 up to June 30 in all locations. The availability of accumulated chilling hours from April, 1st up to September, 15th in 50% of all years varied from 200 HF<7 and 1250 HF<13 in Cachoeira do Sul to 490 HF<7 and 1850 HF<13 in Sobradinho. It was also observed that the variability of chilling hours occurrence among years is large on all locations and the relative difference among the availability of the different locations is proportionally greater at HF<7 than at HF<13. Availability of accumulated chilling hours during April is very low for HF<7, but it is considerable at HF<13. The number of chilling hours decreases drastically from the first 15 days to the last 15 days of September in all locations.
chilling hours; occurrence probability; climatic variability; climatic disponibilities
