In spatial and temporal terms, analysis of the relationship between relative vorticity at 500hPa and precipitation data showed that the extent of, and the strength of the correlation between, relative vorticity and the amounts of monthly precipitation and rain days, are not equal throughout the different seasons of the year. Generally, the results showed that there is a direct and significant relationship between relative vorticity and precipitation amounts. The correlation between relative vorticity and the number of rain days is stronger than the correlation between relative vorticity and precipitation amounts. Increases in the strength of the correlation between relative vorticity and precipitation data (amounts and number of rain days) and the spatial extent of the area over which this correlation is observed correspond closely to increases in the western winds observed during autumn and winter. For the warmer period of May to September, during which a gradual increase in the dominance of anticyclonicity, the subtropical high over Iran at 500hPa and subsidence are observed, there is a significant decrease in the amount of precipitation and the number of rain days. The results of this study reveal that relative vorticity over most regions of Iran decreased in March, July, October and December, and increased over most regions of Iran in November, over southwest regions in January, and over northwest regions in July.
