Spatio-temporal variations of factors affecting the occurrence of precipitation can lead to a change in its amount. The atmospheric moisture is one of the most important factors for precipitation formation. In this study, changes in atmospheric moisture and its relation with occurrence of seasonal wet and dry periods were investigated in Iran. The re-analysis data from the ERA interim European Center for Mid-Term Projections (ECMWF) was used during the period 1981-2011. The z index (ZSI) was used to extract wet and dry periods of autumn, winter and spring seasons. The seasons with the maximum percentage of wetness/drought occurrence during the above periods were selected. Vertical integrated divergence of the moisture flux was extracted in three layers of the lower, middle and upper atmosphere above Iran. The results revealed that in all three layers, moisture flux was maximum during wet period and decreased in dry one. In all layers in wet and dry periods, the moisture content imported to Iran increased during warm season as compared to cold seasons. In addition, the difference in moisture content in the warm season was less than cold seasons and has less variations. There was no significant changes in moisture at high levels in three seasons. In general, there was a significant difference in terms of the winds pattern in the wet and dry periods. The favorable conditions of flow patterns on the water surface of the region provide the condition for increasing transport of moisture to Iran. Although, the moisture transfer reduced due to deviation and change of direction of currents, in dry period especially in the lower layer, and hence increase the occurrence of dry periods in Iran.

In this research, coding the rainfalls, prepares daily 45 stations with the statistical period of 20 years to zero and one codes to realize the daily dry periods in west and north west of the country and then, by establishing the main condition of occurrence of code one for 30 stations, we extracted the dry 4 to 10-day frequencies. And the results gained of considering the atmospheric weathering, shows that the most clear rotational pattern in sea level is related to Siberia-Europe high-pressure panels and sometimes both of them that increase the rotation on the region and also, the local high-pressure reinforcement and there is a high altitude in atmospheric middle level which is derived from sample patterns and is placed on the studied region and these sample patterns are from omega, bi-polar and rex models that they are on Russia and Scandinavian countries with some changes. The 500 level TAVA and omega shows well that in most regions, the air course, has decreasing case and so, we can result that placement of a high altitude in atmospheric middle and upper level on the high-pressures of the earth, causes the weather stability and lack of rainfall and as a result, the stability and durability of these conditions for several days, is related to sample patterns.