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Based on daily data of synoptic maps on the earth surface and level of 500 hp, precipitation floods on the southern part of the Caspian Sea along Iran during the past 30 years ( from 1970 to 1999 ) was analyzed.
Results show that three kinds of air mass and pressure system affect precipitation in the south shores of Caspian Sea: Siberia high pressure, low- pressure systems and emigrant anticyclones. Between those factors, most and strangest precipitation originates from emigrant anticyclones.
Necessary condition for tacking place of density precipitation in area while provide when in the south shores of Caspian Sea in the surface of the earth settle a high pressure ridge that resulted from cold air expansion and exit a trough in the middle and upper level of atmosphere. So, higher of pressure in the surface of the earth and deeper of the trough in the upper level lead to more expectation of more intensity precipitation.
Results show that of density precipitation in north Iran on the level of 500 hp while provide when expansion and exit a ridge on the black sea, east to center of Europe, east Mediterranean Sea and the exit of deep trough on the east of black sea. This trough expansion from 70? N latitude to south-southwest in longitude between 40? to 50? N.
Key words: Caspian Sea, Precipitation of The north coasts of Iran, Synoptic maps, Iceland emigrant low pressure, Siberia high pressure