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A simulated climatology of ducts in the Persian Gulf area was produced with the MM3 atmospheric model. From November to January ducts were sporadic, land and surface based, shallow and weak. From February to October ducts of all types occurred. In the duct season, spatial and temporal variations were related to the land/sea distribution and to day and night. Over land at night, widespread, shallow, weak surface ducts occurred well away from the sea; within about 100 km of the south-western coast in the late evening, ducts were S-shaped. Over land in daytime, the dry, convective boundary layer prevented duct formation. Over the Gulf in the season, duct coverage was complete throughout night and day. A spatial sequence of shallow, weak surface ducts, deeper, stronger S-shaped ducts and deep, strong elevated ducts lay from north-west to south-east over the Gulf. This sequence was related to the growth of a marine internal boundary layer (MIBL) and the effects of land- and sea-breeze circulations. Subsidence in the sea-breeze circulation reduced magnitudes of depth and strength and created gradients in a direction normal to the main growth axis of the MIBL. Ducts growing in the MIBL were tilted upward from west to east. The combined effect gave relatively weak surface ducts in the north-west and strong elevated ducts in the south-east. Duct depth and strength increased as the season progressed, owing to increased wind speed within, and increased depth of, the MIBL. [ABSTRACT FROM AUTHOR]