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ABSTRACT:The Arabian Sea (AS), the Bay of Bengal (BOB) and continental recycling are three principal vapour sources identified for the rain events at Ahmedabad located in semi-arid western India. In the present study, the 48 h backward wind trajectories, at 6 h intervals, converging at an altitude of 1.5 km above the ground, for 120 daily rain events during the monsoon seasons of 2005-2008, have been examined using HYsplit Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Temperature, relative humidity, altitude and rainfall along each trajectory were studied at hourly intervals. The temperature and relative humidity data were used to compute the absolute humidity (g m −3 ) of the air parcel at its location at hourly intervals. The relationship between the height of the air parcel, its relative humidity and rainfall along each trajectory was used to identify the vapour pick-up region. The region around the maximum value of absolute humidity for each trajectory was designated as the dominant vapour pick-up region for each rain event at Ahmedabad. In this manner, principal vapour source identification was unambiguously possible for 93.3% of all the 120 rain events. The fraction of total events and the fractions of total rainfall attributable to the three principal vapour sources, respectively, are: AS, 68.3 and 62.1%; BOB, 2.5 and 1.5%; recycled, 22.5 and 24.5%; mixed, 6.7 and 11.8%. Most of the rain events with recycled vapour contribution occur during the latter part of the monsoon season, namely during September.