A study of the convectively driven monsoon boundary layer has been carried out using the aerological observations collected during the Monsoon Trough Boundary Layer Experiment - 1990. The observations have been obtained over the monsoon trough region for the stations Ranchi, Bhubaneswar and Calcutta during the break conditions of monsoon ( 26 July - 1 August 1990). The analysis technique includes Saturation point concept, mixing line analysis and conserved variable diagrams. The conserved variable plots for the stations Ranchi, Bhubaneswar and Calcutta revealed that the Convective Boundary Layer during the monsoon season is represented by a single mixing line structure with variation in stability. However, the convective boundary layers observed at Ranchi were found to be more saturated as compared to those at Bhubaneswar and Calcutta during the break monsoon conditions. The results of the study are discussed in paper with respect to the position of the monsoon trough