The aerological data of Bombay with the new American type of radiosonde have been examined for the monsoon seasons of 1968, 1969 and 1970. Even during the peak monsoon months of July and August large changes occur in the thermal and humidity structure of the atmosphere. It is found that except during spells of good rainfall over and around the station the depth of moist air over Bombay rarely exceeds 750 mb above which there is often an inversion or layer of stable lapse rate with dry air aloft. On some occasions there is again an increase of humidity towards the 500 mb level and an inversion

or layer of stable lapse rate is also noticed near this level. These features are illustrated with tephigrams for the months of July and August 1968. Examination of upper wind flow patterns shows that the meridional component of the winds at and above 3 km tends to be northerly when the atmosphere is dry with little or no rainfall while it is southerly when the humidity content is high. Both the continental origin of the air as well as subsidence appear to be responsible for the low humidity and inversion. The study shows that it is not correct to assume that a deep moist column of air exists over peninsular India after the monsoon has established. Large space-time variations occur in the vertical distribution of humidity. Vertical transport of moisture beyond the first 2 or 3 kilometres is brought out only by low-level convergence associated with weather systems of various scales and there are compensating areas of descending motion outside the rainfall area.