Particle number size distributions in the diameter range of 5 nm–34 µm were measured in tropical southern India during the transition season from May (pre-monsoon) to June–July (monsoon). Averaged over the entire measurement period, the particle number size distributions were bimodal with a total particle number concentration of NTot = (4.5 ± 2.7) × 103 cm−3 (arithmetic mean ± standard deviation). The ratio of Aitken to accumulation mode particle number concentrations (NAit/NAcc) ranged from 1.06 to 2.07, increasing from May to June–July due to a pronounced decrease in the accumulation mode particles with the onset of the monsoon. Cloud condensation nuclei concentrations at 0.4% supersaturation (CCN0.4) were calculated under the assumption of an average continental hygroscopicity parameter of κ = 0.3. The calculated CCN0.4 concentrations are mostly sensitive to variations in NAcc and vary accordingly between the May and June–July. Using a cloud parcel model, we investigated the formation of cloud droplets using the measured average particle number size distributions, updraft velocities up to 20 m s−1, and initial aerosol particle number concentrations up to 25,000 cm−3. We found different regimes of CCN activation and cloud formation for the month of May (shifting from aerosol-limited regime to transitional regime) and for June–July (transitional regime), which was primarily due to the variability of particle number concentrations in the accumulation mode. © 2020, Springer Nature Switzerland AG.