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Rain drop size distribution (DSD) measurements at different heights were made using a micro rain radar (MRR) at Thiruvananthapuram (latitude: 8.3 • N, longitude: 76.9 • E). Rain DSD data obtained from the MRR have been compared with a Joss-Waldvogel impact-type disdrometer (RD-80) deployed nearby and found to have good agreement. The analysis uses data collected during 16 continuous rainfall episodes during the southwest monsoon (June to September, JJAS) season. Since all the episodes behaved similarly, a single continuous rainfall episode occurring from 1610:01 to 1612:31 hours Indian Standard Time (IST) on 12 August 2006 is presented here. The fall velocity of those drops that contributed most to the rain rate was more or less constant at different altitudes and also with time during this episode, and the average value was 4.65 m s −1 . The rain rate (RR) was below 5 mm h −1 for all the heights throughout the time. At the beginning of the rain episode, the number of drops at any given altitude was lower for larger drops. But towards the end of the episode, the number of drops in the smallest size class had reduced at almost all heights, whereas the number of drops in the larger size classes had increased. This suggests that the larger drops coming from above on colliding with smaller drops could coalesce, thus sweeping out the smaller drops as they fall. The reduction of small drops is seen with a corresponding increase in larger drops and without increase also during the course of a rainfall event. The former is an indication of coalescence while the latter is that of evaporation. All these observed phenomena in the natural rain support the observations made by Low and List in 1982.

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This study aims to investigate the characteristic features of cloud base height (CBH) over Thiruvananthapuram during different seasons. CBH data were used for the present work derived from the Vaisala Laser Ceilometer, CL31 (VLC) installed at the campus of the Centre for Earth Science Studies, Akkulam (8.29 • N, 76.59 • E, 15 m above sea level). The VLC was in operation from the second week of July 2006 onwards. From the study, we found that CBH shows distinct diurnal and seasonal variations during all the seasons (except on rainy days). The diurnal variation for low-level clouds was different from that for the mid-level clouds. A cloud-free layer is evident in the region between 2.5 and 4 km. This cloud-free zone is more prominent during the southwest monsoon period compared to other seasons. Moreover, the monthly variations of cloud frequency and CBH were also described in addition to the different periodicities in cloud frequency. The periodicities found in the cloud frequency were 8 days and 30 days and these are significant at the 5% level. Thermodynamic parameters from the radiosonde were also related to the cloud frequency for various seasons and they were in good agreement.

S U M M A R YAtmospheric electrical conductivities have been monitored at a coastal site having a large deposit of radioactive minerals. Conductivities were also monitored at two other sites, o n e coastal and the other inland, both non-radioactive. T h e ratio of conductivity a t the radioactive site t o that a t the others is about 6, in agreement with the ratio of radioactivity. T h e diurnal variation pattern at this site in May, a pre-monsoon month, agrees with the known variation of radon concentration in air. However, in June, a monsoon month, it is different, a double oscillation similar t o that at the other sites. With t h e onset of the monsoon, conductivity at the radioactive site shows a large decrease, which is not seen a t the other sites. T h e ratio of positive polar conductivity t o negative polar conductivity shows a reduction after the onset of the monsoon rainfall. T h e diurnal variation pattern shows differences between pre-monsoon and monsoon periods at the radioactive site, while there is no change at the non-radioactive sites. The diurnal variation a t all these sites in all seasons shows a sharp decrease starting at about 0400 hrs local time and reaching a minimum at about 0800 hrs. Such a decrease has been attributed t o human activity associated with sunrise. T h e present results, however, suggest that this may be incorrect. Fourier analysis of the data shows that there exists a strong semi-diurnal component that is inversely correlated with the clearly seen semi-diurnal variation in pressure in the tropics.