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, the characteristics of the tropical cirrus clouds observed over Gadanki (13.5°N; 79.2°E) are presented in this paper. Out of 210 nights of observations during this period, cirrus clouds were observed over the lidar site on 170 nights. The cloud mean height is found to be in the range of 8-17 km with peak occurrence at 13-14 km, just below the tropopause. The cloud thickness has values ranging 0.6-4.2 km with maximum occurrence at 0.9-1.2 km. The scattering ratio and the linear depolarization ratio are in the ranges of 1.14-36 and 0.01-0.75 with peak occurrences seen at 1.14-2 and 0.01-0.05, respectively. The optical depth can be as high as 2, but values less than 0.1 account for most (>80%) of the clouds. The clouds with low optical depth occur most frequently during fall equinox and high optical depth during summer. The formations of thin and thick cirrus clouds are seen to be closely related to the minimum tropospheric temperature and cumulonimbus outflows, respectively.

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SUMMARYUsing altitude pro les of temperature in the range 4 to 70 km derived from Mesosphere-StratosphereTroposphere radar and lidar observations at Gadanki (13.5 ± N, 79.2 ± E) from 18 January 1999 to 5 March 1999, characteristics of equatorial waves are studied. Two-dimensional Fourier-transform analysis of the temperature pro les is carried out to identify the periodicities and their vertical wave numbers. From the characteristics obtained, equatorial slow Kelvin waves with periodicities 15.7 d, 9.4 d, 7.8 d and 6.7 d are identi ed in the troposphere and stratosphere regions and among these 7.8 d and 6.7 d periodicities are found to penetrate into the mesosphere. Equatorial waves with smaller periodicities in the range 5.2 d to 3.6 d are also observed. The vertical ux of horizontal momentum (zonal) of the identi ed slow Kelvin-wave periodicities in the altitude region 4-25 km is estimated. It is found that equatorial waves modulate tropical tropopause temperature and altitude.