• Volume 96, Issue 1

      March 1987,   pages  1-79

    • Gneiss-charnockite relation around Ponmudi, southern Kerala: evidences and implications of prograde charnockite formation in southern India

      C Srikantappa G R Ravindra Kumar

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      Isochemical conversion of garnet-biotite bearing paragneiss to charnockite in the Precambrian Khondalite belt of southern Kerala is described from Ponmudi area. Petrographic evidences indicate the formation of hypersthene by the breakdown of biotite in the presence of quartz following the reaction: Biotite + quartz → hypersthene + K-feldspar + vapour. The estimated pressure — temperature conditions of metamorphism are around 5–7 kbars and 750° ± 40°C. Presence of CO2-rich, mixed CO2-H2O and H2O-rich inclusions were noticed in gneiss as well as in charnockites. Charnockites contain abundant CO2-rich inclusions.

    • Analysis of gravity gradients over a thin infinite sheet

      N Sundararajan I Arun Kumar N L Mohan

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      A simple method to interpret gravity gradients over a thin infinite dipping sheet is discussed. The Hilbert transform is used to compute the vertical gradient from the horizontal gradient of the gravity field. The method is illustrated with a theoretical example.

    • Precursory swarm and its application for long range earthquake forecasting in Taiwan region

      V P Singh J Singh

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      At trenches a few earthquake swarms of low magnitude have been observed before the medium size earthquake swarms. The first swarm was designated as precursory swarm and the second as mainshocks. Seismicity fluctuations before six such mainshocks events of medium size earthquakes of magnitudes ranging from 5.3 to 6.1 occurring in the east belt of Taiwan region have been discussed. A precursory gap between the precursory swarm and mainshock events has been observed. The duration of the gaps increases with magnitudes of the mainshocks suggesting a causal relationship between the two. Regression equations between the largest magnitude in the precursory swarms, the largest mainshock magnitude and the precusory gaps have been given.

    • Results from the Indo-USSR ozonesonde intercomparison experiment

      B H Subbaraya K S Appu K P Chatterjee A F Chizhov V D Grinchinkov A Jayaraman G A Khokin V A Kononkov I S Moshnikov V Narayanan S P Perov O V Shtrikov Y V Somayajulu C R Sreedharan K S Zalpuri

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      A total of seventeen vertical profiles of ozone were obtained during an Indo-USSR collaborative experiment on ozonesonde intercomparison conducted at Thumba during March 1983. The vertical distribution of ozone was measured using rocket-borne, balloon-borne as well as ground-based instruments. Four different rocket ozonesondes from India and USSR and the balloon ozonesonde were used to makein situ observations of ozone concentrations in addition to the Dobson spectrophotometric observations of total ozone and Umkehr. The rocket and the balloon launchings were effected in three salvos and measurements were made at different times of the day as well as during night. The results of all these measurements are used to obtain a mean ozone vertical distribution over Thumba foT the spring equinoxial period. The mean profile shows the maximum ozone concentration at 27 km with a value of (3.86±0-52)×1012 molecules per cc. Comparison of this mean profile with available satellite data for the equatorial regions shows that, in general, the Thumba values are lower by 10–15% at altitudes below 40 km and larger at altitudes above 50 km compared to the satellite results. The data also show evidence for a day-to-day variability and a possible day-to-night variability in the ozone vertical distribution with the night-time values higher than the daytime values at all altitudes above 35 km and the difference is found to increase with the increasing altitude.

    • Satellite-observed upwelled region and prime eddy off Somali coast during Monex-79

      P N Mahajan

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      An upwelled region as seen through satellite imagery off the Somali coast is compared with sea surface temperature during summer Monex-79. The relationship between satellite-derived low-level cloud drift winds and the sea-surface temperature is studied. Cloudiness associated with a prime eddy off the Somali coast is also studied. It is observed that the upwelled region has a unique crescent shape and reflects the sea-surface temperature that is driven by low-level strong winds. The prime eddy, as observed through a satellite imagery, shows that low cloud convection tends to be greater over the warm waters of the prime eddy, and the upwelled cold water tends to encircle the eddy leading to the identification of its outer boundary.

    • Some aspects of the 30–50 day oscillation

      S V Kasture R N Keshavamurty

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      The structure and interannual variability of the 30–50 day oscillation over the Indian region have been studied during the monsoon season. The power spectra of the zonal component of wind show large power in the 30–50 day time scale. The oscillation has a meridional wavelength of about 25° latitude and a slow northward phase speed of about 0.7° latitude per day. The oscillation also has some interannual variability. The periods are somewhat longer during the drought years.

    • Variability in the surface wind direction at a coastal site of complex terrain

      Y Sadhuram

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      Variability in the standard deviation of surface wind direction (σθ), under different Pasquill stability regimes on diurnal, seasonal and interannual scales has been investigated making use of a 10-year data set collected at Visakhapatnam (17°42′ N., 82° 18′ E) during January, April, August and October for winter, pre-monsoon, monsoon and post-monsoon seasons respectively. The diurnal scale variability in σθ is more pronounced during day time than in night. The seasonal variability in σθ is only moderate around noon while relatively large fluctuations are noticed on inter-annual scale only during day time in January and August. The seasonal dispersion in σθ decreased from most unstable regime to most stable regime.

    • Seasonal and diurnal variability of thermal structure in the coastal waters off Visakhapatnam

      B Prabhakara Rao V Ramesh Babu P Chandramohan

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      Seasonal and diurnal variability of thermal structure in the coastal waters off Visakhapatnam has been examined in relation to the flow field and surface winds utilizing the hourly data of temperature and currents taken at a fixed location over a tidal cycle at monthly intervals. The coastal currents in the pre-monsoon period and strong near-surface winter cooling processes affect the thermal structure of the coastal sea. Upwelling which is predominant during March to May with an intermittent relaxing event helps in the development of a strong layered thermal structure while convective mixing due to winter inversions during November to February causes weak thermal gradients in the water column.

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