• Volume 93, Issue 1

      March 1984,   pages  1-82

    • Dynamics of the stationary eddies in the lower stratospheric circulation in the wavenumber domain and monsoon activity

      K S Raja Rao S T Awade M V Harindranathan Nair

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      Meridional transports of momentum and sensible heat have been computed using the Fourier coefficients of the geopotential heights at 50 mb and 30 mb levels for each month during the years 1972 to 1979. Seasonal variations in these transports and in the energy conversionsC (KE,KZ) andC (Az,AE) in the contrasting years of monsoon activity have been discussed. The transport and the conversion terms undergo annual variations only with winter maxima and summer minima.

      Large northward transport of momentum and of sensible heat in the low latitude summer and in the high latitude preceeding winter suggests a linkage between monsoon activity and high latitude stratosphere. Seasonal variations in the energy conversion termsC (KE,KZ) andC (AZ,AE) conform with the changes in the system in the years of widely differing monsoon activity both in low and high latitudes. Large conversion of energy fromAZ toAE in low latitude summer indicates weakening of the existing meridional temperature gradient, in bad monsoon years.

    • Effect of relative humidity on the scattering properties of urban aerosol particles

      R K Goel N C Varshneya T S Verma

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      The effect of increase in relative humidity on the angular variation of scattered light intensity distribution function and on asymmetry factor has been studied theoretically for three sizes of particles which are representative of urban aerosols. The increase in relative humidity causes change in physical properties of the particles, like radius (r) and complex refractive index (n−ik). The study is based on these changes in physical characteristics.

      It has been found that there are almost negligible changes in the functioni at low humidities (≤40%) for all the three aerosol sizes studied, which may be due to insignificant changes in the physical properties. At higher values, however, the physical parameters as well as the functioni show significant changes. The changes ini are systematic for the particle very small compared to the wavelength of incident radiation but for the particles comparable or larger it is not so. The asymmetry factor changes with the increase in relative humidity, for smaller particle it increases and for two larger particles changes are of fluctuative nature.

    • On the occurrences of dry and wet sequences in the Brahmaputra valley

      S Kalita S K Sarmah

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      Occurrence of dry and wet days in the Brahmaputra Valley has been studied using a first-order Markov Chain model. The model is fitted to the daily rainfall series recorded at ten stations widely distributed in the valley. The adequacy of the model is tested and found suitable. At all the stations, dry and wet spells having different durations follow geometric distribution. For pre-monsoon and monsoon seasons, the expected dry and wet days, the expected length of a weather cycle and the return period of dry spells having different, durations are calculated, and the results for different stations are compared.

    • On the exact evaluation of displacements within a semi-infinite solid due to a buried line source

      Satyesh K Bhandari Ramesh Chander

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      Garvin’s variant of the Cagniard technique has been used to evaluate exactly the displacements at points within a homogeneous, isotropic, perfectly elastic, solid half space due to a buried explosive line source. The disturbance at any given time is confined to a region bounded by the free surface of the half space and theP wave front within it. The disturbance associated with the Rayleigh pulse affects a smaller region bounded by the free surface and the PP wave front.

    • Alpha-radioactivity-related upper age limit for thermoluminescence dating?

      K S V Nambi

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      A systematic underestimation of the age of mineralisation by the thermoluminescence (tl) technique has been observed in a variety of samples older than Quaternary although their naturaltl was not saturated. The samples included calcites, oozes, lime stones, shales, gypsum, basalts and dolerites. It is shown that thetl build-up in nature reaches a dynamic equilibrium level much ahead of the lifetime of thetl trap concerned and is solely determined by the alpha radioactivity of the sample; the validity fortl dating does not exist once such an equilibrium is reached. For the samples considered, the limiting Paleo-alpha dose fortl dating validity works out to be about 150 kilorads; beyond this dose, thetl age and the geological age bears a ratio given by$$t'/t = a\left[ {\smallint _0^t \dot D_\alpha dt} \right]^{ - b} $$ wherea andb are constants and$$\dot D_\alpha $$ is the annual alpha irradiation rate in the sample. For a suite of samples withb≠1, relative dating seems possible by thetl technique. It may be generalised that samples with 1ppm level of U, Th content cannot betl dated if they are older than about 500 kiloyears even though theirntl trap lifetimes may be 100 myrs; conversely, a 1000 myr-old sample can betl dated only if its U, Th content is much less than ppb levels and itstl trap lifetime greater than 1010 years.

    • Geochemistry and petrogenetic evolution of the diatexites of Central Kerala, India

      P K Rajan M Santosh K K Ramachandran

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      The hornblende-biotite gneisses of Central Kerala which cover approximately 490km2 exhibit schlieric and nebulitic structures, tending towards a homophanous nature and are classified here as diatexites. Mafic protoliths and restite biotite, each representing the refractory residuum of two independent partial melting episodes are widely present in the gneisses. The general mineral assemblage of the gneisses comprise quartz, K-feldspar, oligoclase, biotite and hornblende. Chemically, they are dominantly adamellitic and the behaviour of major and trace elements is consistent with a magmatic parentage. Based on petrochemical criteria, a two-stage evolution model is proposed here, which involves (i) partial melting of mafic granulites under high Archean geothermal gradients and generation of tonalite/trondhjemite through amphibole and plagioclase fractionation and (ii) partial melting and subsequent quartz-alkali feldspar fractionation of the tonalite/trondhjemite under amphibolite facies conditions with synchronous K-enrichment resulting in the diatectic adamellites.

    • A note on the petrochemistry of the Konthainhundi pluton, District Mysore, Karnataka

      S G Karkare R K Ojha R C Srivastava

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      The Konthainhundi suite represents a syn to post orogenic calc-alkalic to alkalicalcic series. The rocks were a result of the collapse of a shallow level magma chamber aided by caldera subsidence.

    • Noble gas composition in Ogi and Siena meteorites

      J T Padia C M Nautiyal M N Rao T R Venkatesan

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      The elemental and isotopic composition of noble gases in two chondritic meteorites which belong to documented “falls” is reported. Ogi, which belongs to the group of H-chondrites, fell in Saga, Kyushu (Japan) at 11am on 8 June 1741 and Siena which belongs to the group of LL-chondrites, fell in Tuscany (Italy) at 7pm on 16 June 1794. The recovered mass of Ogi and Siena were 14·2 and 4 kg respectively. Their exposure ages are 13±2 m.y. and 13±3 m.y. respectively.

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