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61.
au a u naam u a nu¶rt; 1963–1973
. naam, m aum mun ma m mm nam aum n (II) na¶rt;am m u a uu ¶rt; u u,¶rt; ua ma u¶rt;, u u¶rt;a ma mn muna. mu u m ¶rt;u mam nm nmum n¶rt;auma amu m m mm II u a¶rt; ¶rt; n. 相似文献
62.
63.
Catches of perch have strongly decreased in Lake of Geneva from 1975 to 1981. This decrease was attributed to the impact of
overfishing, enhanced by the accelerated growth of perch. Therefore, too many perches were captured before their first reproduction
and the stocks of reproducers were strongly reduced. Furthermore, adverse meteorological conditions occurred probably during
several periods of reproduction. The accelerated growth of perch was attributed to the combined effects of increasing eutrophication
(more food available) and of overfishing (decreased perch density in the shoals).
相似文献
64.
Barbara Barreiro 《Geochimica et cosmochimica acta》1983,47(4):817-822
Pb isotope ratios have been measured in 12 volcanic rocks from the South Sandwich Islands. The ranges are . In and correlation diagrams, the South Sandwich data plot distinctly above the fields for ocean ridge basalts, and yield trends showing apparent mixing with a sedimentary end member similar to South Atlantic pelagic sediments as reported by Chow and Patterson (1962) and this study. Armstrong and Cooper (1971) have likewise shown that volcanics from the Lesser Antilles show mixing trends with North Atlantic sediments in Pb isotope correlation diagrams. The North Atlantic sediments have distinctly higher and ratios compared to the South Atlantic sediments. The parallel relationships between sediments and volcanic island arc rocks of the North and South Atlantic provide strong evidence for a component of Pb from subducted sediments in the lavas of the west Atlantic basin. In contrast to these data, lavas from the Mariana Arc in the western Pacific show little or no component of Pb from pelagic sediments. The reason for the different behaviors in the two settings is speculative. 相似文献
65.
Ice core evidence for secular variability and 200-year dipolar oscillations in atmospheric circulation over East Antarctica during the Holocene 总被引:3,自引:0,他引:3
Barbara Delmonte Jean Robert Petit Gerhard Krinner Valter Maggi Jean Jouzel Roberto Udisti 《Climate Dynamics》2005,24(6):641-654
Two Holocene ice core records from East Antarctica (Vostok and EPICA-Dome C) were analysed for dust concentration and size
distribution at a temporal resolution of 1 sample per ~50 years. A series of volcanic markers randomly distributed over the
common part of the ice cores (from 9.8 to 3.5 kyear BP) ensures accurate relative dating (±33 years). Dust-size records from
the two sites display oscillations structured in cycles with sub-millennial and secular scale frequencies that are apparently
asynchronous. The power spectra of the composite sum (Σ) of the two dust-size records display spectral energy mostly for 150-
to 500-year periodicities. On the other hand, the 200-year band is common to both records and the 200 year components of the
two sites are out-of-phase (100-year lead or lag) over ~5.5 kyear, a phenomenon also reflected by a significant (>99% conf.
lev.) band in the power spectra of the composite difference (Δ) of the two size records. During long-range transport, mineral
dust originating from the Southern Hemisphere continents is graded to a variable extent depending on the altitude and duration
of atmospheric transport. Relatively coarse dust is associated with air mass penetration from the middle–lower troposphere
and conversely relatively fine dust with upper troposphere air masses or the influence of subsidence over the Antarctic plateau,
a hypothesis already proposed for the changes that occurred during the Last Glacial Maximum to Holocene transition (Delmonte
et al. 2004b). Moreover, we assume that the overall fluctuation of air mass advection over Antarctica depends on the meridional pressure
gradient with respect to low latitudes, i.e. the Antarctic Oscillation (AAO). We therefore suggest a regional variability
in atmospheric circulation over East Antarctica. The 150–500 year power spectrum of the composite (Σ) parameter represents
the long term variability of the AAO, imprinted by secular internal oscillations probably related to the southern ocean-climatic
system. On the other hand, the Δ dust composite parameter suggests a persistent atmospheric dipole over East Antarctica delivering
coarser (finer) dust particles alternatively to Vostok and Dome C regions with a bi-centennial periodicity. Indeed, a seesaw
phenomenon in dust size distribution was already observed at three East Antarctic sites during the last deglaciation (Delmonte
et al. 2004b) and was interpreted as a progressive reduction of the eccentricity of the polar vortex with respect to the geographic south
pole. Interestingly, the Δ parameter shows a pronounced 200-year oscillation mode, throwing new light on the unresolved question
of a possible relationship between climate and solar activity. 相似文献
66.
Improving the accuracy of tipping-bucket rain records using disaggregation techniques 总被引:1,自引:0,他引:1
We present a methodology able to infer the influence of rainfall measurement errors on the reliability of extreme rainfall statistics. We especially focus on systematic mechanical errors affecting the most popular rain intensity measurement instrument, namely the tipping-bucket rain-gauge (TBR). Such uncertainty strongly depends on the measured rainfall intensity (RI) with systematic underestimation of high RIs, leading to a biased estimation of extreme rain rates statistics. Furthermore, since intense rain-rates are usually recorded over short intervals in time, any possible correction strongly depends on the time resolution of the recorded data sets. We propose a simple procedure for the correction of low resolution data series after disaggregation at a suitable scale, so that the assessment of the influence of systematic errors on rainfall statistics become possible. The disaggregation procedure is applied to a 40-year long rain-depth dataset recorded at hourly resolution by using the IRP (Iterated Random Pulse) algorithm. A set of extreme statistics, commonly used in urban hydrology practice, have been extracted from simulated data and compared with the ones obtained after direct correction of a 12-year high resolution (1 min) RI series. In particular, the depth–duration–frequency curves derived from the original and corrected data sets have been compared in order to quantify the impact of non-corrected rain intensity measurements on design rainfall and the related statistical parameters. Preliminary results suggest that the IRP model, due to its skill in reproducing extreme rainfall intensities at fine resolution in time, is well suited in supporting rainfall intensity correction techniques. 相似文献
67.
68.
Summary Some errors of method occurring in A. C. measurements of the electrical conductivity of rocks are discussed. It is demonstrated that the difference between A.C. and D.C. conductivities, at given frequency, depends mostly on the magnitude of the D.C. conductivity and magnitude of the dielectric constant. 相似文献
69.
Paleorainfall Reconstructions from Pedogenic Magnetic Susceptibility Variations in the Chinese Loess and Paleosols 总被引:2,自引:0,他引:2
The rock magnetic properties of the Chinese loess and paleosols constitute a unique and sensitive record of East Asian paleoclimate through the Quaternary Period. Systematic variations in the concentration and grain size of the magnetic minerals in these sediments have produced systematic variations in the magnetic susceptibility signal, which can be easily and rapidly measured at many sites across the Loess Plateau. Variations in many other rock magnetic properties can be used to identify the key shifts in ferrimagnetic grain size, but magnetic susceptibility alone is sufficiently sensitive to record stadial and interstadial climate stages, as well as glaciations and interglaciations. Past changes in rainfall and monsoon activity for this region are reconstructed from the susceptibility variations. The susceptibility record is calibrated using the modern relationship between rainfall and pedogenic susceptibility on the Loess Plateau. Our rainfall reconstructions identify enhanced summer monsoonal activity in the Chinese Loess Plateau region in the early Holocene and the last interglaciation. In the presently semiarid western area of the plateau, annual precipitation in interglacial times was up to 80% higher than at present; in the more humid southern and eastern areas, values were up to 20% higher than today's levels. During the last glaciation, precipitation decreased across the entire plateau, typically by 25%. The relationship between pedogenic susceptibility, climate, and weathering age was examined over the Northern Hemisphere temperate zone and the observed positive correlation between rainfall and susceptibility indicates that climate, rather than soil age, is the predominant factor that controls pedogenic susceptibility enhancement in loess soils. 相似文献
70.
Barbara A. Kennedy 《地球表面变化过程与地形》1993,18(1):84-84