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61.
Symplectitic intergrowths of hypersthene (host) with ilmenite and minor magnetite (vermicules) in a gabbroic sill from the Precambrian of southwest Sweden occur as replacement products of olivine, and are thought to have formed simulataneously with the replacement of nearby crystals of ilmenite and Timagnetite by biotite and hornblende (and spinel). These interrelated replacement processes may have taken place during, or immediately after, the final stages of the magmatic crystallization, at temperatures of about 660–680°C, as part of the inherent metamorphism of the gabbroic rock. Another expression of this metamorphic imprint is the occurrence of two-tiered corona shells of hypersthene/hornblende (+spinel) at the interface of olivine and plagioclase crystals. 相似文献
62.
P. B. Storebø T. Bjorvatten K. Hønn»shagen A. Lillegraven C. D. Jones C. J. P. Van Buijtenen 《Boundary-Layer Meteorology》1983,26(2):127-139
Unipolar air ions released into the wind constitute a tracer which can be measured with high resolution. An ion source produces a cloud with homogeneous charge density, insensitive to source strength, dependent on time since formation only. It is well suited for tracing concentration changes due to turbulence, less suited for cloud size tracing. A tight array of 8 sensors has been used to examine turbulently dispersed ions. High-resolution records are presented and discussed. The highest concentrations measured could be undiluted source material. The frequency distribution within a plume did not differ from that in a multitude of puffs. The distribution seems to be log-normal with a geometric standard deviation of about 2.45. The time resolution used corresponds to volume resolutions of 40, 225, and 650 cm3. Sample size had no apparent effect. 相似文献
63.
Low frequency electrostatic waves are studied in magnetized plasmas with an electron temperature which varies with position in a direction perpendicular to the magnetic field. For wave frequencies below the ion cyclotron frequency, the waves need not follow any definite dispersion relation. Instead a band of phase velocities is allowed, with a range of variation depending on the maximum and minimum values of the electron temperature. Simple model equations are obtained for the general case which can be solved to give the spatial variation of a harmonically time varying potential. A simple analytical model for the phenomenon is presented and the results are supported by numerical simulations carried out in a 2.5-dimensional particle-in-cell numerical simulation. We find that when the electron temperature is striated along B0 and low frequency waves (ci) are excited in this environment, then the intensity of these low frequency waves will be striated in a manner following the electron temperature striations. High frequency ion acoustic waves (ci) will on the other hand have a spatially more uniform intensity distribution. 相似文献
64.
Poul Klenz Larsen 《Environmental Geology》2007,52(2):375-383
This paper presents an investigation of the decay of bricks in the chancel vault of Brarup Church located close to the Baltic
Sea at the island of Falster in Denmark. The aim of the work was to study a peculiar phenomenon in order to prescribe appropriate
treatment. Although protected by a lime plaster, some bricks were pulverized up to 50 mm deep from the topside. The decay
occurred in a random pattern over the structure, with undamaged bricks positioned next to deteriorated bricks. The brick structure
was investigated by Mercury Intrusion Porosiometry. All bricks had a majority of pores in the range 1–10 μm, but the decayed
bricks also had a fraction of pores with a diameter less than 100 nm. The difference may be due to a lower firing temperature,
or it may be a consequence of the salt decay. Salt analysis proved that the deteriorated bricks were contaminated with sodium
chloride, which could have degraded the brick structure. However, the powdering of the bricks may be caused by the precipitation
of gypsum, which was found in the brick powder but not in the deteriorated brick itself. According to calculations with the
computer program RUNSALT, the precipitation of gypsum is much influenced by the presence of sodium chloride. The gypsum precipitates
due to changes in temperature between 0 and 30°C and variations in relative humidity between 75 and 95%. Seasonal climatic
changes in the attic were measured to cover this interval. Perhaps sodium chloride acted as a catalyst that facilitated the
accumulation and precipitation of gypsum at the topside of the bricks. A sacrificial plaster, which is the traditional treatment
for salt contaminated structures, would not prevent such decay. 相似文献
65.
Y.-H. Sung C. L. Ciobanu A. Pring J. Brügger W. Skinner N. J. Cook M. Nugus 《Mineralogy and Petrology》2007,90(3-4):249-270
Summary The Cu–Fe–Au–Mo (W) deposits in southeastern Hubei are an important component of the Middle–Lower Yangtze River metallogenic
belt. Molybdenite from the Fengshandong Cu- (Mo), Ruanjiawan W–Cu- (Mo), Qianjiawan Cu–Au, Tongshankou Cu–Mo and Tonglüshan
Cu- (Fe) deposits yielded Re–Os ages of 144.0 ± 2.1 Ma, 143.6 ± 1.7 Ma, 137.7 ± 1.7 Ma, 142.3 ± 1.8–143.7 ± 1.8 Ma and 137.8
± 1.7–138.1 ± 1.8 Ma, respectively. Phlogopite from the Tieshan Fe- (Cu) deposit yielded an Ar–Ar age of 140.9 ± 1.2 Ma. These
data and other published isotopic ages (Re–Os molybdenite and Ar–Ar mica ages) for the Cu–Fe–Au–Mo (W) deposits in the Middle–Lower
Yangtze River metallogenic belt show that Cu–Fe–Au–Mo (W) mineralisation in the Tongling, Anqing, Jiurui and Edong ore districts
developed in a narrow time span between 135.5 and 144.9 Ma, reflecting an important regional metallogenic event. An integrated
study of available petrological and geochronological data, together with relationships to magmatism and the regional geodynamic
framework, indicate that the Cu–Fe–Au–Mo (W) mineralisation in the Middle–Lower Yangtze River belt occurred during a regime
of lithospheric extension. This extension is probably related to Late Mesozoic processes of lower crustal delamination and
lithospheric thinning in East China. 相似文献
66.
The regional ocean model system (ROMS) is used to downscale a 26-year period of the twentieth century 20C3M experiment from
the global coupled Bergen climate model (BCM) for the North Sea. Compared to an observational-based climatology, BCM have
good results on the mean temperature, except for too low winter temperature. This is connected to a too weak inflow of Atlantic
water. The downscaling gives added value to the BCM results by providing regional details, doubling the Atlantic inflow, and
improving the mean winter temperature. For mean salinity, BCM has values very close to the climatology, whereas the downscaling
becomes too fresh. The downscaling, however, improves the sea surface salinity, the vertical structure, and the Norwegian
Coastal Current. It is concluded that the downscaling procedure as presented here is a suitable tool for assessing the future
Atlantic inflow and sea temperature in the North Sea based on a global climate projection. 相似文献
67.
Y. Shao M. Sogalla M. Kerschgens W. Brücher 《Meteorology and Atmospheric Physics》2001,78(3-4):157-181
Summary
In this paper, we examine the effects of land-surface heterogeneity on the calculation of surface-energy and momentum fluxes
in a meso-scale atmospheric model. A series of numerical experiments has been carried out with a combination of different
resolutions for the atmosphere and the land surface, which allows an examination of the aggregation and dynamic effects associated
with land-surface heterogeneity. The numerical results show that for a given atmospheric model resolution, increased land-surface
resolution leads to better estimates of surface-energy and momentum fluxes, and for a given land-surface resolution, increased
atmospheric model resolution also improves the estimates of these fluxes. This latter result contradicts the prevailing view
that subgrid variation in atmospheric data plays only a minor role in estimating the fluxes. It is also shown that subgrid
land-surface heterogeneity leads to increased turbulent fluctuations. The responsible mechanisms of this effect are both the
subgrid variation of surface-energy fluxes and their impact upon the development of convective cells. It is suggested that
subgrid atmospheric motions induced by surface heterogeneity may be an important factor which needs to be considered in subgrid
closure schemes for atmospheric models.
Received August 28, 2000/Revised June 1, 2001 相似文献
68.
Four thousand years of atmospheric lead pollution in northern Europe: a summary from Swedish lake sediments 总被引:8,自引:7,他引:8
This paper presents a large palaeolimnological study of the pre-industrial and industrial history of atmospheric lead pollution deposition in Sweden. Both lead concentrations and 206Pb/207Pb ratios have been analysed in 31 lakes covering most of Sweden, plus one lake in north-west Russia. Four of the lakes have varved (annually-laminated) sediments. Isotope analysis is a sensitive and effective method to distinguish pollution lead from natural catchment lead and to detect early pollution influence, because the 206Pb/207Pb ratio in unpolluted background sediments in Sweden was > 1.3, while that of lead from pollution, derived from ores and coal, was < 1.2. The sediments show a consistent picture of past temporal changes in atmospheric lead pollution. These changes include: the first traces of pollution 3,500-3,000 yrs ago; a pollution peak in Greek-Roman Times (about 0 AD); lower lead fall-out between 400 and 900 AD; a significant and permanent increase in atmospheric lead fall-out from about 1000 AD; an increase with the Industrial revolution; a major increase following World War II; the maximum peak in the 1970s; and decreasing fall-out over the last decades. The four varved sediments provide high-resolution records of atmospheric pollution. They reveal pollution peaks about 1200 and 1530 AD which match the history of metal production in Europe. According to the varve records the lead pollution level in the late 1990s had decreased beneath the level of the 1530s. The pollution level 1200 AD was about 35% of the 1980s, when lead pollution was still near its all time high. About 50% of the total accumulated atmospheric lead pollution deposition through time was deposited in the pre-industrial period. The sediments also show a consistent picture of the geographic distribution of atmospheric lead deposition over time, with higher deposition in south Sweden and declining levels to the north, which supports the hypothesis that the main sources of pre-industrial atmospheric lead pollution in Sweden were cultural areas in mainland Europe and Great Britain. 相似文献
69.
Surface NO and NO2 mixing ratios were measured aboard the research vessel Polarstern during the mission ANT VII/1 from 24 September to 5 October 1988. The measurements were taken along the meridian at 30° W in the Atlantic region covering latitudes between 30° N and 30° S. The average mixing ratios were about 12 pptv NO/30 pptv NO2 in the Northern Hemisphere and about 7 pptv NO/22 pptv NO2 in the Southern. Elevated mixing ratios of 20 pptv NO/70 pptv NO2 were found at 12° N (probably due to air masses originating from the surface of West Africa) and in the region of the ITCZ between 8° N and 5° N. Because of probable contamination by the ship, the measured mixing ratios mostly represent upper limits. 相似文献
70.
Summary During the field experiment FRONTEX 1989 a shallow cold front with a fog field behind its leading edge was observed over the North Sea. The fog field was about 70 km wide, 160 m deep at the front side and 400 m deep at the rear side and was capped by a strong frontal inversion. With increasing height of the inversion the fog broke up into cumulus clouds. The horizontal temperature gradient was largest at 500 m with 9 K/100 km. The observed segment of the front was situated near the col of a geostrophic deformation field with vanishing cross-front wind but non-vanishing cross-front confluence. The front moved at a rate of 5 m/s. It exhibits characteristics of a gravity current which propagates into a stably stratified environment.The physical processes leading to the observed frontal structure and motion are elucidated by experiments with a two-dimensional numerical model. The charateristics of the observed front can be reproduced if both, geostrophic forcing and boundary-layer processes, such as air-sea temperature difference, moisture content and cloud radiational cooling are taken into account. The development of the front under altered conditions is studied by the comparison of different simulations. The reason for the movement of the front is discussed refering to the mechanisms of geostrophic forcing and the forcing by surface friction. The analysis of the forces acting on the cold air mass shows that the Coriolis force is important at some distance behind the frontal head, but has little effect on the processes at the frontal head.With 13 Figures 相似文献