全文获取类型
收费全文 | 88134篇 |
免费 | 1021篇 |
国内免费 | 1473篇 |
专业分类
测绘学 | 2941篇 |
大气科学 | 6444篇 |
地球物理 | 16315篇 |
地质学 | 35180篇 |
海洋学 | 6842篇 |
天文学 | 16661篇 |
综合类 | 2320篇 |
自然地理 | 3925篇 |
出版年
2022年 | 366篇 |
2021年 | 625篇 |
2020年 | 660篇 |
2019年 | 710篇 |
2018年 | 7399篇 |
2017年 | 6603篇 |
2016年 | 5030篇 |
2015年 | 1232篇 |
2014年 | 1834篇 |
2013年 | 2895篇 |
2012年 | 3312篇 |
2011年 | 6268篇 |
2010年 | 5356篇 |
2009年 | 6196篇 |
2008年 | 5144篇 |
2007年 | 6080篇 |
2006年 | 2277篇 |
2005年 | 2177篇 |
2004年 | 2256篇 |
2003年 | 2193篇 |
2002年 | 1874篇 |
2001年 | 1360篇 |
2000年 | 1293篇 |
1999年 | 1011篇 |
1998年 | 1062篇 |
1997年 | 949篇 |
1996年 | 816篇 |
1995年 | 783篇 |
1994年 | 690篇 |
1993年 | 603篇 |
1992年 | 591篇 |
1991年 | 596篇 |
1990年 | 623篇 |
1989年 | 495篇 |
1988年 | 506篇 |
1987年 | 531篇 |
1986年 | 487篇 |
1985年 | 612篇 |
1984年 | 673篇 |
1983年 | 595篇 |
1982年 | 565篇 |
1981年 | 528篇 |
1980年 | 492篇 |
1979年 | 484篇 |
1978年 | 462篇 |
1977年 | 371篇 |
1976年 | 352篇 |
1975年 | 359篇 |
1974年 | 310篇 |
1973年 | 342篇 |
排序方式: 共有10000条查询结果,搜索用时 318 毫秒
701.
A multipole expansion of the velocity potential is described for two- and three-dimensional wave diffraction and radiation problems. The velocity potential is expressed in terms of a series of multipole potentials. The wave terms and the local disturbance terms are represented by separated multipole potentials. Floating bodies and submerged bodies are treated in the same way. This approach differs from that of some other authors, who considered floating bodies and submerged bodies separately and derived entirely different multipoles. Semi-analytical solutions for a circular cylinder in two-dimensional motions are given. It is found that the local disturbance decays rapidly and steadily. The general application of the multipole expansion to arbitrary geometries is also presented, based on a method coupling multipoles to a boundary integral expression. Numerical results for several floating and submerged cylinders are presented. 相似文献
702.
Noriaki Kimura 《Journal of Oceanography》2007,63(4):685-694
This paper examines the mechanism controlling the short time-scale variation of sea ice cover over the Southern Ocean. Sea
ice concentration and ice velocity datasets derived from images of the Defense Meteorological Satellite Program (DMSP) Special
Sensor Microwave Imager (SSM/I) are employed to reveal this mechanism. The contribution of both dynamic and thermodynamic
processes to the change in ice edge location is examined by comparing the meridional velocity of ice edge displacement and
sea ice drift. In the winter expansion phase, the thermodynamic process of new ice production off the ice edge plays an important
role in daily advances of ice cover, whereas daily retreats are mostly due to southward ice drift. On the other hand, both
advance and retreat of ice edges in the spring contraction phase are mostly caused by the dynamic process of the ice drift.
Based on the above mechanism and the linear relation between the degree of ice production at the ice edge and northward wind
speed, the seasonal advance of ice cover can be roughly reproduced using the meridional velocity of ice drift at the ice edge. 相似文献
703.
Two distinct series of slumps deform the upper part of the sedimentary sequence along the continental margin of the Levant.
One series is found along the base of the continental slope, where it overlies the disrupted eastern edge of the Messinian
evaporites. The second series of slumps transects the continental margin from the shelf break to the Levant Basin. It seemed
that the two series were triggered by two unrelated, though contemporaneous, processes. The shore-parallel slumps were initiated
by basinwards flow of the Messinian salt, that carried along the overlying Plio-Quaternary sediments. Seawater that percolated
along the detachment faults dissolved the underlying salt to form distinctly disrupted structures. The slope-normal slumps
are located on top of large canyons that cut into the pre-Messinian sedimentary rocks. A layer of salt is found in the canyons,
and the Plio-Quaternary sediments were deposited on that layer. The slumps are bounded by large, NW-trending faults where
post-Messinian faulted offset was measured. We presume that the flow of the salt in the canyons also drives the slope-normal
slumps. Thus thin-skinned halokynetic processes generated the composite post-Tortonian structural patterns of the Levant margin.
The Phoenician Structures are a prime example of the collapse of a distal continental margin due to the dissolution of a massive
salt layer. 相似文献
704.
Estimates of the characteristics of the horizontal turbulent exchange (coefficients of horizontal exchange, scales of deviations of the velocity of geostrophic currents from the background flows, and horizontal scales of perturbations) in the upper ocean layer were obtained on the basis of the data of expeditions held in 1978 and 1980 in the northwestern part of the Pacific Ocean. It is shown that the characteristics of the horizontal turbulent exchange strongly depend on the structure of the background flows, and one of the causes of their variability is related to tropical cyclones. A tendency to a decrease (to different degrees) of the turbulent coefficients in the wake of a tropical cyclone is noted. 相似文献
705.
N. P. Romanov A. S. Drofa N. S. Kim A. V. Savchenko G. F. Yaskevich 《Izvestiya Atmospheric and Oceanic Physics》2006,42(1):74-83
From analysis of the relationships between the equilibrium state of condensation nuclei and the relative humidity, a conclusion
is made concerning the preferred use of poorly soluble substances for the generation of artificial condensation nuclei (ACN)
designed for modification of warm clouds and fogs with the purpose of their dissipation and precipitation formation. The advantage
of poorly soluble substances over commonly used soluble salts is that the finely dispersed part of the spectrum of the poorly
soluble ACN does not deliquesce and so is not involved in the formation of cloud droplets. For experimental testing of the
conclusions, preference was given to cement, whose main soluble substance is calcium oxide with a solubility of about 1 g/l.
The spectrum of dispersed cement particles was measured and compared with the spectra of pyrotechnic flares widely used for
modification at present. The process of formation of the cloud droplet spectra was simulated in the aerosol chamber of the
Institute of Experimental Meteorology by decreasing the preliminarily generated excess pressure. It was found in these experiments
that, compared to the spectra of particles formed on background condensation nuclei, the introduction of dispersed cement
leads to the broadening of spectra and to a decrease in the concentration of droplets. Even at the early stage of condensation,
droplets with radii of ∼20 μm appear. In this case, no “overseeding” phenomenon is observed, which, for soluble substances,
manifests itself in an increase in the concentration of cloud droplets with a large ACN concentration. These effects indicate
that, according to the existing concepts about the mechanism of warm-cloud modification with hygroscopic substances, the introduction
of poorly soluble ACN (in particular, dispersed cement) below the base of cumulus clouds should stimulate coagulation processes
and accelerate rain-formation processes. Considerations are given that the introduction of poorly soluble ACN into the already
existing cumulus or stratocumulus clouds or fogs should also result in the acceleration of precipitation-formation processes
or cloud dispersal. Comparison of cement powder with the well-known means of warm-cloud and fog modification is carried out.
Original Russian Text ? N.P. Romanov, A.S. Drofa, N.S. Kim, A.V. Savchenko, G.F. Yaskevich, 2006, published in Izvestiya AN.
Fizika Atmosfery i Okeana, 2006, Vol. 42, No. 1, pp. 80–91. 相似文献
706.
Wave forces on a vertical truncated circular cylinder in Stokes waves with the wave slopes ranging from 0.06 to 0.24, are measured in a wave tank. The higher harmonic wave forces are compared with the available values from theories of the FNV (Faltisen–Newman–Vinje) model and Varyani solution. The first harmonic horizontal forces measured are much larger than the theoretical values from the FNV model, while the first harmonic vertical forces are well predicted by the Varyani theory. It was also found that the FNV model significantly overpredicts the second harmonic horizontal forces in high frequency waves, but under predicts the third harmonic forces. The differences between the actual measurement and the theory, in the second and third harmonic horizontal forces, become smaller at low wave frequencies as the wave slope increases. In addition, the transverse instabilities in the incoming waves with high wave slope were observed, which is due to the nonlinear modulation. Measurements were, thus, carried out before the instability occurred. 相似文献
707.
The development of the strongest El Niño event on record in the equatorial Pacific in 1997–1998 and the rapid transition to strong La Niña conditions in 1998–1999 had a large impact on the physical and biological environment of the West Coast. We investigate the evolution of the physical structure and circulation dynamics of the southern California Current System (CCS) during this period based on hydrographic data collected on 25 cruises over a 45-month period (February 1996–October 1999). The El Niño period was characterized by a significant increase in dynamic height, extreme water mass characteristics, a strengthening and broadening of the poleward nearshore flow, and a temporary reversal of net alongshore transport. By early 1999, conditions in the CCS had reversed. The data suggest that remotely driven forcing (propagating oceanic waves) contributed to the anomalies observed during the El Niño period, while the cool-water conditions of 1999 were most likely a result of anomalous local atmospheric forcing. 相似文献
708.
709.
Controls on organic carbon distribution in sediments from the eastern Arabian Sea Margin 总被引:2,自引:0,他引:2
Sediment cores from the upper continental slope of the eastern Arabian Sea have high organic carbon (OC), CaCO3, and sand content at the top. The values decrease with increasing depth in the Holocene and Upper Pleistocene. Topographic
highs show highest OC and lower CaCO3 in the Holocene clayey sediments and vice versa in the Pleistocene sandy sediments. The OC is immature and marine or a mixture
of both marine and terrestrial in the Holocene sediments and is mostly terrestrial and/or reworked marine in the Pleistocene
sediments. Productivity is the main controlling factor for the organic carbon enrichment. Texture and reworking also influence
the organic carbon variations.
Received: 29 May 1996/Revision received: 10 January 1997 相似文献
710.