全文获取类型
收费全文 | 25885篇 |
免费 | 466篇 |
国内免费 | 213篇 |
专业分类
测绘学 | 520篇 |
大气科学 | 1939篇 |
地球物理 | 5200篇 |
地质学 | 9057篇 |
海洋学 | 2106篇 |
天文学 | 6011篇 |
综合类 | 47篇 |
自然地理 | 1684篇 |
出版年
2021年 | 160篇 |
2020年 | 198篇 |
2019年 | 209篇 |
2018年 | 497篇 |
2017年 | 462篇 |
2016年 | 566篇 |
2015年 | 436篇 |
2014年 | 606篇 |
2013年 | 1211篇 |
2012年 | 731篇 |
2011年 | 1015篇 |
2010年 | 875篇 |
2009年 | 1219篇 |
2008年 | 1052篇 |
2007年 | 1053篇 |
2006年 | 978篇 |
2005年 | 818篇 |
2004年 | 837篇 |
2003年 | 799篇 |
2002年 | 739篇 |
2001年 | 688篇 |
2000年 | 637篇 |
1999年 | 571篇 |
1998年 | 576篇 |
1997年 | 576篇 |
1996年 | 440篇 |
1995年 | 419篇 |
1994年 | 380篇 |
1993年 | 335篇 |
1992年 | 311篇 |
1991年 | 276篇 |
1990年 | 304篇 |
1989年 | 281篇 |
1988年 | 235篇 |
1987年 | 311篇 |
1986年 | 257篇 |
1985年 | 346篇 |
1984年 | 384篇 |
1983年 | 371篇 |
1982年 | 335篇 |
1981年 | 310篇 |
1980年 | 312篇 |
1979年 | 282篇 |
1978年 | 311篇 |
1977年 | 263篇 |
1976年 | 267篇 |
1975年 | 275篇 |
1974年 | 234篇 |
1973年 | 238篇 |
1972年 | 158篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude-latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and C2H2, as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North-South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions. 相似文献
113.
Churyumov K. I. Klesachonok V. V. Mussaev F. A. Bikmaev I. F. Galazutdinov G. H. 《Earth, Moon, and Planets》1997,78(1-3):105-110
We present the results of the preliminary study of the comet Hale-Bopp spectrum obtained April 17, 1997 by K. Churyumov and F. Mussayev with the help of the 1-meter Zeiss reflector and the echelle spectrometer (spectral resolutionλ/Δ λ ≈ 50000), CCD and the long slit, oriented along the radius-vector(“Sun-comet direction”). Energy distributions for three selected regions including the C3, C2 (0-0) and CN(Δ ν = 0) molecules emissions of the comet Hale-Bopp spectrum were built. The rotational lines of the CN(Δ ν = 0) band were identified. The nature of the high emission peak near λ 4020 Å in the C3 band is discussed. The presence of the cometary continuum of the nonsolar origin is assumed. 相似文献
114.
The supraregional GIS-supported stochastical model, WEKU, for the determination of groundwater residence times in the upper
aquifers of large groundwater provinces is presented. Using a two-dimensional analytical model of groundwater flow, groundwater
residence times are determined within two extreme cases. In the first case, maximal groundwater residence times are calculated,
representing the part of groundwater, that is drained by the main surface water of a groundwater catchment area. In the second
case, minimal groundwater residence times for drainage into the nearest surface water are determined. Using explicit distribution
functions of the input parameters, mean values as well as potential ranges of variations of the groundwater residence times
are derived. The WEKU model has been used for the determination of groundwater residence times throughout Germany. The model
results – mean values and deviations of the groundwater velocity and the maximal and minimal groundwater residence times in
the upper aquifers – are presented by general maps and discussed in detail. It is shown that the groundwater residence times
in the upper aquifer vary regionally, differentiated between less than 1 year and more than 2000 years. Using this information,
the time scales can be specified, until measures to remediate polluted groundwater resources may lead to a substantial groundwater
quality improvement in the different groundwater provinces of Germany. With respect to its supraregional scale of application,
the WEKU model may serve as a useful tool for the supraregional groundwater management on a state, federal or international
level.
Received: 15 August 1995 · Accepted: 15 October 1995 相似文献
115.
E. Tóth F. Deak C. S. Gyurkócza Z. S. Kasztovszky R. Kuczi G. Marx B. Nagy S. Oberstedt L. Sajó-Bohus C. S. Sükösd G. Toth N. Vajda 《Environmental Geology》1997,31(1-2):123-127
A steady radon exhalation is assumed in most publications. In a village of North-East Hungary, however, high radon concentrations
have been measured, differing strongly in neighbouring houses and varying in time, due to the interplay of geochemical phenomena.
Received: 20 November 1995 · Accepted: 18 June 1996 相似文献
116.
117.
118.
D. Kilkenny L. A. Crause F. van Wyk 《Monthly notices of the Royal Astronomical Society》2005,361(2):559-564
A new statement of the eigenvalue problem of studying small perturbations in arbitrary integrable self-gravitating systems is presented. An example of such a system, a 2D stellar disc, is considered in detail. The theory, based on the general equation for disc eigenmodes, reveals mechanisms for the formation and growth of global galactic structures. This new point of view specifies the limits of the unified theory of bar-like and spiral modes that was based on the assumption that global galactic structures could be understood in terms of low-frequency disc modes. 相似文献
119.
In urban environments, one major concern with deep excavations in soft clay is the potentially large ground deformations in and around the excavation. Excessive movements can damage adjacent buildings and utilities. There are many uncertainties associated with the calculation of the ultimate or serviceability performance of a braced excavation system. These include the variabilities of the loadings, geotechnical soil properties, and engineering and geometrical properties of the wall. A risk‐based approach to serviceability performance failure is necessary to incorporate systematically the uncertainties associated with the various design parameters. This paper demonstrates the use of an integrated neural network–reliability method to assess the risk of serviceability failure through the calculation of the reliability index. By first performing a series of parametric studies using the finite element method and then approximating the non‐linear limit state surface (the boundary separating the safe and ‘failure’ domains) through a neural network model, the reliability index can be determined with the aid of a spreadsheet. Two illustrative examples are presented to show how the serviceability performance for braced excavation problems can be assessed using the reliability index. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
120.
A regular perturbation analysis is presented for natural convection flow over an uniform flux vertical surface with temperature dependent viscosity. Numerical calculations are presented forP
r=6.7 which show that the first-order correction to the local temperature difference and to the local skin-fraction are negative whereas it is positive for the local Nusselt number. The effects of variable viscosity on the temperature, velocity profiles, the local temperature difference, the local Nusselt number and the local skin fraction are discussed. 相似文献