首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2011篇
  免费   67篇
  国内免费   33篇
测绘学   62篇
大气科学   194篇
地球物理   439篇
地质学   656篇
海洋学   155篇
天文学   415篇
综合类   11篇
自然地理   179篇
  2022年   23篇
  2021年   24篇
  2020年   28篇
  2019年   27篇
  2018年   54篇
  2017年   57篇
  2016年   44篇
  2015年   53篇
  2014年   59篇
  2013年   106篇
  2012年   55篇
  2011年   86篇
  2010年   87篇
  2009年   95篇
  2008年   99篇
  2007年   77篇
  2006年   75篇
  2005年   57篇
  2004年   66篇
  2003年   56篇
  2002年   70篇
  2001年   34篇
  2000年   47篇
  1999年   35篇
  1998年   37篇
  1997年   39篇
  1996年   26篇
  1995年   25篇
  1994年   20篇
  1993年   24篇
  1992年   27篇
  1991年   19篇
  1990年   19篇
  1989年   16篇
  1988年   12篇
  1987年   24篇
  1986年   20篇
  1985年   32篇
  1984年   36篇
  1983年   38篇
  1982年   41篇
  1981年   31篇
  1980年   31篇
  1979年   28篇
  1978年   10篇
  1977年   19篇
  1976年   21篇
  1975年   10篇
  1974年   19篇
  1973年   12篇
排序方式: 共有2111条查询结果,搜索用时 9 毫秒
21.
22.
A method for the prediction of ocean waves was developed on the basis of the single-parameter growth equation of wind waves, proposed byToba (1978) on the basis of similarity in growing wind waves. The applicability of the method to actual problems was tested by hindcasting the wave characteristics with the method, for two cases with differing time and space scales, one in Kii Channel Approach, Japan, and the other in the North Atlantic Ocean. The results showed that the present method can predict waves within an error of 1.3 m in wave heights, which ranged from 3 to 12 m.  相似文献   
23.
A comprehensive numerical study on the three-dimensional structure of a turbulent jet in crossflow is performed. The jet-to-crossflow velocity ratio (R) varies in the range of 2 - 16; both vertical jets and inclined jets without excess streamwise momentum are considered. The numerical results of the Standard two-equation k-ε model show that the turbulent structure can be broadly categorised according to the jet-to-crossflow velocity ratio. For strong to moderate jet discharges, i.e. R> 4, the jet is characterized by a longitudinal transition through a bent-over phase during which the jet becomes almost parallel with the main freestream, to a sectional vortex-pair flow with double concentration maxima; the computed flow details and scalar mixing characteristics can be described by self-similar relations beyond a dimensionless distance of around 20-60. The similarity coefficients are only weakly dependent on R. The cross-section scalar field is kidney-shaped and bifurcated, vvith distinct double concentr  相似文献   
24.
阿尔金地区以巨型阿尔金走滑断裂而著称,断裂以西为阿尔金山地区,以东为祁连山地区.近年来人们陆续在阿尔金断裂两侧发现榴辉岩带,即南阿尔金榴辉岩带(车自成等1995;刘良等1996)和柴北缘榴辉岩带(杨经绥等,1998,2000),指出阿尔金断裂两侧的岩石单元可以对比,并推断阿尔金断裂左行走滑位移了约400 km(许志琴等,1999;葛肖虹等,1999).  相似文献   
25.
In this paper, we formulate a finite-element procedure for approximating the coupled fluid and mechanics in Biot’s consolidation model of poroelasticity. We approximate the flow variables by a mixed finite-element space and the displacement by a family of discontinuous Galerkin methods. Theoretical convergence error estimates are derived and, in particular, are shown to be independent of the constrained specific storage coefficient, c o . This suggests that our proposed algorithm is a potentially effective way to combat locking, or the nonphysical pressure oscillations, which sometimes arise in numerical algorithms for poroelasticity.  相似文献   
26.
There is a net discharge of water and nutrients through Long Key Channel from Florida Bay to the Florida Keys National Marine Sanctuary (FKNMS). There has been speculation that this water and its constituents may be contributing to the loss of coral cover on the Florida Keys Reef tract over the past few decades, as well as speculation that changes in freshwater flow in the upstream Everglades ecosystem associated with the Comprehensive Everglades Restoration Plan may exacerbate this phenomenon. The results of this study indicate that although there is a net export of approximately 3,850 (±404) ton N year?1 and 63 (±7) ton P year?1, the concentrations of these nutrients flowing out of Florida Bay are the same as those flowing in. This implies that no significant nutrient enrichment is occurring in the waters of the FKNMS in the vicinity of Long Key Channel. Because of the effect of restricted southwestward water flow through Florida Bay by shallow banks and small islands, the volume of relatively high-nutrient water from central and eastern portions of the bay exiting through the channel is small compared to the average tidal exchange. Nutrient loading of relatively enriched bay waters is mediated by tidal exchange and mixing with more ambient concentrations of the western Florida Bay and Hawk Channel. System-wide budgets indicate that the contribution of Florida Bay waters to the inorganic nitrogen pool of the Keys coral reef is small relative to offshore inputs.  相似文献   
27.
A study of the variation of the spectral relative ratios of reflectivity of selected mare lunar grounds between wavelengths 4000 and 8000 Å is given in comparison with lunar craters. The intensities at different wavelengths of each lunar region are corrected for the angles of illumination and viewing, and they are scaled to unity at =5538Å. Distinct variety in the spectral reflectivity values of mare grounds at short wavelengths are confirmed. The Mare Tranquillitatis type grounds (similar to Apollo-11 site), have relative ratio of reflectivity at short wavelength at =4035 Å; larger than or equal to 1.03 in addition to a bigger difference in reflectivity between the short and the long wavelength. The Mare Serenitatis type grounds (similar to Apollo-12) are characterized to give relative ratio of reflectivity less than 1.03 at =4035 Å, and smaller difference in reflectivity between short and long wavelengths. This is due to the variation in the colour of the Mare Tranquillitatis and Mare Serenitatis type ground due to compositional differences. The mare type grounds are generally different in shape than that of lunar craters grounds.Presented at the IAU-COSPAR Julian Schmidt Symposium on 100 Years of Lunar Mapping held at Lagonissi, Greece, 25–27 May, 1978.  相似文献   
28.
An earlier paper gave solutions for the mean time rates of change of orbital elements of satellite atoms in an exosphere influenced by solar radiation pressure. Each element was assumet to beahve independently. Here the instantaneous rates of change for three elements (e, ω, and θ = ω + Ω) are integrated simultaneously for the case of the inclination i = 0. The results (a) confirm the validity of using mean rates when the orbits are tightly bound to the planet and (b) serve as examples to be reproduced by the complicated numerical solutions required for arbitrary inclination. Strongly bound hydrogen atoms perturbed in Earth orbit by radiation pressure do not seem a likely cause of the geotail extending in the anti-Sun direction. Instead, radiation pressure wil cause those particles' orbits to form a broad fan-shaped tail and to deteriorate into the Earth's atmosphere. Whether loosely bound H atoms are plentiful enough to create the geotail depends on their source function versusr; that question is beyond the scope of this paper.  相似文献   
29.
Mariner 9 ultraviolet spectra of the 1971 dust clouds were analyzed to obtain the phase function times single scattering albedo of the dust particles. The phase functions were matched with Mie scattering calculations for size distributions of spheres of homogeneous and isotropic material. The absorption index of the dust particles was found to increase with decreasing wavelenght from 350 nm down to about 210 nm, and drop off shortward of this wavelength. A structural shoulder occurs in the absorption spectrum between 240 and 250 nm. Titanium dioxide (anatase) has the correct transitions at 210 and 250 nm to match the absorption curve of Martian dust, and is proposed as a candidate constituent in Martian surface material. The spectral neutrality of TiO2 between 0.5 and 4 μm is consistent with visible and infrared observations of Mars. The high refractive index of TiO2 can explain the large refringence of Martian dust. The titanium dioxide content of the dust particles is estimated to be a few percent or less. Uncertainties in the results due to limitations in the model and data are discussed.  相似文献   
30.
Many applied dispersion models require the knowledge of boundary-layer parameters such as sensible heat flux,Q H , friction velocity,u *, and turbulent energy components, w and v . Formulas are suggested for calculating these parameters over a wide variety of types of ground surfaces, based on simple observations of wind speed near the ground and fractional cloud cover, and specification of constants such as roughness length, albedo, and soil moisture availability. Observations ofu *,Q H , w , and v during field experiments in St. Louis and Indianapolis are used to test the formulas for urban sites. Relative errors of about ±20% in the predictions are seen to occur whenu *,Q H , w , and v are large. However, when these quantities are small (e.g.,u * < 0.2 m/s), the errors in the predictions are as large as the mean value of the quantity itself.In addition, it is concluded from studies of available field data and theories that the magnitude of w is not well-known at elevations above about 100m during the late afternoon and night. Some simple parameterizations for w . are suggested that are consistent with the observed steady decrease in ground-level concentration in the afternoon and the sudden increase in concentration that can occur a few hours after sunset due to wind shears associated with a low-level jet, for continuous plumes emitted from moderate to tall stacks.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号