全文获取类型
收费全文 | 30423篇 |
免费 | 675篇 |
国内免费 | 345篇 |
专业分类
测绘学 | 788篇 |
大气科学 | 2447篇 |
地球物理 | 6695篇 |
地质学 | 10638篇 |
海洋学 | 2531篇 |
天文学 | 6058篇 |
综合类 | 60篇 |
自然地理 | 2226篇 |
出版年
2020年 | 192篇 |
2019年 | 207篇 |
2018年 | 406篇 |
2017年 | 379篇 |
2016年 | 556篇 |
2015年 | 434篇 |
2014年 | 607篇 |
2013年 | 1476篇 |
2012年 | 722篇 |
2011年 | 1014篇 |
2010年 | 821篇 |
2009年 | 1180篇 |
2008年 | 1088篇 |
2007年 | 1032篇 |
2006年 | 1024篇 |
2005年 | 892篇 |
2004年 | 920篇 |
2003年 | 884篇 |
2002年 | 879篇 |
2001年 | 709篇 |
2000年 | 726篇 |
1999年 | 678篇 |
1998年 | 647篇 |
1997年 | 644篇 |
1996年 | 540篇 |
1995年 | 535篇 |
1994年 | 506篇 |
1993年 | 468篇 |
1992年 | 440篇 |
1991年 | 389篇 |
1990年 | 441篇 |
1989年 | 353篇 |
1988年 | 391篇 |
1987年 | 434篇 |
1986年 | 363篇 |
1985年 | 546篇 |
1984年 | 594篇 |
1983年 | 596篇 |
1982年 | 483篇 |
1981年 | 486篇 |
1980年 | 488篇 |
1979年 | 432篇 |
1978年 | 431篇 |
1977年 | 396篇 |
1976年 | 431篇 |
1975年 | 382篇 |
1974年 | 429篇 |
1973年 | 409篇 |
1972年 | 255篇 |
1971年 | 200篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
W. J. Chaplin G. Houdek Y. Elsworth D. O. Gough G. R. Isaak R. New 《Monthly notices of the Royal Astronomical Society》2005,360(3):859-868
We investigate the frequency dependence of the power spectral density of low-degree solar p modes by comparing measurements with the results of a stochastic-excitation model. In the past it was common practice to use the total power in such investigations. Using the maximum of the power spectral density instead provides a direct comparison with the measured mode heights in the observed power spectrum. This method permits a more careful calibration of the adjustable parameters in the excitation model, a model which we present here, for the first time, in a format that precisely and unambiguously relates the amplitudes of the modes of oscillation to the Reynolds stress in the equilibrium model. We find that errors in the theory of the linear mode damping rates, particularly at low frequency, have a dramatic impact on the predictions of the mode heights in the spectral density, whereas parameter changes in the stochastic excitation model, within a plausible domain of parameter space, have a comparatively small effect. 相似文献
92.
Donald C. Gordon Peter J. Cranford Con Desplanque 《Estuarine, Coastal and Shelf Science》1985,20(2):205-227
The Cumberland Basin, a 118 km2 estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens. Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalley's method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m?2, respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin. 相似文献
93.
K.A. MAASCH P.A. MAYEWSKI E.J. ROHLING J.C. STAGER W. KARLEN L.D. MEEKER E.A. MEYERSON 《Geografiska Annaler: Series A, Physical Geography》2005,87(1):7-15
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14 C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale. 相似文献
94.
Peter Nielsen 《Coastal Engineering》1985,9(3):261-276
An empirical analysis is performed on the most detailed, recent measurements of turbulent oscillatory boundary layer flow. The measurements show that throughout elevations where the flow can be considered horizontally uniform, one deficit model is sufficient for describing the fundamental mode. Some general properties of the non dimensional velocity deficit D1(z) appear with striking consistency. First of all the identity , which is a theoretical result for smooth laminar flow, seems to hold with great accuracy for a large range of turbulent flow conditions as well. This is of principal theoretical interest because all previous analytical eddy viscosity models as well as numerical mixing length models predict a consistent and fairly large difference between Arg D1 and . If the identity between and Arg D1 extends all the way to the bed, it means that the bed shear stress leads the free stream velocity by 45 degrees. It is also found that the structure of smooth turbulent oscillatory flows as measured by Kalkanis (1964) corresponds to a sharp maximum in the normalized energy dissipation rate. 相似文献
95.
The lifetimes, characteristics of the shapes as well as lengths and perimeters of artificial solar granules (Nordlund, 1982, 1984a) are compared with data from the literature and parameters determined from two different sets of observed granules. No significant differences of the parameters for these sets of granules are detectable.Mitteilungen aus dem Kiepenheuer-Institut Nr. 251. 相似文献
96.
Replicate portions of a Delaware salt marsh were enclosed in cylindrical microcosms and exposed to elevated levels of inorganic arsenic (arsenate). All biotic and abiotic components in dosed cylinders rapidly incorporated arsenic. Spartina blades showed the greatest arsenic enrichment, with dosed plants incorporating arsenic concentrations an order of magnitude higher than controls. Spartina detritus and sediments also exhibited greatly elevated arsenic concentrations. Virtually all of the arsenic was incorporated into plant tissue or strongly sorbed to cell surfaces. Thus, elevated arsenic concentrations in estuarine waters will be reflected in living and non-living components of a salt marsh ecosystem, implying that increased arsenic will be available to organisms within the marsh ecosystem. 相似文献
97.
98.
Alan M Bond Julie R Bradbury Henry A Hudson John S Garnham Peter J Hanna Stanley Strother 《Marine Chemistry》1985,16(1):1-9
Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent. 相似文献
99.
W.J. Boulton 《Planetary and Space Science》1985,33(6):735-756
The influence of aerodynamic drag and the geopotential on the motion of the satellite 1964-52B is considered. A model of the atmosphere is adopted that allows for oblateness, and in which the density behaviour approximates to the observed diurnal variation. A differential equation governing the variation of the eccentricity, e, combining the effects of air drag with those of the Earth's gravitational field is given. This is solved numerically using as initial conditions 310 computed orbits of 1964-52B.The observed values of eccentricity are modified by the removal of perturbations due to luni-solar attraction, solid Earth and ocean tides, solar radiation pressure and low-order long-periodic tesseral harmonic perturbations. The method of removal of these effects is given in some detail. The behaviour of the orbital eccentricity predicted by the numerical solution is compared with the modified observed eccentricity to obtain values of atmospheric parameters at heights between 310 and 430 km. The daytime maximum of air density is found to be at 14.5 hours local time. Analysis of the eccentricity near 15th order resonance with the geopotential yielded values of four lumped geopotential harmonics of order 15, namely: , , , at inclination 98.68°. 相似文献
100.