全文获取类型
收费全文 | 43228篇 |
免费 | 621篇 |
国内免费 | 359篇 |
专业分类
测绘学 | 1307篇 |
大气科学 | 3147篇 |
地球物理 | 8845篇 |
地质学 | 14339篇 |
海洋学 | 3717篇 |
天文学 | 10469篇 |
综合类 | 122篇 |
自然地理 | 2262篇 |
出版年
2021年 | 341篇 |
2020年 | 373篇 |
2019年 | 460篇 |
2018年 | 997篇 |
2017年 | 910篇 |
2016年 | 1208篇 |
2015年 | 677篇 |
2014年 | 1153篇 |
2013年 | 2078篇 |
2012年 | 1263篇 |
2011年 | 1662篇 |
2010年 | 1527篇 |
2009年 | 2097篇 |
2008年 | 1841篇 |
2007年 | 1863篇 |
2006年 | 1753篇 |
2005年 | 1309篇 |
2004年 | 1343篇 |
2003年 | 1217篇 |
2002年 | 1253篇 |
2001年 | 1108篇 |
2000年 | 1036篇 |
1999年 | 909篇 |
1998年 | 903篇 |
1997年 | 902篇 |
1996年 | 741篇 |
1995年 | 682篇 |
1994年 | 648篇 |
1993年 | 559篇 |
1992年 | 493篇 |
1991年 | 500篇 |
1990年 | 480篇 |
1989年 | 499篇 |
1988年 | 450篇 |
1987年 | 518篇 |
1986年 | 469篇 |
1985年 | 566篇 |
1984年 | 682篇 |
1983年 | 605篇 |
1982年 | 593篇 |
1981年 | 533篇 |
1980年 | 460篇 |
1979年 | 469篇 |
1978年 | 475篇 |
1977年 | 399篇 |
1976年 | 364篇 |
1975年 | 366篇 |
1974年 | 345篇 |
1973年 | 389篇 |
1972年 | 269篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
291.
The internal dynamics of an illuminated dust cloud of finite optical thickness is investigated. The dependence of the radiation pressure on the optical depth makes the individual particles oscillate, in one dimension, around the accelerated centre of gravity of the cloud. The cloud moves as an entity, irrespectively of the velocity dispersion of the particles and their efficiency for radiation pressure. If the optical depth does not change, i.e. if the cloud does not expand laterally, its lifetime is unlimited. A contraction caused by energy dissipation in mechanical collissions between the dust particles is expected. The range of particle sizes which can be transported by such a “coherent cloud” is estimated, as well as the acceleration of the whole cloud. The structure of the cloud in real space and in velocity space is investigated. A comparison with the “striae” observed in the dust tails of great comets shows that the parent clouds of these striae may have been of the kind considered. 相似文献
292.
Observations from a surface mooring, in a weak-flow regime over the southeastern Bering Sea shelf, were used to derive surface heat fluxes for the period May–July 1996. Changes in heat content of the water column also were determined from subsurface temperature measurements. Agreement of net surface heat flux and change in heat content was within 2%. This result provides additional evidence that heat advection and diffusion are small in this region. 相似文献
293.
Coastal sand dunes are considered among the most susceptible habitats to recreational use. The aim of this study was to monitor the impact of visitor use on soil and annual plants on long-established trails in the stabilised coastal dunes of the Sharon Park, Israel. The results indicate that:
- 1. The vegetation cover, height and species richness and diversity, as well as soil organic matter content were lower on trails subjected to high visitor use than that on trails under low use. However, soil compaction and moisture on high-use trails were higher than that on low use.
- 2. The rate of change in each of the vegetation properties moving outwards from the centre of the trail towards the undamaged area on its margins and beyond, was higher on trails under high visitor use than on low-use trails.
- 3. The impact of high visitor use is localised and limited to the trail boundaries and their immediate surroundings (6 m axis perpendicular to the trails), while the effect on low-use trails is dispersed over a larger area, apparently because the trail borders are less visually defined to the visitor.
294.
P. A. Klimov G. K. Garipov B. A. Khrenov V. S. Morozenko V. O. Barinova V. V. Bogomolov M. A. Kaznacheeva M. I. Panasyuk K. Yu. Saleev S. I. Svertilov 《Izvestiya Atmospheric and Oceanic Physics》2017,53(9):924-933
In this paper, data obtained by the Vernov satellite are discussed in comparison with the data obtained by Universitetsky–Tatiana-1, 2 satellites. The advanced instrumentation onboard the Vernov satellite improved the capabilities of the data-transmission system, providing an opportunity to acquire more detailed information on transient atmospheric events (TAEs), including a wider range of TAE photon numbers (event brightness) down to lower photon numbers, the opportunity to observe the events in ultraviolet range without the considerable inclusion of red–infrared photons (which indicates the substantial contribution of electric discharges, lightning precursors, at the altitudes of thunderstorm clouds), and the observation of TAE series at a rate of about 10–100 occurrences per second with a period from tens of seconds to several minutes. 相似文献
295.
P. N. Makkaveev A. A. Polukhin A. V. Kostyleva E. A. Protsenko S. V. Stepanova Sh. Kh. Yakubov 《Oceanology》2017,57(1):48-57
During cruise 65 of the R/V Akademik Mstislav Keldysh in the Kara Sea, three transects were executed: one eastwards from the Novaya Zemlya Archipelago and two in the St. Anna and Voronin troughs. It was noted that the continental runoff affected the entire surveyed aquatic area, even at the northern extremity of the Novaya Zemlya Archipelago. The transect along the St. Anna Trough showed the presence of a slope frontal zone overlaid at the surface by a desalinated layer. The Voronin Trough was characterized by sliding of slope waters. The hydrochemical parameters show that the surveys were carried out during a recession of biological activity of the waters and that the peak bloom was over by that time. The hydrochemical structure of waters conformed to early autumn conditions, but before the beginning of intense cooling of surface waters. 相似文献
296.
Water Resources - A procedure for assessing the protection of groundwater against pollution is considered. This is a basic procedure for tentative forecast estimates of changes in the... 相似文献
297.
Geomagnetism and Aeronomy - The magnetic fields of a subterranean source observed on the Earth’s surface or from the orbit of a satellite are studied. We analyze the space of all harmonic... 相似文献
298.
The diffusion of an ion in porewaters cannot occur independently of the other ions in solution as a result of Coulombic coupling, as well as from other effects not considered here. Unfortunately, a longstanding disagreement exists about the correct form and meaning of the equations that describe Coulombic coupling in porewaters, i.e., Ben-Yaakov [Am. J. Sci. 281 (1981) 974] vs. Lasaga [Am. J. Sci. 281 (1981) 981]. This paper re-examines this controversy by reformulating the problem starting from fundamental concepts of mass and charge conservation. We show that these antagonistic formulations are both valid and, in fact, equivalent, when the different interpretations of charge balance are resolved. Most of the disagreements between Ben-Yaakov and Lasaga are then shown to result from differing methods of solution, not fundamental disparities in their models. We note, however, that the explanation for the concept of “stationary” gradients of nonreacting ions as given Ben-Yaakov is inaccurate, and such gradients do lead to diffusive fluxes that are counterbalanced by electrochemical migrational fluxes to produce no net flux (excluding advective flux). We further find that the bicarbonate diffusive flux will not balance the diffusional charge flux of sulfate during its reduction if advection is present. This latter imbalance generates compensating fluxes in the other nonreacting ions. We have applied our theory to a simplified case of sulfate reduction in a marine sediment. The results show that nonreacting ions do diffuse and that with normally expected values of porewater advection, the ratio of the bicarbonate to the sulfate flux can be far different than the ideal value of −2. 相似文献
299.
Tunnel behaviour under earthquake loading is affected by many factors such as shape, depth and stiffness of the tunnel lining and the nature of the input motion. However, current knowledge on the effects of these parameters on the seismic behaviour of tunnels is limited to lack of experimental or field data. Existing analytical methods are based on assumptions, the validity of which needs to be established using carefully conducted experimental studies and numerical analyses. This paper focuses on the effects of input motion characteristics on seismic behaviour of circular and square tunnels. Dynamic centrifuge tests were carried out on model tunnels using input motions of different amplitude and frequency. Accelerations and earth pressures around the tunnels were measured. Complementary Finite Element analyses were conducted with different types of input motions. Results show that magnitude of the maximum input acceleration plays a crucial role on the maximum and residual lining forces, which the tunnel experiences. 相似文献
300.
K. SchroederA.C. Haza A. Griffa T.M. Özgökmen P.M. PoulainR. Gerin G. PeggionM. Rixen 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(3):209-228
Relative dispersion in the Liguro-Provençal basin (a subregion of the Mediterranean Sea) is investigated using clusters of surface drifters deployed during two Marine Rapid Environment Assessment (MREA) experiments covering different months in 2007 and 2008, respectively. The clusters have initial radii of less than 1 km, or an order of magnitude below a typical deformation radius (approximately 10-20 km). The data set consists of 45 original pairs and more than 50 total pairs (including chance ones) in the spatial range between 1 and 200 km. Relative dispersion is estimated using the mean square separation of particle pairs and the Finite Scale Lyapunov Exponents (FSLEs). The two metrics show broadly consistent results, indicating in particular a clear exponential behaviour with an e-folding time scale between 0.5 and 1 days, or Lyapunov exponent ?? in the range of 0.7-1 days−1. The exponential phase extends for 4-7 days in time and between 1 and 10-20 km in separation space. To our knowledge, this is only the third time that an exponential regime is observed in the world ocean from drifter data. This result suggests that relative dispersion in the Liguro-Provençal basin is nonlocal, namely controlled mainly by mesoscale dynamics, and that the effects of the sub-mesoscale motions are negligible in comparison. NCOM model results are used to complement the data and to quantify errors arising from the sparse sampling in the observations. 相似文献