全文获取类型
收费全文 | 1910篇 |
免费 | 76篇 |
国内免费 | 19篇 |
专业分类
测绘学 | 41篇 |
大气科学 | 131篇 |
地球物理 | 527篇 |
地质学 | 671篇 |
海洋学 | 181篇 |
天文学 | 301篇 |
综合类 | 6篇 |
自然地理 | 147篇 |
出版年
2021年 | 20篇 |
2020年 | 25篇 |
2019年 | 25篇 |
2018年 | 47篇 |
2017年 | 39篇 |
2016年 | 65篇 |
2015年 | 38篇 |
2014年 | 49篇 |
2013年 | 91篇 |
2012年 | 51篇 |
2011年 | 73篇 |
2010年 | 80篇 |
2009年 | 80篇 |
2008年 | 75篇 |
2007年 | 85篇 |
2006年 | 68篇 |
2005年 | 58篇 |
2004年 | 55篇 |
2003年 | 55篇 |
2002年 | 55篇 |
2001年 | 36篇 |
2000年 | 35篇 |
1999年 | 33篇 |
1998年 | 31篇 |
1997年 | 36篇 |
1996年 | 27篇 |
1995年 | 31篇 |
1994年 | 33篇 |
1993年 | 16篇 |
1992年 | 32篇 |
1991年 | 30篇 |
1990年 | 39篇 |
1989年 | 26篇 |
1988年 | 24篇 |
1987年 | 19篇 |
1986年 | 22篇 |
1985年 | 35篇 |
1984年 | 42篇 |
1983年 | 28篇 |
1982年 | 25篇 |
1981年 | 33篇 |
1980年 | 28篇 |
1979年 | 23篇 |
1978年 | 22篇 |
1977年 | 17篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1974年 | 18篇 |
1973年 | 15篇 |
1971年 | 12篇 |
排序方式: 共有2005条查询结果,搜索用时 31 毫秒
91.
92.
The coastal ocean model FVCOM is applied to quantify the changes in circulation, flushing, and exposure time in Great South Bay, New York, after Superstorm Sandy breached the barrier island in 2012. Since then, the lagoon system is connected to the Atlantic via five instead of four inlets. The model simulations are run on two high-resolution unstructured grids, one for the pre-breach configuration, one including the new inlet, with tidal-only forcing, and summer and winter forcing conditions. Despite its small cross-sectional size, the breach has a relatively large net inflow that leads to a strengthening of the along-bay through-flow in Great South Bay (GSB); the tidally driven volume transport in central GSB quadrupled. The seasonal forcing scenarios show that the southwesterly sea breeze in summer slows down the tidally driven flow, while the forcing conditions in winter are highly variable, and the circulation is dependent on wind direction and offshore sea level. Changes in flushing and exposure time associated with the modified transport patterns are evaluated using a Eulerian passive tracer technique. Results show that the new inlet produced a significant decrease in flushing time (approximately 35% reduction under summer wind conditions and 20% reduction under winter wind conditions). Maps of exposure time reflect the local changes in circulation and flushing. 相似文献
93.
Barbara A. Muhling Carlos F. Gaitán Charles A. Stock Vincent S. Saba Desiree Tommasi Keith W. Dixon 《Estuaries and Coasts》2018,41(2):349-372
Estuaries are productive and ecologically important ecosystems, incorporating environmental drivers from watersheds, rivers, and the coastal ocean. Climate change has potential to modify the physical properties of estuaries, with impacts on resident organisms. However, projections from general circulation models (GCMs) are generally too coarse to resolve important estuarine processes. Here, we statistically downscaled near-surface air temperature and precipitation projections to the scale of the Chesapeake Bay watershed and estuary. These variables were linked to Susquehanna River streamflow using a water balance model and finally to spatially resolved Chesapeake Bay surface temperature and salinity using statistical model trees. The low computational cost of this approach allowed rapid assessment of projected changes from four GCMs spanning a range of potential futures under a high CO2 emission scenario, for four different downscaling methods. Choice of GCM contributed strongly to the spread in projections, but choice of downscaling method was also influential in the warmest models. Models projected a ~2–5.5 °C increase in surface water temperatures in the Chesapeake Bay by the end of the century. Projections of salinity were more uncertain and spatially complex. Models showing increases in winter-spring streamflow generated freshening in the Upper Bay and tributaries, while models with decreased streamflow produced salinity increases. Changes to the Chesapeake Bay environment have implications for fish and invertebrate habitats, as well as migration, spawning phenology, recruitment, and occurrence of pathogens. Our results underline a potentially expanded role of statistical downscaling to complement dynamical approaches in assessing climate change impacts in dynamically challenging estuaries. 相似文献
94.
Charles?R.?SternEmail author Julien?M.?Allaz Markus?B.?Raschke G.?Lang?Farmer M.?Alexandra?Skewes Jeremy?T.?Ross 《Contributions to Mineralogy and Petrology》2018,173(8):65
Aplite dikes intruding the Proterozoic 1.42(±?3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride?+?phosphate (+?S?+?CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride?+?phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(±?25) and 1.442(±?8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, εNd1.42Ga values for the granites and related granitic pegmatites range from ??3.3 to ??4.7 (average ??3.9), and differ from the values for both the aplites and REE-rich segregations, which range from ??1.0 to ??2.2 (average ??1.6). Furthermore, the granites and pegmatites have (La/Yb)N <50 with significant negative Eu anomalies, which contrast with higher (La/Yb)N >100 and absence of an Eu anomaly in both the aplites and segregations. These data are consistent with the aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher εNd1.42Ga values for the aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite. 相似文献
95.
Kenneth?S.?BefusEmail author Benjamin?J.?Andrews 《Contributions to Mineralogy and Petrology》2018,173(11):92
High-temperature decompression experiments demonstrate that crystal textures preserve a record of the style and rate of magmatic ascent. To reinforce this link, we performed a suite of isothermal decompression experiments using starting material from the climactic 1991 Pinatubo eruption. We decompressed experiments from 220 MPa to final, quench pressures of 75 or 30 MPa using continuous decompression rates of 100, 30, 10, 3, 1, and 0.3 MPa h?1. Amphibole, clinopyroxene, and plagioclase crystallized during the experiments, with plagioclase microlites dominating the assemblage. Total microlite number densities range from 107.6±0.4 up to 108.2±0.2 cm?3, with plagioclase accounting for up to 65% of the total number. Plagioclase microlite area increased systematically from 19?±?8 to 937?±?487 µm2 with increasing experiment duration. Our textures provide time-integrated records of crystal kinetics. Average nucleation and areal growth rates of plagioclase are highest in the fastest decompressions (~?107.5 cm?3 h?1 and 10.1?±?4.1 µm2 h?1, respectively) and more than an order of magnitude lower in the slowest experiments (~?105.5 cm?3 h?1 and 0.8?±?0.2 µm2 h?1, respectively). Both nucleation and growth rates are highest at high degrees of disequilibrium. We find that peak supersaturation-dependent instantaneous rates are generally more than an order of magnitude faster than average rates. We use those instantaneous nucleation and growth rates to introduce an iterative model to evaluate the effects of different decompression rates, decompression paths (continuous, single-step or multistep), and the presence of phenocrysts on final crystallinity and microlite size distribution. 相似文献
96.
97.
Jonathan Charles Iliffe 《制图学和地理信息科学》2018,45(3):270-283
Map projections are an essential component of coordinate systems used in applications such as surveying, topographic mapping, and engineering, and care needs to be taken to select ones that minimize distortion for each case. This article explores the selection process for near-linear features on the surface of the Earth and derives limits for the extent of a project that can be projected within specified distortion tolerances. It is then demonstrated that a multifaceted set of projections of the Earth may be used to extend this concept to the mapping of features such as highways and railways that are quasi-linear but do not exactly follow a standard geometrical line (a great circle or a small circle) on the surface of the Earth. A continuous, conformal coordinate system may be derived in such situations, extending to indefinite length and applicable over a swath of several kilometers width, but it cannot be extended to cover situations with extensive variations in height. Instead, the Snake Projection is analyzed, and it is shown that this can be used to develop continuous (non-zonal) projected coordinate systems for major engineering projects extending for hundreds of kilometers and having extensive height ranges. Examples are shown of the application to railway projects. 相似文献
98.
Climatic Change - The severity and frequency of climate change hazards are increasing around the world. Because the impacts are most acutely felt in local communities, it is critical to improve... 相似文献
99.
100.
AbstractMeasurements by free fall instruments, in the San Diego Trough, the Florida Current, and the central Pacific, reveal the detailed structure of the vertical component of the oceanic temperature gradient. The temperature changes are concentrated into regions on the order of a meter thick wherein the measured gradients are often more than ten times the average gradient. The horizontal extent of the regions of high gradient is greater than 750 meters in the seasonal thermocline off San Diego, but is only a few hundred meters at depths greater than 400 meters.Fine scale measurements show that the layers of high gradient consist of even finer fluctuations in gradient which are only a few centimeters thick. Time scales of the thinnest of these regions of high gradient are of the order of five minutes. The data also yields an estimate of the entropy generation. According to the results of an idealized model relating entropy generation to the turbulent heat transport, only 240 to 700 ergs per cm.2 per sec were transported in a 25 meter vertical section measured in the San Diego Trough. This value compared with 3600 ergs per cm.2 per sec estimated from the mean gradient and an eddy coefficient of 1 cm.2 per sec. 相似文献