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111.
Some 20% of Titan’s surface is covered in large linear dunes that resemble in morphology, size and spacing (1-3 km) those seen on Earth. Although gravity, atmospheric density and sand composition are very different on these two worlds, this coincident size scale suggests that the controlling parameter limiting the growth of giant dunes, namely the boundary layer thickness (Andreotti et al., 2009). Nature, 457, 1120-1123], is similar. We show that a ∼3 km boundary layer thickness is supported by Huygens descent data and is consistent with results from Global Circulation Models taking the distinctive thermal inertia and albedo of the dune sands into account. While the boundary layer thickness on Earth controlling dunes can vary by an order of magnitude depending on the proximity of oceans, which have very different thermal properties from dry land, the relative invariance of dune spacing on Titan is consistent with relatively uniform thermal properties near the dunes and no prominent variation with latitude is seen. 相似文献
112.
Threshold of wave generation on Titan’s lakes and seas: Effect of viscosity and implications for Cassini observations 总被引:1,自引:0,他引:1
Motivated by radar and near-infrared data indicating that Titan’s polar lakes are extremely smooth, we consider the conditions under which a lake surface will be ruffled by wind to form capillary waves. We evaluate laboratory data on wind generation and derive, without scaling for surface tension effects, a threshold for pure methane/ethane of ∼0.5-1 m/s. However, we compute the physical properties of predicted Titan lake compositions using the National Institute for Standards Technology (NIST) code and note that dissolved amounts of C3 and C4 compounds are likely to make Titan lakes much more viscous than pure ethane or methane, even without allowing for suspended particulates which would increase the viscosity further. Wind tunnel experiments show a strong dependence of capillary wave growth on liquid viscosity, and this effect may explain the apparent absence so far of waves, contrary to prior expectations that generation of gravity waves by wind should be easy on Titan. On the other hand, we note that winds over Titan lakes predicted with the TitanWRF Global Circulation Model indicate radar observations so far have in any case been when winds have been low (∼0.5-0.7 m/s), possibly below the wave generation threshold, while peak winds during summer may reach 1-2 m/s. Thus observations of Titan’s northern lakes during the coming years by the Cassini Solstice mission offer the highest probability of observing wind-roughening of lake surfaces, while observations of Ontario Lacus in the south will likely continue to show it to be flat and smooth. 相似文献
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We present three dimensional simulations of the interaction of a light hypersonic jet with an inhomogeneous thermal and turbulently supported disk in an elliptical galaxy. These simulations are applicable to the GPS/CSS phase of some extragalactic radio sources. We identify four generic phases in the evolution of such a jet with the interstellar medium. The first is a “flood and channel” phase, dominated by complex jet interactions with the dense cloudy medium close to the nucleus. This is characterized by high pressure jet gas finding changing weak points in the ISM and flowing through channels that form and reform over time. A spherical, energy driven, bubble phase ensues, wherein the bubble is larger than the disk scale, but the jet remains fully disrupted close to the nucleus, so that the jet flux is thermalised and generates a smooth isotropic energy-driven bubble. In the subsequent, rapid, jet break-out phase the jet breaks free of the last obstructing dense clouds, becomes collimated and pierces the more or less spherical bubble. In the final classical phase, the jet propagates in a momentum-dominated fashion similar to jets in single component hot haloes, leading to the classical jet–cocoon–bow-shock structure. 相似文献
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Iron and manganese oxide coatings are actively forming on stream substrates in the White Oak Creek watershed in East Tennessee. Although oxidizing conditions are required for Fe-Mn oxide precipitation, coatings accumulate only if dissolved iron and manganese exceed 50 μg/L. Below this, coatings are lost by abrasion as fast as or faster than they form. Annual rates of formation of 3 mg/g on substrate (gravel) were observed. Manganese is dissolved from coatings between Eh values of 100 to 300 mV and below 1 mg/L dissolved O2 at pH 6.5 to 7.5. Iron oxides can be precipitated under these conditions.Uncontaminated gravels with oxide coatings (composed of illite, quartz, and feldspar) adsorbed radionuclides rapidly from waters below MPC (Maximum Permissible Concentration) levels. Contaminated gravels placed in uncontaminated waters lost 60Co by abrasion in oxidizing conditions and by dissolution of manganese coatings in reducing conditions. Exchangeable 90Sr was completely lost after one month whereas nonexchangeable 90Sr was lost more slowly; 137Cs was totally retained by the gravels. Gravels such as these can be used to monitor the radionuclide content of waters in the environment. 相似文献
117.
To investigate land–sea interactions during deglaciation, we compared proxies for continental (pollen percentages and accumulation rates) and marine conditions (dinoflagellate cyst percentages and alkenone-derived sea surface temperatures). The proxies were from published data from an AMS-radiocarbon-dated sedimentary record of core GeoB 1023-5 encompassing the past 21,000 years. The site is located at ca. 2000 m water depth just north of the Walvis Ridge and in the vicinity of the Cunene River mouth. We infer that the parallelism between increasing sea surface temperatures and a southward shift of the savanna occurred only during the earliest part of the deglaciation. After the Antarctic Cold Reversal, southeast Atlantic sea surface temperatures no longer influenced the vegetation development in the Kalahari. Stronger trade winds during the Antarctic Cold Reversal and the Younger Dryas period probably caused increased upwelling off the coast of Angola. A southward shift of the Atlantic anti-cyclone could have resulted in both stronger trade winds and reduced impact of the Westerlies on the climate of southwestern Africa. 相似文献
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Ralph C. Foster Francois Vianey Philippe Drobinski Pierre Carlotti 《Boundary-Layer Meteorology》2006,120(2):229-255
The near-surface flow of a well-resolved large-eddy simulation of the neutrally-stratified planetary boundary layer is used to explore the relationships between coherent structures and the vertical momentum flux. The near-surface flow is characterized by transient streaks, which are alternating bands of relatively higher and lower speed flow that form parallel to the mean shear direction in the lower part of the boundary layer. Although individual streaks are transient, the overall flow is in a quasi-equilibrium state in which the streaks form, grow, decay and regenerate over lifetimes on the order of tens of minutes. Coupled with the streaky flow is an overturning circulation with alternating bands of updrafts and downdrafts approximately centered on the streaks. The surface stress is dominated by upward ejections of slower moving near-surface air and downward sweeps of higher speed air from higher in the boundary layer. Conditional sampling of the ejection and sweep events shows that they are compact, coherent structures and are intimately related to the streaks: ejections (sweeps) preferentially form in the updrafts (downdrafts) of the three-dimensional streak flow. Hence, consistent with other recent studies, we propose that the streak motion plays an important role in the maintenance of the surface stress by establishing the preferential conditions for the ejections and sweeps that dominate the surface stress. The velocity fluctuation spectra in the model near the surface have a k
−1 spectral slope over an intermediate range of wavenumbers. This behaviour is consistent with recent theoretical predictions that attempt to evaluate the effects of organized flow, such as near-surface streaks, on the variance spectra. 相似文献