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91.
A simulation of collisional and gravitational interaction in the early solar system generates planets ~500 km in diameter from an initial swarm of kilometer-sized planetesimals, such as might have resulted from gravitational instabilities in the solar nebula. The model treats collisions according to experimental and theoretical impact results (such as rebound, cratering, and catastrophic fragmentation) for a variety of materials whose parameters span plausible values for early solid objects. Ad hoc sticking mechanisms are avoided. The small planets form in ~104 yr, during which time most of the mass of the system continues to reside in particles near the original size. The relative random velocities remain of the order of a kilometer-sized body's escape velocity, with random velocities of the largest objects somewhat depressed because of damping by the bulk of the material. The simulation is terminated when the largest objects' random motion is of smaller dimension than their collision cross sections, so that the “particle-in-a-box” statistical methods of the model break down. The few 500-km planets, in a swarm still dominated by kilometer-scale planetesimals, may act as “seeds” for the subsequent, gradual, accretional growth into full-sized planets. 相似文献
92.
Entrepreneurship in the northern periphery in Israel should be viewed as a response to the crisis in rural agriculture during the 1980’s. Most entrepreneurs left their farms for salaried employment for a few years and they took professional courses in order to learn necessary skills before they opened their enterprises. They have developed new small entreprizes using local resources at times informally as means to reduce risks and they specialize mainly in internal tourism and construction related branches. While Jewish entrepreneurs develop mainly tourism activities oriented toward the national market, Arab entrepreneurs develop mainly construction related branches to local and home regional markets. Both represent two styles of peripheral activities. It seems that both styles has only limited potential to overcome their marginality. 相似文献
93.
Elyakom Vadislavsky Yoav Yair Carynelisa Erlick Colin Price Eran Greenberg Roy Yaniv Baruch Ziv Nàama Reicher Adam Devir 《Journal of Atmospheric and Solar》2009,71(17-18):1835-1839
We report synchronized optical observations of sprites in Israel during the winters of 2006/7–2007/8. Based on several events, we suggest that the elements of columniform sprites are organized in spaced intervals on the circumference of a circle centered directly above, or a little offset, to the vertical direction from the parent lightning. In 2D images most of the cases show columns to be arranged in highly eccentric elliptical forms or in straight rows. The analysis of the optical images provided the geometrical dimensions of the columns and their spatial organization. We used an electrostatic model of the QE field with reasonable assumptions on the location and magnitude of the cloud charge center, constrained by ELF evaluation of the Charge Moment Change in the parent flash, to show that the observed diameter of the columnar arrangement closely matches the conventional breakdown field line contour at the same altitude. 相似文献
94.
Cytochrome P450 monooxygenases constitute an enzyme superfamily, with at least 74 known families. Members of CYP families 1–4 are important in the phase 1 metabolism of lipophilic xenobiotics, such as those found in contaminated marine environments. Previous studies (James et al. Arch. Biochem. Biophys. (1996) 329, 31–38) showed that a major form of P450 in spiny lobster, Panulirus argus, hepatopancreas was CYP2L1, a new sub-family, and that there was evidence for other P450 forms in hepatopancreas. We now report the sequence of a second member of this subfamily, named CYP2L2, present in P. Argus hepatopancreas. The deduced amino acid sequences of CYP2L1 and CYP2L2 share 54.7% sequence identity and an additional 13.6% of the sequences show conservative substitutions. Analysis of the sequences of CYP2L1, CYP2L2 and other representative CYP2 family members (from rat and mouse sub-families 2A, 2B, 2D and 2E) showed that the crustacean sequences clustered together. In addition to CYP2L2, cDNA clones of 66 to 117 base pairs from the 5′ coding region of two more P450 isoforms were isolated from the spiny lobster cDNA library. The deduced amino acid sequence of one of these additional cDNA clones was identical to the first 22 amino acids of the N-terminal sequence of a P450 protein previously isolated from hepatopancreas microsomes. These studies confirm earlier biochemical evidence that the hepatopancreas contains multiple forms of cytochrome P450. 相似文献
95.
Shmueli Deborah F. Segal Ehud Ben Gal Michal Feitelson Eran Reichman Amnon 《Natural Hazards》2019,98(2):405-423
Natural Hazards - The Jordan Rift Valley is a high-risk low-occurrence zone for earthquakes, with documented incidences within the last millennium causing widespread destruction. This research... 相似文献
96.
Stream temperature response to variable glacier coverage in coastal watersheds of Southeast Alaska 总被引:1,自引:0,他引:1
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Jason B. Fellman Sonia Nagorski Sanjay Pyare Andrew W. Vermilyea Durelle Scott Eran Hood 《水文研究》2014,28(4):2062-2073
We measured stream temperature continuously during the 2011 summer run‐off season (May through October) in nine watersheds of Southeast Alaska that provide spawning habitat for Pacific salmon. The nine watersheds have glacier coverage ranging from 0% to 63%. Our goal was to determine how air temperature and watershed land cover, particularly glacier coverage, influence stream temperature across the seasonal glacial meltwater hydrograph. Multiple linear regression models identified mean watershed elevation (related to glacier extent) and watershed lake coverage (%) as the strongest landscape controls on mean monthly stream temperature, with the weakest (May) and strongest (July) models explaining 86% and 97% of the temperature variability, respectively. Mean weekly stream temperature was significantly correlated with mean weekly air temperature in seven streams; however, the relationships were weak to non‐significant in the streams influenced by glacial run‐off. Streams with >30% glacier coverage showed decreasing stream temperatures with rising summer air temperatures, whereas those with <30% glacier coverage exhibited summertime warming. Glaciers also had a cooling effect on monthly mean stream temperature during the summer (July through September) equivalent to a decrease of 1.1 °C for each 10% increase in glacier coverage. The maximum weekly average temperature (an index of thermal suitability for salmon) in the six glacial streams was substantially below the lower threshold for optimum salmon growth. This finding suggests that although glaciers are important for moderating summer stream temperatures, future reductions in glacier run‐off may actually improve the thermal suitability of some glacially dominated streams in Southeast Alaska for salmon. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
97.
植物挥发性有机物的分析方法 总被引:6,自引:0,他引:6
为了分析植物排放到大气中的挥发性有机物,在2000年1-6月期间,与美国国家大气研究中心(NCAR)的科学家共同发展和研究了一套自动的气相色谱系统和分析方法。该系统引入一个小型冰箱以除去空气样品进样时的水分,得到了对挥发性有机物样品进行分析时比较好的准确度。在5℃以及稳定的进样速率的条件下,可以得到比较好的除水效果。对NCAR附近的大气进行了采样,并利用此系统做了分析。本简单介绍此自动系统以及样品分析结果。实验结果表明,此套系统具有良好的重复性和稳定性,主要的挥发性有机物的成分如异戊二烯和单萜烯等具有明显的日变化。 相似文献
98.
Simulating the three-dimensional circulation and hydrography of Halifax Harbour using a multi-nested coastal ocean circulation model 总被引:2,自引:2,他引:0
Shiliang Shan Jinyu Sheng Keith Richard Thompson David Alexander Greenberg 《Ocean Dynamics》2011,61(7):951-976
Halifax Harbour is located on the Atlantic coast of Nova Scotia, Canada. It is one of the world’s largest, ice-free natural
harbours and of great economic importance to the region. A good understanding of the physical processes controlling tides,
flooding, transport and dispersion, and hydrographic variability is required for pollution control and sustainable development
of the Harbour. For the first time, a multi-nested, finite difference coastal ocean circulation model is used to reconstruct
the three-dimensional circulation and hydrography of the Harbour and its variability on timescales of hours to months for
2006. The model is driven by tides, wind and sea level pressure, air-sea fluxes of heat, and terrestrial buoyancy fluxes associated
with river and sewage discharge. The predictive skill of the model is assessed by comparing the model simulations with independent
observations of sea level from coastal tide gauges and currents from moored instruments. The simulated hydrography is also
compared against a new monthly climatology created from all available temperature and salinity observations made in the Harbour
over the last century. It is shown that the model can reproduce accurately the main features of the observed tides and storm
surge, seasonal mean circulation and hydrography, and wind driven variations. The model is next used to examine the main physical
processes controlling the circulation and hydrography of the Harbour. It is shown that non-linear interaction between tidal
currents and complex topography occurs over the Narrows. The overall circulation can be characterized as a two-layer estuarine
circulation with seaward flow in the thin upper layer and landward flow in the broad lower layer. An important component of
this estuarine circulation is a relatively strong, vertically sheared jet situated over a narrow sill connecting the inner
Harbour to the deep and relatively quiescent Bedford Basin. Local wind driven variability is strongest in winter as expected
but it is also shown that a significant part of the temperature and salinity variability is driven by physical processes occurring
on the adjacent inner continental shelf, especially during storm and coastal upwelling events. 相似文献
99.
Daisuke Hasegawa Jinyu Sheng David A. Greenberg Keith R. Thompson 《Ocean Dynamics》2011,61(11):1845-1868
The Bay of Fundy in eastern Canada has the highest tides in the world. Harnessing the tidal energy in the region has long
been considered. In this study, the effects of tidal in-stream energy extraction in the Minas Passage on the three-dimensional
(3D) tidal circulation in the Bay of Fundy (BoF) and the Gulf of Maine (GoM) are examined using a nested-grid coastal ocean
circulation model based on the Princeton Ocean Model (POM). The nested-grid model consists of a coarse-resolution (~4.5 km)
parent sub-model for the GoM and a high-resolution (~1.5 km) child sub-model for the BoF. The tidal in-stream energy extraction
in the model is parameterized in terms of nonlinear Rayleigh friction in the momentum equation. A suite of numerical experiments
are conducted to determine the ranges of extractable tidal in-stream energy and resulting effects on the 3D tidal circulation
over the Bay of Fundy and the Gulf of Maine (BoF-GoM) in terms of the Rayleigh friction coefficients. The 3D model results
suggest that the maximum energy extraction in the Minas Passage increases tidal elevations and tidal currents throughout the
GoM and reduces tidal elevations and circulation in the upper BoF, especially in the Minas Basin. The far-field effect of
tidal energy extraction in the Passage on the 3D tidal circulation in the BoF-GoM is examined in two cases of harnessing tidal
in-stream energy from (a) the entire water column and (b) the lower water column within 20 m above the bottom in the Passage.
The 3D model results demonstrate that tidal in-stream energy extraction from the lower water column has less impact on the
tidal elevations and circulation in the BoF-GoM than the energy extraction from the whole water column in the Minas Passage. 相似文献
100.
Richard Greenberg 《Icarus》2004,167(2):313-319
The dilemma of the surface-area budget on Europa is resolved by identification of sites of crustal convergence, which have balanced the continual and common creation of new surface along dilational bands and pull-aparts. Convergence bands are characterized by a distinctive, albeit subdued, morphology. The prominent, unusual lineament Agenor is one of several examples. We also find diametrically opposite Agenor a similar bright linear feature surrounded by markings that allow reconstruction, which shows it to be a convergence feature. Until recently, identification of convergence sites was difficult because these features are subtle and do not exhibit structures (like the Himalayas or plate subduction) familiar from convergence of thick solid crusts on terrestrial planets. 相似文献