全文获取类型
收费全文 | 1687篇 |
免费 | 117篇 |
国内免费 | 82篇 |
专业分类
测绘学 | 90篇 |
大气科学 | 99篇 |
地球物理 | 452篇 |
地质学 | 880篇 |
海洋学 | 81篇 |
天文学 | 129篇 |
综合类 | 28篇 |
自然地理 | 127篇 |
出版年
2024年 | 3篇 |
2023年 | 13篇 |
2022年 | 56篇 |
2021年 | 60篇 |
2020年 | 67篇 |
2019年 | 78篇 |
2018年 | 150篇 |
2017年 | 130篇 |
2016年 | 176篇 |
2015年 | 81篇 |
2014年 | 182篇 |
2013年 | 173篇 |
2012年 | 92篇 |
2011年 | 103篇 |
2010年 | 61篇 |
2009年 | 73篇 |
2008年 | 57篇 |
2007年 | 39篇 |
2006年 | 54篇 |
2005年 | 30篇 |
2004年 | 23篇 |
2003年 | 25篇 |
2002年 | 23篇 |
2001年 | 18篇 |
2000年 | 15篇 |
1999年 | 6篇 |
1998年 | 13篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1978年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有1886条查询结果,搜索用时 31 毫秒
201.
P. D. Safai S. Kewat G. Pandithurai P. S. Praveen K. Ali S. Tiwari P. S. P. Rao K. B. Budhawant S. K. Saha P. C. S. Devara 《Journal of Atmospheric Chemistry》2008,61(2):101-118
Simultaneous measurements on physical, chemical and optical properties of aerosols over a tropical semi-arid location, Agra
in north India, were undertaken during December 2004. The average concentration of total suspended particulates (TSP) increased
by about 1.4 times during intense foggy/hazy days. Concentrations of SO4
2−, NO3
−, NH4
+ and Black Carbon (BC) aerosols increased by 4, 2, 3.5 and 1.7 times, respectively during that period. Aerosols were acidic
during intense foggy/hazy days but the fog water showed alkaline nature, mainly due to the neutralizing capacity of NH4 aerosols. Trajectory analyses showed that air masses were predominantly from NW direction, which might be responsible for
transport of BC from distant and surrounding local sources. Diurnal variation of BC on all days showed a morning and an evening
peak that were related to domestic cooking and vehicular emissions, apart from boundary layer changes. OPAC (Optical properties
of aerosols and clouds) model was used to compute the optical properties of aerosols. Both OPAC-derived and observed aerosol
optical depth (AOD) values showed spectral variation with high loadings in the short wavelengths (<1 μm). AOD value at 0.5 μm
wavelength was significantly high during intense foggy/hazy days (1.22) than during clear sky or less foggy/hazy days (0.63).
OPAC-derived Single scattering albedo (SSA) was 0.84 during the observational period, indicating significant contribution
of absorbing aerosols. However, the BC mass fraction to TSP increased by only 1% during intense foggy/hazy days and thereby
did not show any impact on SSA during that period. A large increase was observed in the shortwave (SW) atmospheric (ATM) forcing
during intense foggy/hazy days (+75.8 W/m2) than that during clear sky or less foggy/hazy days (+38 W/m2), mainly due to increase in absorbing aerosols. Whereas SW forcing at surface (SUF) increased from −40 W/m2 during clear sky or less foggy/hazy days to −76 W/m2 during intense foggy/hazy days, mainly due to the scattering aerosols like SO4
2-. 相似文献
202.
Previous experimental studies have found that people generally misperceive the basic dynamics of renewable resources, and in particular the accumulation of greenhouse gases (GHGs) in the atmosphere. The purpose of the present laboratory experiment is to find out why people misperceive the dynamics of CO2 accumulation and how misperceptions could be avoided. Using a simulator, 242 subjects were each asked to control total global emissions of CO2 to reach a given target for the stock of CO2 in the atmosphere. Consistent with previous investigations we find a strong tendency for people to overshoot the stated goal. Furthermore, our results point out that people need help to develop proper mental models of CO2 accumulation and they need motivation to reconsider inappropriate decision heuristics. Based on these results and the literature on conceptual change a new information strategy is designed. To motivate, it imposes cognitive conflict; and to facilitate new understanding, it provides simple analogies. A new test shows promising learning effects. The results have important implications for the Intergovernmental Panel on Climate Change (IPCC), governments, and media covering the climatic change issue as well as for general education. 相似文献
203.
The 3+1 spacetime split is used in a preliminary investigation of waves propagating in a medium of isothermal plasma in the vicinity of event horizon of Reissner-Nordström planar analogue. The general relativistic magnetohydrodynamic equations for isothermal have been investigated there. The dispersion relations are obtained by using linear perturbation and Fourier analyze techniques for non-rotating and rotating, non-magnetized and magnetized environment. The wave number, phase velocity, group velocity etc. are shown to discuss the nature of the waves. 相似文献
204.
The Souss Basin in SW Morocco is filled by Pliocene–Quaternary fluvial, fluvio-lacustrine and aeolian sediments, representing an excellent archive of palaeohydrology, palaeoclimate and the effects of crustal deformation. In general these sediments indicate stream-dominated alluvial systems, influenced by fluctuations in climate (humidity/aridity). Lakes developed within the basin around the Pliocene–Pleistocene transition and persisted into the Early Pleistocene. During this early period, relatively humid conditions are indicated by the dominance of coarse-grained sedimentation in the upper reaches of fluvial systems, the existence of large lakes and the considerable sediment thicknesses in the centre of the basin. Uplift of the surrounding mountain ranges contributed to piedmont formation by providing large amounts of coarse-grained material that accumulated at the lowland margin. Climatic deterioration in the Middle Pleistocene was accompanied by progressively more irregular and disrupted fluvial regimes. These trends were evident in the Late Pleistocene and became clearer after the mid-Holocene, with aeolian activity becoming the dominant sedimentary agent. Differences between upstream and downstream depositional regimes became marked: while coarse-grained sedimentation has characterized the upper reaches of wadi catchments, fine-grained sedimentation has prevailed downstream. Hiatuses in sedimentation throughout the Pliocene and Quaternary are marked by palaeosol horizons interbedded within the sedimentary sequences, indicating alternate vegetated (stable) and unvegetated (unstable/active) phases (biostasy and ‘rhexistasy’). 相似文献
205.
Field reconnaissance of the Anti-Atlas coastline, Morocco: Fluvial and marine evidence for Late Cenozoic uplift 总被引:2,自引:2,他引:0
The available evidence regarding the disposition and chronology of Pliocene–Pleistocene fluvial terraces, coastal rock flats, raised beaches and lacustrine sediments adjoining the Anti-Atlas coastline of Morocco has been reviewed and supplemented by additional information from our own field reconnaissance. It is thus suggested that the study region has experienced uplift by 130 m since the Mid-Pliocene climatic optimum ( 3.1 Ma), by 90 m since the latest Pliocene ( 2 Ma), and by 45 m since the Mid-Pleistocene Revolution ( 0.9 Ma). Each of these phases of uplift correlates with a phase of global climate change known independently, and it is thus inferred that the observed uplift is being driven by climate through mechanisms such as erosional isostasy and the associated induced lower-crustal flow. Numerical modelling of the observed uplift history indicates that the mobile lower-crustal layer in the study region is 9 km thick, with a temperature at its base of 500 °C. The base of this mobile layer is inferred to be at 24 km depth, the deepest crust consisting of a layer of mafic underplating that does not flow under ambient conditions. The principal landform in the study region, the coastal rock platform at 60 m a.s.l., thus formed during a succession of interglacial marine highstands in the late Early Pleistocene when uplift rates were low. Although control on the ages of young sediments and landforms is currently extremely limited, being dependent on regional correlation schemes rather than on absolute dating, the study region fits the pattern, emerging worldwide, that climate change is driving the systematic growth of topographic relief evident during the Late Cenozoic. 相似文献
206.
Edwin GNOS Silvio LORENZETTI Otto EUGSTER A. J. Timothy JULL Beda A. HOFMANN Ali Al‐KATHIRI Manuel EGGIMANN 《Meteoritics & planetary science》2009,44(3):375-387
Abstract— The recently discovered Jiddat al Harasis (JaH) 073 strewn field is the largest found so far in the Sultanate of Oman, covering an area of 19 × 6 km. The 3463 single stones collected range in weight from 52.2 kg down to <1 g (total weight 600.8 kg) and show a pronounced mass sorting. The strewn field shape can be approximated by a NW‐SE‐oriented ellipsoid, indicating an atmospheric entry from SE at a low angle relative to the surface. The meteorite belongs to the L6 ordinary chondrite group and shows S4 average shock grade. Smaller stones generally show a higher weathering grade resulting in a spread from W2 and W4. Enhanced weathering of the stones causing fragmentation after the fall is observed in sandy depressions. Five 14C measurements on stones of variable size and weathering grade yielded 14C from 3.8 to 49.9 dpm/kg. Three samples give a 14C/ 10Be age consistent with about 14.4 ka. For two samples the cosmogenic, trapped, and radiogenic noble gases were measured. The ratio of the 4He and 40Ar gas retention ages of 0.29 ± 0.10 and that of the 3He and 21Ne cosmic ray exposure ages of 0.36 ± 0.08 Ma indicate that JaH 073 experienced a complex exposure history and lost 4He and 40Ar due to a major collision. Fragmentation statistics indicate a single major atmospheric disruption and an originally relatively spherical shape of the object. Assuming the material collected represents the majority of fallen mass, and 90–99% of the original weight was lost by ablation, the pre‐atmospheric minimum radius of the meteoroid with density 3.4 g cm?3would have been at least 75 cm. 相似文献
207.
A hybrid discrete–finite element model is introduced for simulation of mechanical behavior of geomaterials. The soil or rock is modeled as a system of discrete balls that interact through normal and shear springs. The balls can be bonded at the contact points to withstand the applied deviatoric stresses. The important feature of this model is that the confining walls that can be imagined for example as the surrounding membrane or the mold in a physical test are modeled by deformable finite elements. This allows simulation of laboratory test features more realistically compared to the situations where the surrounding walls are rigid. The relationships between micro- and macro-properties are investigated in this paper as well. These relationships and the corresponding curves are helpful tools in calibration of the numerical model for the macroscopic elastic properties. 相似文献
208.
Stefano Urbini Iacopo Nicolosi Antonio Zeoli Sami El Khrepy Ahmed Lethy Mahfooz Hafez Mohamed El Gabry Ahmed El Barkooky Aly Barakat Mahomoud GOMAA Ali M. Radwan Mohamed El Sharkawi Massimo D’orazio Luigi Folco 《Meteoritics & planetary science》2012,47(11):1842-1868
Abstract– We detail the Kamil crater (Egypt) structure and refine the impact scenario, based on the geological and geophysical data collected during our first expedition in February 2010. Kamil Crater is a model for terrestrial small‐scale hypervelocity impact craters. It is an exceptionally well‐preserved, simple crater with a diameter of 45 m, depth of 10 m, and rayed pattern of bright ejecta. It occurs in a simple geological context: flat, rocky desert surface, and target rocks comprising subhorizontally layered sandstones. The high depth‐to‐diameter ratio of the transient crater, its concave, yet asymmetric, bottom, and the fact that Kamil Crater is not part of a crater field confirm that it formed by the impact of a single iron mass (or a tight cluster of fragments) that fragmented upon hypervelocity impact with the ground. The circular crater shape and asymmetries in ejecta and shrapnel distributions coherently indicate a direction of incidence from the NW and an impact angle of approximately 30 to 45°. Newly identified asymmetries, including the off‐center bottom of the transient crater floor downrange, maximum overturning of target rocks along the impact direction, and lower crater rim elevation downrange, may be diagnostic of oblique impacts in well‐preserved craters. Geomagnetic data reveal no buried individual impactor masses >100 kg and suggest that the total mass of the buried shrapnel >100 g is approximately 1050–1700 kg. Based on this mass value plus that of shrapnel >10 g identified earlier on the surface during systematic search, the new estimate of the minimum projectile mass is approximately 5 t. 相似文献
209.
Sk. Saiyad Ali Somnath Bharadwaj Jayaram N. Chengalur 《Monthly notices of the Royal Astronomical Society》2008,385(4):2166-2174
Foreground subtraction is the biggest challenge for future redshifted 21-cm observations to probe reionization. We use a short Giant Meter Wave Radio Telescope (GMRT) observation at 153 MHz to characterize the statistical properties of the background radiation across ∼1° to subarcmin angular scales, and across a frequency band of 5 MHz with 62.5 kHz resolution. The statistic we use is the visibility correlation function, or equivalently the angular power spectrum C l . We present the results obtained from using relatively unsophisticated, conventional data calibration procedures. We find that even fairly simple-minded calibration allows one to estimate the visibility correlation function at a given frequency V 2 ( U , 0) . From our observations, we find that V 2 ( U , 0) is consistent with foreground model predictions at all angular scales except the largest ones probed by our observations where the model predictions are somewhat in excess. On the other hand, the visibility correlation between different frequencies κ( U , Δν) seems to be much more sensitive to calibration errors. We find a rapid decline in κ( U , Δν) , in contrast with the prediction of less than 1 per cent variation across 2.5 MHz. In this case, however, it seems likely that a substantial part of the discrepancy may be due to limitations of data reduction procedures. 相似文献
210.
The Crab Pulsar was observed at 1540 MHz with the 25m radio telescope at Urumqi with a filterbank de-dispersion backend. A total of 2436 giant pulses with pulse energies larger than 4300 Jy μs were detected in two observing sets. All of these giant pulses are located in the main pulse (MP) and inter pulse (IP) windows of the average profile of the Crab Pulsar. The ratio of the numbers of giant pulses detected in the IP and MP windows is about 0.05. Our results show that, at 1540 MHz, the emission in the IP is contributed by giant and normal pulses, while that in the MP is almost dominated by giant pulses. The distribution of energy of the 2436 giant pulses at 1540 MHz can be described by a power-law with index α=3.13±0.09. The intrinsic threshold of giant pulse energy in the MP window is about 1400 Jy μs at 1540 MHz. 相似文献