首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   837篇
  免费   31篇
  国内免费   8篇
测绘学   13篇
大气科学   49篇
地球物理   231篇
地质学   325篇
海洋学   70篇
天文学   79篇
综合类   5篇
自然地理   104篇
  2023年   3篇
  2022年   3篇
  2021年   8篇
  2020年   13篇
  2019年   14篇
  2018年   23篇
  2017年   20篇
  2016年   20篇
  2015年   16篇
  2014年   19篇
  2013年   46篇
  2012年   39篇
  2011年   33篇
  2010年   22篇
  2009年   37篇
  2008年   42篇
  2007年   39篇
  2006年   40篇
  2005年   26篇
  2004年   40篇
  2003年   14篇
  2002年   32篇
  2001年   17篇
  2000年   26篇
  1999年   27篇
  1998年   30篇
  1997年   15篇
  1996年   15篇
  1995年   10篇
  1994年   10篇
  1993年   9篇
  1992年   13篇
  1991年   11篇
  1990年   13篇
  1989年   14篇
  1988年   10篇
  1987年   9篇
  1986年   6篇
  1985年   14篇
  1984年   7篇
  1983年   6篇
  1981年   6篇
  1980年   5篇
  1979年   7篇
  1978年   4篇
  1977年   7篇
  1976年   8篇
  1975年   6篇
  1974年   7篇
  1969年   4篇
排序方式: 共有876条查询结果,搜索用时 31 毫秒
91.
During the 1st Lagrangian experiment of the North Atlantic Regional Aerosol Characterisation Experiment (ACE‐2), a parcel of air was tagged by releasing a smart, constant level balloon into it from the Research Vessel Vodyanitskiy . The Meteorological Research Flight's C‐130 aircraft then followed this parcel over a period of 30 h characterising the marine boundary layer (MBL), the cloud and the physical and chemical aerosol evolution. The air mass had originated over the northern North Atlantic and thus was clean and had low aerosol concentrations. At the beginning of the experiment the MBL was over 1500 m deep and made up of a surface mixed layer (SML) underlying a layer containing cloud beneath a subsidence inversion. Subsidence in the free troposphere caused the depth of the MBL to almost halve during the experiment and, after 26 h, the MBL became well mixed throughout its whole depth. Salt particle mass in the MBL increased as the surface wind speed increased from 8 m s−1 to 16 m s−1 and the accumulation mode (0.1μm to 3.0 μm) aerosol concentrations quadrupled from 50 cm−3 to 200 cm−3. However, at the same time the total condensation nuclei (>3 nm) decreased from over 1000 cm−3 to 750 cm−3. The changes in the accumulation mode aerosol concentrations had a significant effect on the observed cloud microphysics. Observational evidence suggests that the important processes in controlling the Aitken mode concentration which, dominated the total CN concentration, included, scavenging of interstitial aerosol by cloud droplets, enhanced coagulation of Aitken mode aerosol and accumulation mode aerosol due to the increased sea salt aerosol surface area, and dilution of the MBL by free tropospheric air.  相似文献   
92.
93.
Analysis of soil samples from above and below trimlines representing the upper limit of glacial erosion at the Last Glacial Maximum demonstrates that soils with prolonged weathering histories above such trimlines yield significantly different mineral magnetic signatures from soils below trimlines. The nature of the contrast is conditioned by lithology. Basalt soils above the trimline yield significantly higher values of concentration‐dependent magnetic parameters (χ, χarm, IRM3T, soft IRM and hard IRM) than those below the trimline, due probably to transformation of non‐magnetic iron‐bearing minerals into magnetic forms. Conversely, for sandstone soils most magnetic parameters yield significantly lower values for above‐trimline samples, probably reflecting loss of ferrimagnetic minerals by dissolution and oxidation to aniferrimagnetic forms. These significant contrasts represent a new approach to validating high‐level weathering limits as periglacial trimlines cut at the Last Glacial Maximum. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
94.
95.
Cosmic ray exposure ages of frost-weathered bedrock from mountain summits in the Outer Hebrides exceed the age of Late Devensian glaciation. Exposure ages of most glacially-abraded bedrock surfaces at low and intermediate elevations are younger than the age of maximum Late Devensian glaciation. These results confirm that previously mapped periglacial trimlines in the Outer Hebrides define the upper limit of bedrock erosion by Late Devensian ice. They are consistent with the interpretation, based on geomorphological evidence, that the trimlines mark the approximate upper limit of a Late Devensian Outer Hebrides Ice Cap. A postglacial exposure age from the summit of Oreval (662 m) suggests that this mountain was overrun during the last glaciation, indicating thicker ice cover and a lower surface gradient west of the ice-cap divide than previously inferred. Although bedrock surfaces below the trimlines are strongly ice-moulded, some show evidence of prior cosmic ray exposure, which we attribute to limited erosion during Late Devensian glaciation. If this interpretation is correct, the youngest apparent ages from these surfaces give the most reliable dates for deglaciation, at ca. 14.5–14 ka. This implies that ice persisted at favourable sites through the warm opening phase of the Windermere Interstade. Comparison with radiocarbon-dated evidence from offshore cores suggests net ice margin retreat of ∼74 km eastwards across the adjacent shelf in > 2.3 ± 1.0 ka. The dating evidence is consistent with relatively rapid retreat of calving margins to the coast, then slower withdrawal of ice margins to high ground. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
96.
Palaeoclimates across Europe for 6000 y BP were estimated from pollen data using the modern pollen analogue technique constrained with lake-level data. The constraint consists of restricting the set of modern pollen samples considered as analogues of the fossil samples to those locations where the implied change in annual precipitation minus evapotranspiration (PE) is consistent with the regional change in moisture balance as indicated by lakes. An artificial neural network was used for the spatial interpolation of lake-level changes to the pollen sites, and for mapping palaeoclimate anomalies. The climate variables reconstructed were mean temperature of the coldest month (T c ), growing degree days above 5  °C (GDD), moisture availability expressed as the ratio of actual to equilibrium evapotranspiration (α), and PE. The constraint improved the spatial coherency of the reconstructed palaeoclimate anomalies, especially for PE. The reconstructions indicate clear spatial and seasonal patterns of Holocene climate change, which can provide a quantitative benchmark for the evaluation of palaeoclimate model simulations. Winter temperatures (T c ) were 1–3 K greater than present in the far N and NE of Europe, but 2–4 K less than present in the Mediterranean region. Summer warmth (GDD) was greater than present in NW Europe (by 400–800 K day at the highest elevations) and in the Alps, but >400 K day less than present at lower elevations in S Europe. PE was 50–250 mm less than present in NW Europe and the Alps, but α was 10–15% greater than present in S Europe and PE was 50–200 mm greater than present in S and E Europe. Received: 3 January 1996 / Accepted: 15 July 1996  相似文献   
97.
Fractal geostatistics are being applied to subsurface geological data as a way of predicting the spatial distribution of hydrocarbon reservoir properties. The fractal dimension is the controlling parameter in stochastic methods to produce random fields of porosity and permeability. Rescaled range (R/S)analysis has become a popular way of estimating the fractal dimension, via determination of the Hurst exponent (H). A systematic investigation has been undertaken of the bias to be expected due to a range of factors commonly inherent in borehole data, particularly downhole wireline logs. The results are integrated with a review of previous work in this area. Small datasets. overlapping samples, drift and nonstationariry of means can produce a very large bias, and convergence of estimates of H around 0.85–0.90 regardless of original fractal dimension. Nonstationarity can also account for H>1, which has been reported in the literature but which is theoretically impossible for fractal time series. These results call into question the validity of fractal stochastic models built using fractal dimensions estimated with the R/Smethod.  相似文献   
98.
Lubbock Lake (Southern High Plains of Texas) contains a cultural, faunal, and floral record within a virtually complete geological record spanning the past 11 100+ years. More than 88 archaeological occurrences have been excavated from five major stratigraphic units. The Paleoindian record (11 500–6500yr BP) begins with Clovis-age occupation (ca. 11 100yr BP) found within fluvial deposits (stratum 1). Subsequent Paleoindian occupations are found in lake and marsh sediments (stratum 2). Archaic occupations (8500-2000yr BP) are contained within aeolian and marsh deposits (strata 3 and 4). Ceramic occupations (2000-500yr BP) are found on a soil developed in stratum 4, in marsh sediments (strata 4 and 5), and in slopewash and aeolian sediments (stratum 5). The Protohistoric (500-300yr BP) and Historic (300-100yr BP) remains are in slopewash, aeolian, and marsh sediments (stratum 5) and associated soils. The Southern High Plains remained a grasslands throughout the last 11 500 years and neither man nor bison abandoned the region. The successive local faunas reflect changing ecosystems under pluvial to arid to more mesic to semiarid conditions. The occupation of Lubbock Lake through time appears to have been by small groups of people for both economic and short-term residential uses. These hunter-gatherer peoples underwent adaptive change brought about by climatic stress and alterations to food resources.  相似文献   
99.
100.
A physically-based semidistributed model, TOPMODEL, is applied to a 340 hectare spruce forested catchment in mid-Wales. the model is calibrated to runoff and the subsurface flow is divided into components of flow identified by depth of origin. in addition, chemical mixing techniques are used to provide a hydrograph separation between acidic soil waters and well buffered deep waters. the short-term variations in the components of flow identified by these two approaches compare well.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号