全文获取类型
收费全文 | 970篇 |
免费 | 40篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 88篇 |
地球物理 | 211篇 |
地质学 | 292篇 |
海洋学 | 108篇 |
天文学 | 180篇 |
综合类 | 7篇 |
自然地理 | 101篇 |
出版年
2021年 | 12篇 |
2020年 | 10篇 |
2019年 | 20篇 |
2018年 | 13篇 |
2017年 | 16篇 |
2016年 | 26篇 |
2015年 | 18篇 |
2014年 | 22篇 |
2013年 | 51篇 |
2012年 | 28篇 |
2011年 | 48篇 |
2010年 | 40篇 |
2009年 | 51篇 |
2008年 | 31篇 |
2007年 | 44篇 |
2006年 | 42篇 |
2005年 | 24篇 |
2004年 | 19篇 |
2003年 | 29篇 |
2002年 | 20篇 |
2001年 | 20篇 |
2000年 | 21篇 |
1999年 | 21篇 |
1998年 | 18篇 |
1996年 | 10篇 |
1995年 | 18篇 |
1994年 | 10篇 |
1993年 | 19篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 9篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 10篇 |
1985年 | 20篇 |
1984年 | 19篇 |
1983年 | 22篇 |
1982年 | 13篇 |
1981年 | 18篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1978年 | 15篇 |
1977年 | 13篇 |
1976年 | 12篇 |
1975年 | 16篇 |
1974年 | 8篇 |
1973年 | 16篇 |
1972年 | 7篇 |
1970年 | 6篇 |
排序方式: 共有1016条查询结果,搜索用时 187 毫秒
131.
132.
133.
Jacques Derome Gilbert Brunet André Plante Normand Gagnon George J. Boer Francis W. Zwiers 《大气与海洋》2013,51(4):485-501
Abstract Two dynamical models are used to perform a series of seasonal predictions. One model, referred to as GCM2, was designed as a general circulation model for climate studies, while the second one, SEF, was designed for numerical weather prediction. The seasonal predictions cover the 26‐year period 1969–1994. For each of the four seasons, ensembles of six forecasts are produced with each model, the six runs starting from initial conditions six hours apart. The sea surface temperature (SST) anomaly for the month prior to the start of the forecast is persisted through the three‐month prediction period, and added to a monthly‐varying climatological SST field. The ensemble‐mean predictions for each of the models are verified independently, and the two ensembles are blended together in two different ways: as a simple average of the two models, denoted GCMSEF, and with weights statistically determined to minimize the mean‐square error (the Best Linear Unbiased Estimate (BLUE) method). The GCMSEF winter and spring predictions show a Pacific/North American (PNA) response to a warm tropical SST anomaly. The temporal anomaly correlation between the zero‐lead GCMSEF mean‐seasonal predictions and observations of the 500‐hPa height field (Z500) shows statistically significant forecast skill over parts of the PNA area for all seasons, but there is a notable seasonal variability in the distribution of the skill. The GCMSEF predictions are more skilful than those of either model in winter, and about as skilful as the better of the two models in the other seasons. The zero‐lead surface air temperature GCMSEF forecasts over Canada are found to be skilful (a) over the west coast in all seasons except fall, (b) over most of Canada in summer, and (c) over Manitoba, Ontario and Quebec in the fall. In winter the skill of the BLUE forecasts is substantially better than that of the GCMSEF predictions, while for the other seasons the difference in skill is not statistically significant. When the Z500 forecasts are averaged over months two and three of the seasons (one‐month lead predictions), they show skill in winter over the north‐eastern Pacific, western Canada and eastern North America, a skill that comes from those years with strong SST anomalies of the El Niño/La Niña type. For the other seasons, predictions averaged over months two and three show little skill in Z500 in the mid‐latitudes. In the tropics, predictive skill is found in Z500 in all seasons when a strong SST anomaly of the El Niño/La Niña type is observed. In the absence of SST anomalies of this type, tropical forecast skill is still found over much of the tropics in months two and three of the northern hemisphere spring and summer, but not in winter and fall. 相似文献
134.
P. Riley M. Ben-Nun J. A. Linker Z. Mikic L. Svalgaard J. Harvey L. Bertello T. Hoeksema Y. Liu R. Ulrich 《Solar physics》2014,289(3):769-792
The observed photospheric magnetic field is a crucial parameter for understanding a range of fundamental solar and heliospheric phenomena. Synoptic maps, in particular, which are derived from the observed line-of-sight photospheric magnetic field and built up over a period of 27 days, are the main driver for global numerical models of the solar corona and inner heliosphere. Yet, in spite of 60 years of measurements, quantitative estimates remain elusive. In this study, we compare maps from seven solar observatories (Stanford/WSO, NSO/KPVT, NSO/SOLIS, NSO/GONG, SOHO/MDI, UCLA/MWO, and SDO /HMI) to identify consistencies and differences among them. We find that while there is a general qualitative consensus, there are also some significant differences. We compute conversion factors that relate measurements made by one observatory to another using both synoptic map pixel-by-pixel and histogram-equating techniques, and we also estimate the correlation between datasets. For example, Wilcox Solar Observatory (WSO) synoptic maps must be multiplied by a factor of 3?–?4 to match Mount Wilson Observatory (MWO) estimates. Additionally, we find no evidence that the MWO saturation correction factor should be applied to WSO data, as has been done in previous studies. Finally, we explore the relationship between these datasets over more than a solar cycle, demonstrating that, with a few notable exceptions, the conversion factors remain relatively constant. While our study was able to quantitatively describe the relationship between the datasets, it did not uncover any obvious “ground truth.” We offer several suggestions for how this may be addressed in the future. 相似文献
135.
The strongest observed solar magnetic fields are found in sunspot umbrae and associated light bridges. We investigate systematic
measurements of approximately 32 000 sunspot groups observed from 1917 through 2004 using data from Mt. Wilson, Potsdam, Rome
and Crimea observatories. Isolated observations from other observatories are also included. Corrections to Mt. Wilson measurements
are required and applied. We found 55 groups (0.2%) with at least one sunspot with one magnetic field measurement of at least
4000 G including five measurements of at least 5000 G and one spot with a record field of 6100 G. Although typical strong-field
spots are large and show complex structure in white light, others are simple in form. Sometimes the strongest fields are in
light bridges that separate opposite polarity umbras. The distribution of strongest measured fields above 3 kG appears to
be continuous, following a steep power law with exponent about −9.5. The observed upper limit of 5 – 6 kG is consistent with
the idea that an umbral field has a more or less coherent structure down to some depth and then fragments. We find that odd-numbered
sunspot cycles usually contain about 30% more total sunspot groups but 60% fewer >3 kG spots than preceding even-numbered
cycles. 相似文献
136.
137.
C. Hans Nelson Andres Maldonado Francis Coumes Henri Got Andre Manaco 《Geo-Marine Letters》1984,3(2-4):125-131
The Ebro Fan System consists of en echelon channel-levee complexes, 50×20 km in area and 200-m thick. A few strong reflectors
in a generally transparent seismic facies identify the sand-rich channel floors and levee crests. Numerous continuous acoustic
reflectors characterize overbank turbidites and hemipelagites that blanket abandoned channel-levee complexes. The interlobe
areas between channel complexes fill with homogeneous mud and sand from mass flow and overbank deposition; these exhibit a
transparent seismic character. The steep continental rise and sediment “drainage” of Valencia Trough at the end of the channel-levee
complexes prevent the development of distributary channels and midfan lobe deposits.
Margin setting represents fan and/or source area 相似文献
138.
Evidence for Precambrian fossil eukaryotic microorganisms has been based on: (1) the presence of internal ‘spots’ which have been variously interpreted to be remains of nuclei or pyrenoids of photosynthetic plastids or other organelles; (2) tetrahedral tetrad arrangement of cells; (3) trilete scars interpreted to be indicative of meiotic division: (4) large cell diameters; and (5) putative mitotic cell divisions. These features have been reported in fossils preserved in Precambrian cherts. We have studied modern microbial mats, thought to be analogues of Precambrian fossil communities, and found they may be silicified by laboratory procedures. In microbial mats from Baja California we have found many ‘spot cells’ that we could identify as remains of cyanophytes. We have silicified the newly discovered large prokaryotic coccoid green alga Prochloron and have found that it, like many cyanophytes previously silicified, preserves its structure and maintains its initial dimensions. In laboratory-silicified prokaryotic organisms we have found that all of the above criteria, supposedly characteristic of eukaryotes, can be observed. We conclude that there is no compelling morphological evidence for fossil eukaryotic microbes from Precambrian cherts. 相似文献
139.
Volatile fatty acid (VFA) apparent turnover rates in organic-rich marine sediments were determined by measuring whole sediment VFA concentration and the corresponding first-order reaction rate constants. In order to measure VFA concentrations, bulk wet sediment samples were basified, freeze-dried, extracted with methanol, derivatized to form methyl esters of the VFAs, and analyzed by packed-column gas chromatography using hexanoic acid as an internal standard. The detection limits for acetate, propionate, iso-butyrate and butyrate were 1.0, 0.4, 0.2 and 0.2 μmol l?1s, respectively, for 600 ml samples. Rate constants for acetate and propionate were determined by anaerobically incubating samples at in-situ temperatures with tracer levels of 14C-labelled VFAs. Metabolized label was recovered as CO2, CH4, cellular material, water-soluble material, and VFA (ether-soluble) fractions. Apparent turnover rates measured during summer and winter in anoxic Cape Lookout Bight, North Carolina (U.S.A.) sediments were in the range of 19–330 μmol l?1s h?1 for acetate and 0.7–7.0 μmol l?1s h?1 for propionate. 相似文献
140.