全文获取类型
收费全文 | 838篇 |
免费 | 37篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 30篇 |
大气科学 | 48篇 |
地球物理 | 216篇 |
地质学 | 225篇 |
海洋学 | 134篇 |
天文学 | 174篇 |
综合类 | 5篇 |
自然地理 | 47篇 |
出版年
2021年 | 9篇 |
2020年 | 12篇 |
2019年 | 5篇 |
2018年 | 15篇 |
2017年 | 18篇 |
2016年 | 19篇 |
2015年 | 16篇 |
2014年 | 21篇 |
2013年 | 44篇 |
2012年 | 19篇 |
2011年 | 30篇 |
2010年 | 31篇 |
2009年 | 40篇 |
2008年 | 32篇 |
2007年 | 32篇 |
2006年 | 39篇 |
2005年 | 19篇 |
2004年 | 27篇 |
2003年 | 26篇 |
2002年 | 21篇 |
2001年 | 17篇 |
2000年 | 20篇 |
1999年 | 14篇 |
1998年 | 20篇 |
1997年 | 13篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 17篇 |
1991年 | 10篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 13篇 |
1986年 | 7篇 |
1985年 | 21篇 |
1984年 | 32篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 22篇 |
1980年 | 13篇 |
1979年 | 10篇 |
1978年 | 13篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1974年 | 7篇 |
1973年 | 11篇 |
1971年 | 5篇 |
1966年 | 3篇 |
排序方式: 共有879条查询结果,搜索用时 31 毫秒
791.
Observations show that at middle and high latitudes, the magnitude of stochastic wind stress forcing due to atmospheric weather is comparable to that of the seasonal cycle and will likely exert a significant influence on the ocean circulation. The focus of this work will be the contribution of the North Atlantic Oscillation (NAO) to the stochastic forcing in the North Atlantic and its influence on the large-scale, wind-driven ocean circulation. To this end, a QG model of the North Atlantic Ocean was forced with the stochastic component of wind stress curl associated with the NAO signal. The ocean response is localized primarily in the western boundary region and can be conveniently understood using generalized stability analysis. Much of the variability is associated with the nonnormal influence of the bathymetry and inhomogeneities in the western boundary flow on the large-scale circulation. A more traditional statistical analysis of the circulation, however, reveals that there are very small and insignificant correlations between the NAO forcing and the ocean response within the western boundary region. This suggests that the dynamics of the ocean response to stochastic forcing may obscure any obvious coherence between the forcing and the response which is equally difficult to identify from observations. 相似文献
792.
Atmospheric nitrogen deposition in the northern Chihuahuan desert: Temporal trends and potential consequences 总被引:9,自引:0,他引:9
S. Bez J. Fargione D.I. Moore S.L. Collins J.R. Gosz 《Journal of Arid Environments》2007,68(4):640-651
Rates and impacts of atmospheric nitrogen (N) deposition are poorly understood in arid land ecosystems where soils are typically low in plant available N. To address this issue, we quantified long-term trends in N deposition and estimated its impact on plant community structure in the northern Chihuahuan desert of Central New Mexico, USA. Annual and seasonal rates of N deposition were strongly positively correlated with precipitation. When precipitation effects were removed statistically, N deposition increased at an annual rate of 0.049 kg ha−1 yr−1 between 1989 and 2004. Based on two independent fertilization studies at our desert grassland field site, continued atmospheric inputs are likely to increase grass cover, decrease legume abundance, and may favor blue grama (Bouteloua gracilis) at the expense of the current dominant species, black grama (Bouteloua eriopoda). We conclude that, although arid lands have low rates of N deposition and are primarily water limited, observed trends in N deposition rates may lead to significant changes in plant community structure. 相似文献
793.
Rosemary Charlton Rowan Fealy Sonja Moore John Sweeney Conor Murphy 《Climatic change》2006,74(4):475-491
Estimates are made of changes in effective runoff at a high spatial resolution for the island of Ireland under different climate
change scenarios. The first part of the investigation examines changes in annual and seasonal effective runoff for the whole
land area of Ireland. The rainfall-runoff model HYSIM is used to carry out the hydrological simulations. The output from the
HadCM3 Global Climate Model (GCM) is downscaled using statistical techniques to provide precipitation and evaporation data
at a 10 km × 10 km resolution; this data is then used to drive the HYSIM model. Simulations are carried out for each of the
825 10 km × 10 km grid cells covering Ireland for the baseline period (1961–1990) and two future scenarios; 2041–2070 and
2061–2090. Parameter values are derived for each square using data from the Soil Survey of Ireland and the CORINE land use
database and validation is carried out for selected catchments. The results of these simulations indicate a decrease in annual
runoff that is most marked in the east and southeast of the country, whereas an increase is likely for the extreme northwest.
The reduction in effective runoff for the east of the country is particularly marked during the summer months. It is these
areas that have highest population density and also where greatest population growth is likely to occur. During the winter
months an increase in effective runoff is suggested for the western half of country which could have implications for flood
frequency, as well as the extent and duration of winter flooding. 相似文献
794.
Four bogs in New Zealand were investigated in order to understand the relationship between peat type and depositional environment. This relationship is important because peat type translates into coal type, and coal types can ultimately be used to infer how and under what conditions the original peat bog formed. In our study, no correlation was found between peat type and depositional environment in the four bogs examined. Moreover, no correlation was found between peat type and either tectonic setting or climate. Water table level and degree of fluctuation are the only parameters which seem to have a good causative relationship on peat type.The bogs, Whangamarino, Moanatuatua and Kopouatai in the North Island and Sponge Swamp in the South Island, all have different depositional settings ranging from coastal plain, to fluvial-meandering and fluvial-braided river floodplain. We found no diagnostic peat types that would allow those different environments to be distinguished from studies of the peat. Data from other tropical and temperate climate peat bogs also support our contention that no diagnostic peat types can distinguish particular depositional settings. However, the level and variability of water table does have a correlation, one that is also seen in bogs elsewhere.From our observations, we infer that the validity of using maceral ratios (directly related to coal type) to indicate depositional environment should be questioned. At best, coal type only represents to what degree the original plant components were degraded, but not how they were degraded. To infer other parameters such as depositional environment, tectonic setting or climate, other data (e.g. distribution of surrounding sediment types, palynology, etc.) must be collected and assessed. 相似文献
795.
By using two spacecraft equipped with multi-bandpass X-ray telescopes, it is possible to obtain direct 3-dimensional morphology of coronal structures which is essential for understanding the energetics and dynamics of the solar atmosphere. X-ray observations taken only in orbit about the Earth are inadequate to fully resolve the 3-dimensional nature of the solar corona. These Earth-orbit observations produce 2-dimensional images and an appropriate model must be included to derive the 3-dimensional structures from the line-of-sight information. Stereoscopic observations from space will remove this limitation and are needed if we are to improve our knowledge of the 3-dimensional morphology of the corona.Several important points regarding a stereoscopic mission are investigated and illustrated using model coronal flux tubes and image-rendering techniques. Synthesized images are formed by integrating the emission from volume elements along the line-of-sight path through a 3-dimensional volume in which a set of model flux tubes are located. The flux tubes are defined by (1) a plasma model defining the emissivity for a specific density, temperature, and pressure distribution, and (2) a magnetic field model from which a set of field lines are selected to define the geometry of the flux tubes. The field lines are used to define the flux-tube volume by assuming an initial base radius and conservation of flux. An effective instrumental spectral-response function is folded into the integration. Analysis of pairs of these synthesized images with various angular perspectives are used to investigate the effect of angular separation on mission objectives. The resulting images and analysis provide guidelines for developing a stereoscopic mission.Our study produced four important results, namely: (1) An angular separation of 30 degrees maximizes the scientific return by direct triangulation analysis because of the tradeoff between increased line-of-sight resolution of position and decreased recognition of individual loop structures arising from the overlapping of multiple loops with increasing angular separation. (2) The analysis benefits from the use of time-differential images to select flux tubes from the collection of numerous overlapping systems by selecting only recently heated or cooled flux tubes. (3) An analysis needs to be developed for algebraic reconstruction techniques applying a priori information, specific to the solar coronal structures, i.e., flux-tube continuity, maximum emission strength, non-negative emission, previous history, and maximum gradients of emission. (4) An analysis strategy combining triangulation, modeling techniques, and algebraic restoration is necessary to derive a complete understanding of the 3-dimensional morphology of the magnetic field. In the same way that helioseismology is classical viewing of the Sun with a tailored set of analysis tools for probing the interior of the Sun, heliostereoscopy is classical viewing of the X-ray emitting corona and requires a tailored set of analysis tools to deduce the true 3-dimensional structure of the corona. 相似文献
796.
Moist potential voracity (MPV) and Its generation may be important in the development of mesoscale structures such as rainbands within cyclones. In an adiabatic and frictionless flow, MPV generation is possible if the flow is three-dimensional and the air is unsaturated. Moist potential vorticity can be generated through the combined effects of gradients in the potential temperature and moisture fields. The diagnosis of MPV generation in an extratropical cy-clone was performed with the ECMWF objectively analyzed fields for a system that developed during February 1992. It was found that at various stages during the development of the cyclone, negative MPV was generated: at the north end of the cold front; along the occluded front and the cold front; and in the region of the warm core. This pattern of negative MPV generation is in excellent agreement with the predictions of previous theoretical and numerical studies. After the cyclone ceased to deepen, the region of negative MPV generated in the cyclone was horizontally adverted in-to a saturated area. The area of negative MPV generated both along the occluded front in this case study and in the region of the bent-back warm front in a numerical simulation showed a mesoscale structure with a width of about 200-500 km. It was found that the intrusion of moist or dry air into baroclinic zones was important for MPV genera-tion. In addition, baroclinicity increase (adjacent to the area of condensation) in the regions of high moisture gra-dients led to significant MPV production. 相似文献
797.
We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO2) and a basaltic andesite (55 wt% SiO2) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst
assemblages of the lavas, is found in water saturated liquids, suggesting that the CO2 content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite,
plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with
a water content of 3.0–5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with
only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars pressure with a dissolved
water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst
assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a
wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition
with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures
below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the
andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages
for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of
initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite
parent by hornblende-plagioclase fractionation under the same hydrous conditions.
Received: 10 December 1996 / Accepted: 21 August 1997 相似文献
798.
799.
J. McMahon Moore 《Mineralium Deposita》1978,13(1):123-129
The district of Mahawiyah in the Proterozoic shield of Arabia contains a group of Zn-Cu-Au-Ag-Ba mineral prospects in folded meta-sedimentary, volcanoclastic and volcanic rocks, ranging in composition from basalt to rhyolite. The mineralization occurs in veins and as strata-bound, disseminated orebodies associated with intense argillic alteration of adjacent rocks. An intrusive rhyolite dome or laccolith is situated at the centre of an eight square kilometre area of slight but pervasive alteration whose outline can be traced from aerial photographs and within which many of the ore mineral occurrences lie. A model is proposed to explain the pattern of alteration in the volcano-sedimentary pile and formation of the volcanogenic mineralization, based on a concept of the dome acting as a heat source to drive a geothermal "cell". Circulating connate-hydrothermal fluids could have caused alteration and redistribution of trace metals within the volcanics and sediments which mantle the sub-volcanic, rhyolite intrusion. The ore genetic model implies that clusters of veins, disseminated strata-bound and stratiform massive sulphide orebodies occur in distinct areas of the shield, marked by tracts of pervasive alteration which can be identified in aerial photographs and satellite images. 相似文献
800.