全文获取类型
收费全文 | 988篇 |
免费 | 118篇 |
国内免费 | 135篇 |
专业分类
测绘学 | 85篇 |
大气科学 | 317篇 |
地球物理 | 176篇 |
地质学 | 205篇 |
海洋学 | 134篇 |
天文学 | 99篇 |
综合类 | 54篇 |
自然地理 | 171篇 |
出版年
2024年 | 6篇 |
2023年 | 12篇 |
2022年 | 21篇 |
2021年 | 33篇 |
2020年 | 34篇 |
2019年 | 42篇 |
2018年 | 28篇 |
2017年 | 36篇 |
2016年 | 50篇 |
2015年 | 38篇 |
2014年 | 50篇 |
2013年 | 55篇 |
2012年 | 54篇 |
2011年 | 70篇 |
2010年 | 62篇 |
2009年 | 81篇 |
2008年 | 58篇 |
2007年 | 60篇 |
2006年 | 62篇 |
2005年 | 38篇 |
2004年 | 45篇 |
2003年 | 46篇 |
2002年 | 35篇 |
2001年 | 29篇 |
2000年 | 23篇 |
1999年 | 23篇 |
1998年 | 21篇 |
1997年 | 18篇 |
1996年 | 16篇 |
1995年 | 18篇 |
1994年 | 17篇 |
1993年 | 9篇 |
1992年 | 15篇 |
1991年 | 9篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1988年 | 1篇 |
1987年 | 6篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1977年 | 2篇 |
排序方式: 共有1241条查询结果,搜索用时 250 毫秒
1.
Frdric Zagury 《New Astronomy》2001,6(8):345-486
This paper examines the relationship between spectra of stars of same spectral type with extremely low reddenings. According to the standard theory, the relationship between the spectrum of stars with same spectral type and small, but different reddenings should be different in the optical and in the UV. This difference is not observed: the ratio of the spectra of two stars in directions where the reddening is large enough to be detected and low enough not to give a noticeable 2200 Å bump is an exponential of 1/λ from the near-infrared to the far-UV. This result is in conformity with the ideas introduced in preceding papers: the exponential optical extinction extends to the UV, and the spectrum of stars with enough reddening is contaminated by light scattered at close angular distance from the stars. An application will be the determination of the spectrum of a non-reddened star from the spectrum of a star of same spectral type with little reddening. 相似文献
2.
3.
The characteristics of Asian dust events in Northeast Asia during the springtime from 1993 to 2004 总被引:3,自引:1,他引:2
The characteristics of Asian dust events that occurred in Northeast Asia during the springtime from 1993 to 2004 are investigated using 3-hourly SYNOP reports (World Meteorological Organization). Occurrences of blowing sand and dust storm are low in 1997 and 1999, but have increased rapidly since 2000. The maximum occurrence was recorded in 2002. Wind velocity of 6.5 m s− 1 as a threshold wind velocity is not so exactly consistent with the occurrence of blowing sand. However, wind velocity of 14 m s− 1 as a strong wind causing dust storm had similar tendency to those of dust storm and Dust Storm Index.Source regions of Asian dust are divided into three regions (A: dry arid, B: semi-arid, and C: cultivated), based upon the occurrence of blowing sand and dust storm. Eight meteorological stations are selected in three regions, which have frequent occurrences of blowing sand. Source regions of Asian dust that affect the Korean peninsula are gradually extending eastward. Positive anomalies of NDVI occurred in 1994, 1995, and 1998 when temperature was high and precipitation was heavy. However, the frequent occurrence of the dust phenomena is not always consistent with lots of vegetation, high temperature, and much precipitation in this study. 相似文献
4.
Lightning discharge generated in the protoplanetary nebula is viewed as a temporally isolated surge in the flow of electrically charged particles, similar to that of terrestrial lightning. If the current is intense enough, a powerful circular impulse magnetic field is generated around the instantaneous virtual electric conductor. Such magnetic field is capable of magnetizing dust grains containing ferromagnetic components present in its vicinity to their saturation levels. As a result, dust grains attract one another, forming the aggregates. This magnetically driven attraction suggests an important process possibly operational at an early stage of the planetary accretion. Based on both a classical model for electric conductor, and the theory of Lienard–Wiechert electromagnetic potentials, our calculations show that the magnetic impulse due to a discharge channel of a few cm in diameter transferring a charge of about 104 electrons reaches as high as 10 T. At these magnetic fields, the ferromagnetic dust grains, and possibly the already-formed larger aggregates as well, are easily magnetized to the saturation levels, producing compact clusters exhibiting permanent magnetic moments. 相似文献
5.
Gregory P. Marchildon Suren Kulshreshtha Elaine Wheaton Dave Sauchyn 《Natural Hazards》2008,45(3):391-411
Agriculture in the southern Great Plains of Canada has been particularly vulnerable to prolonged episodes of drought. Using
climate data and a precipitation minus potential evapotranspiration index, the extent of the region’s exposure to drought
is examined. Between 1914 and 1917, the Dry Belt was particularly vulnerable to drought, whereas after 1928, a much larger
region known as the Palliser Triangle covering most of southern Alberta and Saskatchewan was much more exposed to drought.
These droughts provoked major institutional adaptation, in particular the establishment of the Special Areas Board by the
Government of Alberta, and the creation of the Prairie Farm Rehabilitation Administration by the Government of Canada. Both
organizations have proved to be relatively permanent public adaptations to the natural hazard of drought in the region. Moreover,
these earlier experiences with prolonged drought as well as institution-building may be of value in helping the residents
of the Palliser Triangle adapt to predicted climate changes in the future as well as anticipate some of the barriers to effective
institutional adaptation. 相似文献
6.
A.R. Salgueiro P. Freire vila H. Garcia Pereira J.M. Santos Oliveira 《Journal of Geochemical Exploration》2008,98(1-2):15
Based on an environmental geochemistry case study carried out in the neighbourhood of a W–Sn abandoned mine, the pollution in stream sediments was modelled through a Global Contamination Index. Such an index permits one to summarize the combination of deleterious elements in a single variable, obtained by the projection of samples onto the first axis of a PCASD (Principal Components Analysis of Standardized Data) applied to the entire n × p matrix containing the available concentrations of p = 16 elements in the set of n = 220 collected samples.In order to provide a sound basis for a coherent planning of the remediation process which will be put in operation in the affected area, it is necessary to balance the costs of reclaiming with the probabilities of exceeding the upper limits accepted for concentrations of environmentally harmful elements in sediments. Given these limits, they are back-transformed in the index values, providing a practical threshold between ‘clean’ and ‘contaminated’ samples. On the other hand, the minimum dimension of the cell to be reclaimed is restrained by the selected remediation process to be applied in the affected area. Hence, to meet the constraints of such a remediation process, it is required to estimate the probabilities of exceeding the index threshold in technologically meaningful sub-areas. For this end, the Indicator Block Kriging technique was applied, producing a series of maps where sub-areas to be reclaimed can be spotted for different probability levels. These maps, on which the decision making remediation agency can rely for its cost-benefit analysis, take into account both the spatial structure of ‘clean’ vs. ‘contaminated’ samples and the constraints of the reclaiming process. 相似文献
7.
The influence of cyclic loading on the strength and deformation behavior of cemented marine clay has been studied. This marine clay is of recent Pleistocene origin and deposited in a shallow water marine environment. Open pits were dug in sheeted enclosures and from these pits, undisturbed samples were taken for strength testing. A series of standard triaxial shear tests and stress controlled one-way cyclic load tests were conducted at consolidation stress ranges below and above the preconsolidation pressure. For the stress levels below the preconsolidation pressure, the cyclic loading has brought about the collapse of the cementation bond through an increase in strains, and at higher pressure ranges, the soil behaves like typical soft clay. This experiment studied the rate of development of strain and pore water pressure and shows that rate is a function of number of cycles, applied stress, and stress history. In addition, soil degradation during cyclic loading is studied in terms of Degradation Index. Attempt has been made to predict stain, pore water pressure, and degradation index through an empirical model. 相似文献
8.
2.4拟庸蝶食性的对应分析2.4.1观察量和变量的分组观察量(捕食鱼类)依不同季节(冬、夏季)、不同性别(雌雄性)和不同性成熟状态(幼鱼和成鱼)来分组,因此得到8(23)个观察量。对应分析的变量(饵料)首先根据个体生态的不同分为6个类型:游走多毛类,隐居多毛类,双壳类,不大游动的甲壳类(端足目和等足目),爬行甲壳类(十足国短尾部)和棘皮动物。其他6个类型的饵料生物(类、纽虫类、小型游泳甲壳类、腔肠动物、鱼类及苔藓虫类顺在分组时剔除,其原因为它们的出现可能少了些。考虑到捕食鱼类和饵料双壳类贝壳厚度… 相似文献
9.
10.
Bridge scour modeling requires storm surge hydrographs as open ocean boundary conditions for coastal waters surrounding tidal inlets. These open coast storm surge hydrographs are used to accurately determine both horizontal and vertical circulation patterns, and thus scour, within the inlet and bay for an extreme event. At present, very little information is available on the effect that tidal inlets have on these open coast storm surge hydrographs. Furthermore, current modeling practice enforces a single design hydrograph along the open coast boundary for bridge scour models. This study expands on these concepts and provides a more fundamental understanding on both of these modeling areas. 相似文献