全文获取类型
收费全文 | 72772篇 |
免费 | 724篇 |
国内免费 | 558篇 |
专业分类
测绘学 | 1962篇 |
大气科学 | 4473篇 |
地球物理 | 13383篇 |
地质学 | 28935篇 |
海洋学 | 6168篇 |
天文学 | 16077篇 |
综合类 | 272篇 |
自然地理 | 2784篇 |
出版年
2022年 | 536篇 |
2021年 | 829篇 |
2020年 | 897篇 |
2019年 | 1016篇 |
2018年 | 4159篇 |
2017年 | 3725篇 |
2016年 | 3259篇 |
2015年 | 1072篇 |
2014年 | 1952篇 |
2013年 | 3109篇 |
2012年 | 2889篇 |
2011年 | 4560篇 |
2010年 | 4069篇 |
2009年 | 4707篇 |
2008年 | 3966篇 |
2007年 | 4569篇 |
2006年 | 2679篇 |
2005年 | 1853篇 |
2004年 | 1799篇 |
2003年 | 1770篇 |
2002年 | 1585篇 |
2001年 | 1319篇 |
2000年 | 1235篇 |
1999年 | 930篇 |
1998年 | 925篇 |
1997年 | 931篇 |
1996年 | 652篇 |
1995年 | 700篇 |
1994年 | 695篇 |
1993年 | 563篇 |
1992年 | 562篇 |
1991年 | 518篇 |
1990年 | 596篇 |
1989年 | 496篇 |
1988年 | 473篇 |
1987年 | 505篇 |
1986年 | 400篇 |
1985年 | 586篇 |
1984年 | 606篇 |
1983年 | 587篇 |
1982年 | 555篇 |
1981年 | 498篇 |
1980年 | 522篇 |
1979年 | 398篇 |
1978年 | 431篇 |
1977年 | 395篇 |
1976年 | 341篇 |
1975年 | 357篇 |
1974年 | 337篇 |
1973年 | 358篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
51.
Projections of climate change are essential to guide sustainable development plans in the tropical Andean countries such as Peru. This study assessed the projections of precipitation and potential evaporation, rain erosive potential, and precipitation concentration in the Mantaro River Basin, in the Peruvian Andes, which is important for agriculture and energy production in Peru. We assumed the Intergovernmental Panel on Climate Change (IPCC) A1B greenhouse gas emission scenario and simulated the global climate change by the HadCM3 global climate model. Due to the steepness of the mountain slopes and the narrowness of the river valley, this study uses the downscaling of the global model simulations by the regional Eta model down to 20-km resolution. The downscaling projections show decrease in the monthly precipitation with respect to the baseline period, especially during the rainy season, between February and April, until the end of the 21st century. Meanwhile, a progressive increase in the monthly evaporation from the baseline period is projected. The Modified Fournier Index (MFI) shows a statistically significant downward trend in the Mantaro River Basin, which suggests a possible reduction in the rain erosive potential. The Precipitation Concentration Index (PCI) shows a statistically significant increasing trend, which indicates increasingly more irregular temporal distribution of precipitation towards the end of the century. The results of this study allow us to conclude that there should be a gradual increase in water deficit and precipitation concentration. Both changes can be negative for agriculture, power generation, and water supply in the Mantaro River Basin in Peru. 相似文献
52.
53.
In this paper the steady lateral growth of three-dimensional turbulent inclined turbidity current is investigated. To simulate the current, an experimental setup is developed to analyze the turbidity current for different regimes in the particle laden density currents environment. The Buckingham's π theorem together with a dimensional analysis is implemented to derive the appropriate non-dimensional variables. The experimental results were normalized and plotted in the form of non-dimensional graphs from which a theoretical model is developed and analyzed. Based on the results obtained for the steady lateral growth, three different regimes, namely, inertia-viscous one as the first regime, buoyancy-viscous and gravity-viscous as the second and third regimes are distinguished within the current.In these regimes, the force balance is between the driving and resisting forces. Namely, in the first regime, the force balance is between the inertia and viscous forces, in the second regime, the buoyancy and viscous forces, and in the third regime, gravity and viscous forces are balanced. The experimental results indicate that the lateral growth rate in the first regime is smaller than that in the second and third regimes due to the magnitude and type of the forces involved in those regimes. According to the graphical results, the three different lateral growth rates appear when the normalized current length is smaller than about 3, between about 3 and 10, and larger than about 10. In those regions,the slopes of the data are different with respect to one another. 相似文献
54.
Impact angle plays a significant role in determining the fate of the projectile. In this study, we use a suite of hypervelocity impact experiments to reveal how impact angle affects the preservation, distribution, and physical state of projectile residues in impact craters. Diverse types of projectiles, including amorphous silicates, crystalline silicates, and aluminum, in two sizes (6.35 and 12.7 mm), were launched into blocks of copper or 6061 aluminum at speeds between 1.9 and 5.7 km s−1. Crater interiors preserve projectile residues in all cases, including conditions relevant to the asteroid belt. These residues consist of projectile fragments or projectile-rich glasses, depending on impact conditions. During oblique impacts at 30° and 45°, the uprange crater wall preserves crystalline fragments of the projectile. The fragments of water-rich projectiles such as antigorite remain hydrated. Several factors contribute to enhanced preservation on the uprange wall, including a weaker shock uprange, uprange acceleration as the shock reflects off the back of the projectile, and rapid quenching of melts along the projectile–target interface. These findings have two broader implications. First, the results suggest a new collection strategy for flyby sample return missions. Second, these results predict that the M-type asteroid Psyche should bear exogenic, impactor-derived debris. 相似文献
55.
Fully nonlinear features of heavy ion-acoustic solitary waves (HIASWs) have been investigated in an astrophysical degenerate relativistic quantum plasma (ADRQP) containing relativistically degenerate electrons and non-relativistically degenerate light ion species, and non-degenerate heavy ion species. The pseudo-energy balance equation is derived from the fluid dynamical equations by adopting the well-known Sagdeev-potential approach, and the properties of arbitrary amplitude HIASWs are examined. The small amplitude limit for the propagation of HIASWs is also recovered. The basic features (width, amplitude, polarity, critical Mach number, speed, etc.) of HIASWs are found to be significantly modified by the relativistic effect of the electron species, and also by the variation of the number density of electron, light ion, and heavy ion species. The basic properties of HIASWs, that may propagated in some realistic astrophysical plasma systems (e.g., in white dwarfs), are briefly discussed. 相似文献
56.
Izvestiya, Atmospheric and Oceanic Physics - The problem of atmospheric correction for shortwave channels of a multispectral low-resolution scanning radiometer onboard the Meteor-M No. 2 satellite... 相似文献
57.
We present our observations of the galaxy UGS 5600 with a long-slit spectrograph (UAGS) and a multipupil field spectrograph (MPFS) attached to the 6-m Special Astrophysical Observatory telescope. Radial-velocity fields of the stellar and gaseous components were constructed for the central region and inner ring of the galaxy. We proved the existence of two nearly orthogonal kinematic subsystems and conclude that UGC 5600 is a galaxy with an inner polar ring. In the circumnuclear region, we detected noncircular stellar motions and suspected the existence of a minibar. The emission lines are shown to originate in H II regions. We estimated the metallicity from the intensity ratio of the [N II]λ6583 and Hα lines to be nearly solar, which rules out the possibility that the polar ring was produced by the accretion of gas from a dwarf companion. 相似文献
58.
59.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars. 相似文献
60.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk. 相似文献