全文获取类型
收费全文 | 91826篇 |
免费 | 1209篇 |
国内免费 | 1329篇 |
专业分类
测绘学 | 2916篇 |
大气科学 | 7077篇 |
地球物理 | 17696篇 |
地质学 | 37033篇 |
海洋学 | 6599篇 |
天文学 | 15992篇 |
综合类 | 2272篇 |
自然地理 | 4779篇 |
出版年
2021年 | 466篇 |
2020年 | 518篇 |
2019年 | 523篇 |
2018年 | 7355篇 |
2017年 | 6623篇 |
2016年 | 4822篇 |
2015年 | 1224篇 |
2014年 | 1578篇 |
2013年 | 2830篇 |
2012年 | 3177篇 |
2011年 | 6284篇 |
2010年 | 5441篇 |
2009年 | 6207篇 |
2008年 | 5129篇 |
2007年 | 5993篇 |
2006年 | 2178篇 |
2005年 | 2219篇 |
2004年 | 2248篇 |
2003年 | 2214篇 |
2002年 | 1871篇 |
2001年 | 1439篇 |
2000年 | 1359篇 |
1999年 | 1109篇 |
1998年 | 1105篇 |
1997年 | 1107篇 |
1996年 | 890篇 |
1995年 | 884篇 |
1994年 | 856篇 |
1993年 | 695篇 |
1992年 | 687篇 |
1991年 | 625篇 |
1990年 | 687篇 |
1989年 | 615篇 |
1988年 | 578篇 |
1987年 | 670篇 |
1986年 | 585篇 |
1985年 | 765篇 |
1984年 | 809篇 |
1983年 | 835篇 |
1982年 | 749篇 |
1981年 | 699篇 |
1980年 | 733篇 |
1979年 | 600篇 |
1978年 | 622篇 |
1977年 | 562篇 |
1976年 | 548篇 |
1975年 | 545篇 |
1974年 | 521篇 |
1973年 | 514篇 |
1972年 | 338篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
D.J. Curtis C.G. Galbraith J.C. Smyth D.B.A. Thompson 《Estuarine, Coastal and Shelf Science》1985,21(1):75-90
The number of black-headed gulls (Larus ridibundus) in the Clyde Estuary is large. In summer the average density has reached 1350 gulls km?2 and in winter 180 gulls km?2. This paper compares prey selection and feeding efficiency in gulls during summer and winter on tidal flats, and considers how seasonal differences may be adaptations to cope with seasonal changes in prey availability.Gross and net rates of energy intake were highest in summer because gulls captured more of the polychaete N. diversicolor than the amphipod C. volutator. In winter, gulls selected for C. volutator and therefore an energetically less profitable diet. Throughout the year gulls selected more C. volutator relative to N. diversicolor than expected on energetic grounds and so apparently did not maximize potential net rate of energy intake.Gulls used three techniques to capture prey and made most intensive use of the ‘crouch’ technique. Crouching gulls attained a much higher net rate of energy intake than ‘upright’ or ‘paddling’ gulls.A log-linear model showed that (a) season, water depth and gull density determined feeding technique and (b) feeding technique and season independently determined foraging success and prey selection. Thus gull density and water depth acted on prey selection through imposed variations in feeding technique.Reasons for gulls selecting energetically unprofitable C. volutator and for the use of several distinct feeding techniques are discussed. 相似文献
142.
The change in the darkness of the Great Red Spot (GRS) of Jupiter (1894–1974) has been analysed with Fourier (FFT), Maximum Entropy and Power spectrum (Blackman-Tukey window) (PSA) methods of spectrum analysis. Significance, non-randomness and stationarity tests assigned high variance to periodicities of 33 ± 4, 13–15, about 11, 9 and 3 yrs. The highest correlation between solar activity and GRS darkness was found for the 14th and 16th solar cycle. The periodicities obtained are interpreted as the combined eftects of solar activity, planetary resonances and internal jovian mechanisms. 相似文献
143.
144.
Two-dimensional (2D) compressible magnetohydrodynamic simulations are performed to explore the idea that the asymmetric reconnection between newly emerging intranetwork magnetic field flux and pre-existing network flux causes the explosive events in the solar atmosphere. The dependence of the reconnection rate as a function of time on the density and temperature of the emerging flux are investigated. For a Lundquist number of L
u= 5000 we find that the tearing mode instability can lead to the formation and growth of small magnetic islands. Depending on the temperature and density ratio of the emerging plasma, the magnetic island can be lifted upward and convected out of the top boundary, or is suppressed downward and convected out of the top boundary, or is suppressed downward nad submerged below the bottom boundary. The motions of the magnetic islands with different direction are accompanied respectively with upward or downward high velocity flow which might be associated with the red- and blue-shifted components detected in the explosive events. 相似文献
145.
Tidal effects on temperature front in the Yellow Sea 总被引:5,自引:0,他引:5
Temperature front (TF) is one of the important features in the Yellow Sea, which forms in spring, thrives in summer, and fades
in autumn as thermocline declines. TF intensity ⋎S
T
⋎ is defined to describe the distribution of TF. Based on the MASNUM wave-tide-circulation coupled model, temperature distribution
in the Yellow Sea was simulated with and without tidal effects. Along 36°N, distribution of TF from the simulated results
are compared with the observations, and a quantitative analysis is introduced to evaluate the tidal effects on the forming
and maintaining processes of the TF. Tidal mixing and the circulation structure adapting to it are the main causes of the
TF.
Supported by the National Basic Research Program of China (No. G1999043809) and the National Science Foundation of China (No.
49736190). 相似文献
146.
147.
Non-Newtonian topographic relaxation on Europa 总被引:1,自引:0,他引:1
Models of topographic support on Europa by lateral shell thickness variations have previously assumed a Newtonian ice viscosity. Here I show that using a more realistic stress-dependent viscosity gives relaxation times which can be significantly different. Topography of wavelength 100 km cannot be supported by lateral shell thickness variations for ∼50 Myr, unless the shell thickness is <10 km or the ice grain size >10 mm. Shorter wavelength topography would require even thinner shells, but may be supported elastically. Global-scale variations in shell thickness, however, can be supported for geological timescales if the shell thickness is O(10 km). 相似文献
148.
After briefly recalling basic facts about the metallicity gradients inspiral galaxies, we summarize two recent N-body gas-dynamical modelsthat implement a non-instantaneous gas recycling and point our their potential for an improved treatment of the chemical evolution ingalactic disks. 相似文献
149.
150.