全文获取类型
收费全文 | 81004篇 |
免费 | 1093篇 |
国内免费 | 682篇 |
专业分类
测绘学 | 2412篇 |
大气科学 | 5871篇 |
地球物理 | 15554篇 |
地质学 | 30270篇 |
海洋学 | 6668篇 |
天文学 | 17922篇 |
综合类 | 280篇 |
自然地理 | 3802篇 |
出版年
2021年 | 485篇 |
2020年 | 557篇 |
2019年 | 698篇 |
2018年 | 3986篇 |
2017年 | 3681篇 |
2016年 | 3088篇 |
2015年 | 1105篇 |
2014年 | 1700篇 |
2013年 | 3077篇 |
2012年 | 2733篇 |
2011年 | 4720篇 |
2010年 | 4335篇 |
2009年 | 5191篇 |
2008年 | 4331篇 |
2007年 | 4842篇 |
2006年 | 2573篇 |
2005年 | 2346篇 |
2004年 | 2258篇 |
2003年 | 2170篇 |
2002年 | 1972篇 |
2001年 | 1581篇 |
2000年 | 1538篇 |
1999年 | 1335篇 |
1998年 | 1320篇 |
1997年 | 1299篇 |
1996年 | 1041篇 |
1995年 | 986篇 |
1994年 | 963篇 |
1993年 | 808篇 |
1992年 | 730篇 |
1991年 | 697篇 |
1990年 | 721篇 |
1989年 | 717篇 |
1988年 | 654篇 |
1987年 | 736篇 |
1986年 | 659篇 |
1985年 | 808篇 |
1984年 | 937篇 |
1983年 | 841篇 |
1982年 | 818篇 |
1981年 | 727篇 |
1980年 | 687篇 |
1979年 | 621篇 |
1978年 | 626篇 |
1977年 | 562篇 |
1976年 | 506篇 |
1975年 | 515篇 |
1974年 | 485篇 |
1973年 | 508篇 |
1972年 | 362篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
171.
172.
Historical Changes in the Flood Hazard at Khartoum, Sudan: Lessons and Warnings for the Future 总被引:1,自引:0,他引:1
The 1988 floods at Khartoum were frequently described as "unprecedented". However, an examination of evidence for floods caused by the Nile and rain storms during the nineteenth and twentieth centuries leads to the conclusion that neither the Nile flood nor the rain storms nor their coincidence in one season were unprecedented and that similar situations will recur.We argue that most problems were created by complacency, lack of planning and mismanagement. We conclude with some simple suggestions for flood amelioration in the future and warn that complacency is the single most important phenomenon likely to recur to the detriment of Greater Khartoum in the future. 相似文献
173.
The restricted problem of 2 + 2 bodies when one of the infinitesimal masses is further acted upon by the light pressure of the two primaries, is considered. The stationary solutions of this problem are found out. A short discussion is devoted to the stability of these solutions. 相似文献
174.
Swath bathymetric, gravity, and magnetic studies were carried out over a 55 km long segment of the Central Indian Ridge.
The ridge is characterized by 12 to 15 km wide rift valley bounded by steep walls and prominent volcanic constructional ridges
on either side of the central rift valley. A transform fault at 7°45′S displaces the ridge axis. A mantle Bouguer anomaly
low of −14 mGals and shallowing of rift valley over the middle of the ridge segment indicate along axis crustal thickness
variations. A poorly developed neovolcanic zone on the inner rift valley floor indicate dominance of tectonic extension. The
off-axis volcanic ridgs suggest enhanced magmatic activity during the recent past.
Received: 24 May 1996 / Rivision received: 13 January 1997 相似文献
175.
Pittichovâ J. Sekenina Z. Birkle K. Boehnhardt J. Engels D. Keller P. 《Earth, Moon, and Planets》1997,78(1-3):329-338
The Sekanina-Farrell particle fragmentation model for the striated tails of dust comets is successfully applied to two images
of comet Hale-Bopp to study the motions of 12 striae in a time span of March 12–15, 1997. There is evidence for recurring
outbursts with a periodicity of 11h21m, consistent with results based on analysis of dust jets. The ejecta in all the striae appear to have been released from one
source on the nucleus between the end of January and the second half of February 1997, some 60 to 40 days before perihelion.
The parent particles were subjected to a radiation pressure acceleration of βp ≃ 0.55 and their fragmentation lifetimes in 11 of the 12 striae were practically constant and equal to 13–15 days, when normalized
to 1 AU from the Sun. Brief analysis of Watanabe et al.'s measurements of striae on their images from March 5–9, 1997 shows
even shorter fragmentation lifetimes for the parent particles, mostly about 7–11 days at1 AU.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
176.
177.
178.
Theoretical line ratios involving 2s
2
S - 3p
2
P, 2p
2
P - 3s
2
S, and 2p
2
S - 3d
2
D transitions inCiv between 312 and 420 Å are presented. A comparison of these with solar active region observational data obtained during a rocket flight by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals good agreement between theory and experiment, with discrepancies that average only 22%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations, and also resolves discrepancies found previously when the theoretical results were compared with solar data from the S082A instrument on boardSkylab. The potential usefulness of theCIV line ratios as electron temperature diagnostics for the solar transition region is briefly discussed. 相似文献
179.
D.W. Crawford D.A. Purdie A.P.M. Lockwood P. Weissman 《Estuarine, Coastal and Shelf Science》1997,45(6):799-812
In the Southampton Water estuary (southern England, U.K.), red-tides caused by the planktonic, phototrophic ciliateMesodinium rubrum(=Myrionecta rubra) occur during most summers and sometimes in autumn. These events were investigated in detail between 1985 and 1987 and were characterized by levels of chlorophylla(chl a) of over 100 μg l−1, cell numbers ofM. rubrumof over 1×103 ml−1, oxygen saturations of around 150%, and depleted numbers of macrozooplankton. Initiation of red-water did not appear to be triggered by irradiance or nutrients, but coincided with an increase in temperature and water column stability. This enhanced stability was promoted by increased surface to bottom gradients of both temperature and salinity, and by reduced mixing during neap tides. Development of red-water was accompanied by removal of most of the dissolved NH+4from the water column, whereas some NO−3persisted, presumably maintained by freshwater input. NO−3and NH+4gradually returned to pre-bloom concentrations as the red-water declined in late summer. Maximal biomass ofM. rubrumappeared to be limited by irradiance, and self-shading probably imposed an upper limit of around 300 mg chl a m−2within the water column. At the observed levels of chl a, irradiance values within the population maximum between 1 and 3 m depth were only just of the order (≈15 μmol photons m−2 s−1) required to balance estimated respiratory demands. Oxygen concentration became undersaturated during the late bloom phase, with minimal values of 20–30% saturation recorded in deeper waters; however, despite this and reduced numbers of macrozooplankton, direct deleterious effects on other organisms were not observed. 相似文献
180.
A. Bhatnagar R. M. Jain J. T. Burkepile I. M. Chertok A. Magun H. Urbarz P. Zlobec 《Astrophysics and Space Science》1996,243(1):209-213
The powerful cosmic ray flare of Sept. 29, 1989 occurred behind the limb and was observed over a wide spectral range. The analysis of optical, radio, and other relevant data suggest two phases of energy release. After an impulsive phase a prolonged post eruption energy release occurred in an extended region of the corona following the eruption of a large coronal mass ejection (CME). This phase is responsible for numerous coronal and interplanetary phenomena including the ground-level increase of cosmic rays. 相似文献