首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   14456篇
  免费   694篇
  国内免费   126篇
测绘学   362篇
大气科学   1251篇
地球物理   3566篇
地质学   5453篇
海洋学   1079篇
天文学   2266篇
综合类   47篇
自然地理   1252篇
  2022年   86篇
  2021年   209篇
  2020年   249篇
  2019年   218篇
  2018年   462篇
  2017年   474篇
  2016年   638篇
  2015年   484篇
  2014年   546篇
  2013年   965篇
  2012年   644篇
  2011年   665篇
  2010年   622篇
  2009年   691篇
  2008年   561篇
  2007年   476篇
  2006年   500篇
  2005年   385篇
  2004年   453篇
  2003年   387篇
  2002年   389篇
  2001年   273篇
  2000年   257篇
  1999年   213篇
  1998年   214篇
  1997年   201篇
  1996年   169篇
  1995年   183篇
  1994年   196篇
  1993年   155篇
  1992年   141篇
  1991年   144篇
  1990年   161篇
  1989年   113篇
  1988年   108篇
  1987年   164篇
  1986年   128篇
  1985年   196篇
  1984年   224篇
  1983年   174篇
  1982年   160篇
  1981年   171篇
  1980年   127篇
  1979年   148篇
  1978年   137篇
  1977年   126篇
  1976年   110篇
  1975年   108篇
  1974年   83篇
  1973年   104篇
排序方式: 共有10000条查询结果,搜索用时 609 毫秒
101.
102.
103.
Barite has been found for the first time in the Swiss Jura (Hirni, Passwang). Small, white crystals of barite with yellow ankerite grow on calcite in different chambers of a Bajocian ammonite. Chemical data (by microprobe analysis) and lattice parameters (refined by leastsquares from powder patterns) are given for the two minerals. There is a distinct correlation between the cell dimensions and the chemical composition for both of them. The genesis is discussed.  相似文献   
104.
Summary Correlation between some physical and chemical variables, measured at 28 stations of the United States Weather Bureau Network, and seasonal and annual frequencies of precipitation, has been attempted with the aim of gaining insight into the bearing of such variables, on occurrence of precipitation. — Concurrent trends of frequencies with local temperature functions, altitude parameters, precipitable water vapour increments, and some chemical species have been found.Contribution of the «Centro Nucleazione Aerosoli» of the National Research Council of Italy, Via Vettore 4 (Monte Sacro),Roma.  相似文献   
105.
Summary According to the author the reason of the subsistence of the 11 years' sunspot cycle is to be found in the conjunction of the planets. As a result of the investigations it has been stated that from among the nine planets the joint tidal effect of Venus-Jupiter-Earth is a decisive factor in the variations of sunspot activity. The above three planets are every 10.4 years and 12.0 years respectively, in a close conjunction. The mean value (11.2 years) is in an almost full accordance with the average cycle-period. The fluctuations of the period of the cycle come from the fact that the planets' getting into approximately one line takes a different time within each cycle. This time delay is the cause of the stronger or weaker sunspot activity, and of the shorter or longer cycles.  相似文献   
106.
107.
108.
Unusual large-scale phytoplankton blooms in the equatorial Pacific   总被引:1,自引:0,他引:1  
Unusual large-scale accumulations of phytoplankton occurred across 10,000 km of the equatorial Pacific during the 1998 transition from El Niño to La Niña. The forcing and dynamics of these phytoplankton blooms were studied using satellite-based observations of sea surface height, temperature and chlorophyll, and mooring-based observations of winds, hydrography and ocean currents. During the bloom period, the thermocline (nutricline) was anomalously shallow across the equatorial Pacific. The relative importance of processes that enhanced nutrient flux into the euphotic zone differed between the western and eastern regions of the blooms. In the western bloom region, the important vertical processes were turbulent vertical mixing and wind-driven upwelling. In contrast, the important processes in the eastern bloom region were wave-forced shoaling of nutrient source waters directly into the euphotic zone, along-isopycnal upwelling, and wind-driven upwelling. Advection by the Equatorial Undercurrent spread the largest bloom 4500 km east of where it began, and advection by meridional currents of tropical instability waves transported the bloom hundreds of kilometers north and south of the equator. Many processes influenced the intricate development of these massive biological events. Diverse observations and novel analysis methods of this work advance the conceptual framework for understanding the complex dynamics and ecology of the equatorial Pacific.  相似文献   
109.
110.
Ringnet fishing began in the early 20th century and is practised worldwide, mainly to target nearshore pelagic species. The method was introduced to Kenya’s coastal waters by migrant fishers from Tanzania. However, the impacts of this fishing gear remain poorly assessed. We assessed the spatial distribution of ringnet fishing effort and its possible effects on ecosystem components, such as coral reefs, marine megafauna and marine protected areas, on the south coast of Kenya. We tracked 89 ringnet fishing trips made from December 2015 to January 2016 and used spatial multicriteria analysis to determine hotspots of possible environmental risks. The results showed that habitat type and bathymetric profile influenced the spatial distribution of ringnet fishing effort. Mixed seagrass and coral habitats had the highest concentration of the effort. Most of the habitats in the study area were moderately exposed to the impacts of the ringnet fishery. The study identifies high-risk areas that require spatial measures to minimise possible environmental risks of the gear both to habitats and to endangered sea turtles.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号