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   75470篇
  免费   1510篇
  国内免费   702篇
测绘学   1813篇
大气科学   5421篇
地球物理   15108篇
地质学   26755篇
海洋学   6572篇
天文学   17140篇
综合类   219篇
自然地理   4654篇
  2022年   405篇
  2021年   732篇
  2020年   819篇
  2019年   881篇
  2018年   1964篇
  2017年   1826篇
  2016年   2323篇
  2015年   1420篇
  2014年   2283篇
  2013年   4011篇
  2012年   2421篇
  2011年   3318篇
  2010年   2781篇
  2009年   3749篇
  2008年   3436篇
  2007年   3213篇
  2006年   3045篇
  2005年   2500篇
  2004年   2410篇
  2003年   2257篇
  2002年   2077篇
  2001年   1895篇
  2000年   1821篇
  1999年   1473篇
  1998年   1569篇
  1997年   1446篇
  1996年   1176篇
  1995年   1202篇
  1994年   1022篇
  1993年   928篇
  1992年   903篇
  1991年   825篇
  1990年   907篇
  1989年   771篇
  1988年   697篇
  1987年   885篇
  1986年   711篇
  1985年   915篇
  1984年   1004篇
  1983年   918篇
  1982年   893篇
  1981年   780篇
  1980年   723篇
  1979年   658篇
  1978年   675篇
  1977年   602篇
  1976年   584篇
  1975年   540篇
  1974年   545篇
  1973年   503篇
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
891.
Some aspects of the upper ocean layer dynamics are considered, including the Ekman transport, the relation of the thickness of the quasi-homogeneous layer to the thickness of the layer of rotation of the drift current vector, and the correlation between the level of the turbulent energy dissipation and the thickness of the quasi-homogeneous layer.  相似文献   
892.
893.
An extensive experimental and computational investigation of the combined and separate effects of free surface and body on the lift characteristics of a pair of fins attached to a strut and fin alone is conducted. The results reveal that the free-surface effect becomes significant when the depth of submergence to chord ratio (H/c) is less than three. The effect of the strut is also realized for shallower depth of submergence of the fins through free-surface deformation leading to a significant change in the incidence angle of the flow to the fins. The numerical results based on the Higher Order Boundary Element Method with the linearized free-surface condition show good agreement with the experimental results for fin (foil) alone even at shallow submergence, but some discrepancies appear for the fin attached to the strut at higher speeds mostly due to the neglect of the nonlinear free-surface effect.  相似文献   
894.
The burrowing and emergence behavior of sand worms, Nereis (Neanthes) virens Sars, in sediment contaminated with sublethal concentrations of Prudhoe Bay crude oil ranging from 74 to 5222 ppm, was studied in the laboratory. Initial burrowing in oiled sediment did not differ from that observed in unoiled sediment. Emergence of exposed worms was related to the oil concentration and the extent of weathering of oiled sediment. Sand worms buried in unoiled sediment did not emerge. Worms dug from oiled sediment after 12 hours' exposure and placed on unoiled sediment were impaired, resulting in a significant increase in time to burrow. However, after 12h, they recovered and burrowed at normal rates. Worms held in oiled sediment for 96 h also recovered. Possible causes for recovery are discussed. Results suggest that oil-induced aberrations may increase vulnerability to predation.  相似文献   
895.
896.
897.
898.
A global survey of the distribution of dissolved CO2 taking advantage of sampling opportunities provided by the World Ocean Circulation Experiment: World Hydrographic Program (WOCE-WHP) is being carried out through 1995. Goals include the measurement of oceanic inorganic carbon transport and the development of a data base from which future fossil-fuel CO2 build-up can be monitored. The analytical method selected for total carbon dioxide (CT) is gas extraction of acidified seawater with coulometric titration of the acid formed by the resultant carbon dioxide and monoethanolamine. To combine high accuracy and precision (± 1.5 μmol/kg for CT ≥ 2000 μmol/kg) with a high rate of analysis, we have modified an automated single-parameter system. Following prototype development between 1987 and 1990, an instrument emerged with the acronym Somma standing for single-operator multiparameter metabolic analyzer. Improved functional and operating procedures have integrated electronic calibration, CO2 gas calibration, and sample analysis with automated pressure, temperature, and conductivity (salinity) sensing into a single convenient transportable package.  相似文献   
899.
900.
During the EPOS leg 2 cruise (European Polarstern Study, November 1988–January 1989), the production rate of biogenic silica in the euphotic zone was measured by the 30Si method at stations in the Scotia and Weddell Seas.The highest integrated production rates were observed in the Scotia Sea (range: 11.2–20.6 mmol Si m−2 day−1), the marginal ice zone of the Weddell Sea exhibiting somewhat lower values (range: 6.0–20.0 mmol Si m−2day−1).Our results demonstrate that as far as biogenic silica production is concerned the marginal ice zone of the Weddell Sea is considerably less productive than that of the Ross Sea. Our results also indicate that the water of the Antarctic Circumpolar Current (ACC) could be more productive in late spring and early summer than at the beginning of spring. Possible reasons for the differences among the three subsystems (Ross Sea, Weddell Sea and ACC) are discussed.  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